Hard Probes 2012

27 May 2012, 14:30 → 1 Jun 2012, 13:30 Europe/Rome

Cagliari - Italy

Antinori Federico (INFN), Scomparin Enrico (INFN), Usai Gianluca (University of Cagliari)

IMPORTANT DATES: - Start of registration: 16 January - Deadline for early registration: 10 April - Start of abstract submission: 16 January - Deadline for abstract submission: 4 March - Deadline for support request: 4 March

Sunday 27 May

14:30 → 16:30 Student lectures I

14:30 Quarkonium production: experimental aspects

Speaker: Hermine Woehri (CERN)

Slides HKWoehri-HP2012-Cagliari.pdf

15:30 Quarkonium production: theory aspects

Speaker: elena Ferreiro (Universidade Santiago de Compostela)

Slides FerreiroHP2012students.pptx

16:30 → 17:00 Coffee break

17:00 → 19:00 Student lectures II

17:00 Jets: experimental aspects

Speaker: Mateusz Ploskon (Lawrence Berkeley National Laboratory)

Slides quenchinglectureALL.pdf

18:00 Jet quenching: theory

Speaker: Guilherme Milhano (CENTRA-IST (Lisbon) and CERN-PH-TH)

Slides quenchinglectureALL.pdf

Monday 28 May

09:00 → 09:15 Welcome

09:15 → 10:45 Plenary 1A

09:15 No Pain, no Gain? Hard Probes of Quark-Gluon Plasma Coming of Age

Hard QCD and QED probes hold the promise of providing controlled, rigorously calculable means of interrogating hot and dense QCD Matter, measuring its properties and determining its structure. While the RHIC has opened hard probes to experimental investigation, the LHC has extended the range of hard probe measurements by an order of magnitude. Data on jets, high-pT hadrons, quarkonia, photons, and lepton pairs over a wide kinematic range are now putting the challenge back into the theorists' court: Can we use these hard probes as hoped to characterize the quark-gluon plasma created in heavy ion collisions model independently and determine its structure? Related questions concern the investigation of the low-x parton structure on large nuclei and of the thermalization mechanism by means of hard probes. In my lecture I will review the status of experimental results and of theoretical efforts to understand and interpret them prior to Hard Probes 2012. I will also identify issues that need to be clarified before the full promise of hard probes for hot QCD matter can be realized.

Speaker: Berndt Mueller (Duke University)

Slides MuellerB_1A.pdf

09:45 Jet quenching and heavy flavor production in ATLAS

Measurements of inclusive jet and heavy quark jet suppression in relativistic heavy ion collisions are presented. The measurements were performed using Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$~TeV recorded with the ATLAS detector at the LHC during the 2010 Pb ion run. Results are obtained using calorimetrically reconstructed jets using the anti-kt algorithm with a per-event background subtraction procedure. Measurements of the single inclusive jet spectrum with jet radius parameters R = 0.2, 0.3, 0.4 and 0.5, are presented. The spectra are unfolded to correct for the finite energy resolution introduced by both detector effects and underlying event fluctuations. Single jet production, through the central-to-peripheral ratio RCP, is presented as a function of jet pT, centrality and jet radius. Measurements of the single inclusive muons are also presented over the range 4 < pT < 14 GeV, where the production is dominated by the semi-leptonic decays of open heavy flavor hadrons. The muon spectra and RCP are presented as a function of pT and centrality, which provide sensitivity to the quenching of heavy quarks.

Speaker: Aaron Angerami (Columbia University)

Slides AngeramiA_1A.pdf

10:15 Jet quenching and energy loss in CMS

The energy loss of fast partons in hot and dense nuclear matter is studied in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV with the CMS detector at the LHC. Detailed studies of the dijet momentum balance and angular correlations are performed as a function of collision centrality and leading jet transverse momentum up to 350 GeV/c. Dijets in central collisions are found to be more unbalanced than the corresponding dijets in pp data up to the highest values of the leading jet transverse momenta studied. Concurrently, the dijet angular correlations in PbPb collisions remain consistent with the pp reference. Further, the transverse momentum balance in isolated photon-jet pairs is studied. Using the isolated photon as an unbiased measure of the initial parton energy, the ratio of the jet and photon transverse momenta is studied relative to a corresponding pp reference. A significant decrease of the jet to photon momentum ratio with increasing collision centrality is observed. Track-jet correlations and jet fragmentation functions are also studied. A large fraction of the momentum of unbalanced jets is found to be carried by low-pT particles at large radial distance from the jet axis, while the hard component of the fragmentation function evaluated with respect to the reconstructed jet momentum is not strongly modified in comparison to jet fragmentation in vacuum.

Speaker: Christof Roland (MIT)

Slides RolandC_1A.ppt

10:45 → 11:15 Coffee break

11:15 → 12:45 Plenary 1B

11:15 High pT identified particle production in ALICE

The production of particles at high $p_T$ in pp collisions can be described using perturbative QCD. In Pb-Pb collisions the observed yield of high $p_T$ particles is much smaller than expected from binary scaling because of strong final state interactions with the surrounding dense medium. Experiments at RHIC have shown that this modification is very different for mesons and baryons. In the ALICE experiment it is possible to identify particles with very high transverse momentum, $p_T \gg 3$~GeV/c. Charged pions and kaons+protons (together) can be identified from the $dE/dx$, measured in the Time Projection Chamber, thanks to the separation on the relativistic rise. $K^0_s$ and $\Lambda$ can be identified from their V0 weak decay topology. In this talk preliminary results from measurements in pp at $\sqrt{s} = 2.76$~TeV and Pb-Pb at $\sqrt{s_{NN}} = 2.76$~TeV for $3.0 < p_T < 20.0$~GeV/c will be shown. The evolution of $R_{AA}$ for identified particles with centrality and $p_T$ will be discussed and compared to unidentified charged particles, theoretical predictions, and measurements at RHIC.

Speaker: Peter Christiansen (Lund University)

Slides ChristiansenP_1B.pptx

11:45 Perturbative vs non-perturbative aspects of jet quenching: the role of color flow

Providing a unified description of the various "jet-quenching" observable nowadays available represents a deep challenge. Several model calculations are available on the market, mostly based on the factorization between an energy loss occurring at the partonic level in the medium and a standard time-delayed non-perturbative hadronization stage taking place in the vacuum. Two important features of QCD were so far ignored in the literature of jet-quenching, namely the correlations between successive gluon emissions (what is known in the vacuum as angular-ordering) and the analysis of color-flow. I will briefly illustrate how the above features are essential to understand important QCD results in elementary collisions, so that studying how they are modified in the presence of a medium looks mandatory: this will be the main topic of my talk. I will start reminding (very briefly) the audience recent findings on the ``anti-angular ordering'' of QCD antenna radiation in the medium. Then I will move to the problem of medium modification of color flow. I will show how radiative energy-loss calculations can be given a rigorous color-differential formulation and illustrate the implications of the modified color connections of the radiated gluons (with respect to a shower developing in the vacuum) at the hadronization stage. The phenomenological relevance for the interpretation of experimental results on quenching of hadron spectra and jet-fragmentation in AA collisions will be discussed.

Speaker: Andrea Beraudo (CERN)

Slides BeraudoA_1B.pdf

12:15 Advancing QCD-based calculations of energy loss

As a first step towards an understanding the physics of jet modifications in heavy-ion collisions, we analyze the radiation off a QCD antenna propagating through a quark-gluon plasma. The resulting spectrum is characterized in terms of the hardest scale of the problem. We show that this scale is either 1) the inverse size of the antenna as probed by the medium or 2) the maximal momentum transfer from the medium, given by the saturation scale of the QGP. In the former case, called the ``dipole" regime, the antenna preserves its color correlation during the passage through the QGP and radiates coherently. In the latter situation, called the ``decoherence" regime, which applies to the case of a dense medium, the antenna constituents de-cohere rapidly and the resulting spectrum is predominantly the superposition of the independent spectra off the two components. In both cases, however, vacuum coherence is restored for gluons emitted with transverse momenta larger than the hard scale of the associated regime. We explore the typical timescales relevant for emissions in both cases and discuss shortly about the implications for multi-gluon emissions in the QGP, relevant for jet studies in heavy-ion collisions. References Y. Mehtar-Tani, C.A. Salgado, K. Tywoniuk, PRL 106 (2011) 122002 Y. Mehtar-Tani, C.A. Salgado, K. Tywoniuk, PLB 707 (2012) 156 Y. Mehtar-Tani, C.A. Salgado, K. Tywoniuk, arXiv:1102.4317 [hep-ph] Y. Mehtar-Tani, C.A. Salgado, K. Tywoniuk, in preparation

Speaker: Konrad Tywoniuk (Lund University)

Slides HP2012_Tywoniuk.pdf

12:45 → 14:00 Lunch

14:00 → 16:00 Parallel IA: Quarkonia

14:00 Measurement of charmonium production in PbPb collisions at 2.76TeV with CMS

The Compact Muon Solenoid (CMS) is fully equipped to measure hard probes in the di-muon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Such probes are especially relevant for studying the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS has measured the nuclear modification factors of non-prompt J/psi (from b-hadron decays) and prompt J/psi in PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV. For prompt J/psi with relatively high pt (pt=6.5-30 GeV/c), a strong, centrality-dependent suppression is observed in PbPb collisions, compared to the yield in pp collisions scaled by the number of inelastic nucleon-nucleon collisions. In the same kinematic range, a suppression of non-prompt J/psi, which is sensitive to the in-medium b-quark energy loss, is measured for the first time. Results from the 2010 data taking period are reported and an outlook on the 2011 data analysis will be given.

Speaker: Moon Dong Ho (Korea University)

Slides MoonDH_1A.pdf

14:20 Nuclear modification of $J/\psi$ production in Pb--Pb collisions at $\sqrt{s_{NN}}=2.76$\,TeV

Heavy quarkonium states, such as the $J/\psi$, are expected to provide essential information on the properties of high-energy heavy-ion collisions where the formation of a Quark-Gluon Plasma is expected. The impact on the $J/\psi$ production of such a hot and dense medium formed in the early times of the collision has been extensively studied at SPS and RHIC energies. It is expected that due to colour screening mechanisms $J/\psi$ production is suppressed in a plasma of quarks and gluons. At LHC energies, however, charm is produced abundantly in central Pb--Pb collisions allowing for scenarios where originally uncorrelated charm and anti-charm quarks (re)combine at the phase boundary. Measuring the $J/\psi$ production at LHC will help to disentangle between the different mechanisms. ALICE is the dedicated heavy-ion experiment at the LHC. Due to the unique particle identification capabilities of the central barrel detectors ($|\eta|<0.9$), $J/\psi$ can be measured in the di electron channel in the very demanding environment of central Pb--Pb collisions at LHC. In addition $J/\psi$ is measured at forward rapidity ($2.5 < y < 4$) with a dedicated muon spectrometer. ALICE is the only LHC experiment with a $J/\psi$ acceptance that reaches down to $p_t = 0$ at both mid- and forward-rapidity. First results on the nuclear modification factor of the inclusive $J/\psi$ production at mid-rapidity in Pb--Pb collisions at $\sqrt{s_{NN}}=2.76$\,TeV will be presented. The mid-rapidity measurement is complemented with the results obtained at forward-rapidity.

Speaker: Jens Wiechula (Physikalisches Institut, Universität Tübingen)

Slides WiechulaJ_IA.pdf

14:40 Heavy-quarkonium suppression from medium-induced gluon radiation

Quarkonium nuclear suppression in p-A collisions at large xF can be accurately described in a model based on first principles (medium-induced gluon radiation) and depending on a single free parameter. Our results strongly support `parton energy loss' as a dominant effect in quarkonium nuclear suppression, and moreover give some clue on the quarkonium hadroproduction mechanism.

Speaker: Francois Arleo (LAPTH)

Slides hp12_arleo_v2.pdf

15:00 The relation between cross-section, decay width and imaginary potential of heavy quarkonium in a quark-gluon plasma.

Computations with weakly-coupled plasmas and some lattice results suggest that the heavy quarkonium potential has an imaginary part that is important in order to study dissociation. This imaginary part is due to the scattering with partons in the medium in a process called quasi-free dissociation. At temperatures much below dissociation another process that is known to be important is the gluo-dissociation. The aim of the talk is to clarify in a perturbative framework the relation of the different expressions for the quarkonium cross-sections that can be found in the literature with the quarkonium thermal width. Finally, with the use of effective field theories we evaluate the quasi-free and gluo-dissociation cross-sections in a wide range of temperatures ranging from the binding energy to the dissociation temperature.

Speaker: Miguel Angel Escobedo (Technische Universität München)

Slides hp.pdf

15:20 Improving the J/psi Production Baseline at RHIC and the LHC

We assess the theoretical uncertainties on the inclusive J/psi production cross section in the Color Evaporation Model using values for the charm quark mass, renormalization and factorization scales obtained from a fit to the charm production data [1]. We use our new results to provide improved baseline comparison calculations at RHIC and the LHC. We calculate the fraction of J/psi production from B decays as a function of pT with these parameters. We also study cold matter effects on J/psi production at leading relative to next-to-leading order in the CEM within this approach [2]. [1] A. D. Frawley, R. Nelson and R. Vogt, in preparation. [2] R. Nelson and R. Vogt, in progress.

Speaker: Ramona Vogt (LLNL and UC Davis)

Slides HP12_vogt_talk.pdf.pdf

15:40 J/psi elliptic flow measurement in Pb-Pb collisions at forward rapidity in the ALICE experiment

J/psi suppression induced by color screening of its constituents quarks was proposed 26 years ago as a signature of the formation of a quark gluon plasma in heavy-ion collisions. Recent results from ALICE in Pb-Pb collisions exhibit a smaller suppression with respect to SPS and RHIC previous measurements. The study of azimuthal anisotropy in particle production gives information on the collective hydrodynamic expansion of the QGP. In particular, J/psi elliptic flow v2 is important to test the degree of thermalization of particles containing heavy quarks. Together with the production yields, the elliptic flow is a powerful observable to address the question of suppression and regeneration of J/psi in QGP. We present the first J/psi elliptic flow measurement obtained at forward rapidity and in the low transverse momentum region. v2 is measured as a function of transverse momentum and as a function of rapidity in mid-central collisions. Comparison with recent STAR results is performed.

Speaker: Laure MASSACRIER (Subatech Nantes)

Slides HardProbes_massacrier.pdf

14:00 → 16:00 Parallel IB: Jet quenching and energy loss

14:00 Weakness or Strength in the Golden Years of RHIC and LHC?

We present comparisons of the latest pQCD- and AdS/CFT-based energy loss models with the newest high-pT measurements from RHIC and LHC. Zero parameter predictions of energy loss from WHDG rigorously constrained to PHENIX pi0 RAA data show quantitative agreement with the measured azimuthal anisotropy and D meson suppression at LHC. pQCD predictions follow the qualitative trend of the LHC light hadron suppression, and we report on progress in including next-to-leading order effects which could provide the key ingredient needed for quantitative agreement. LHC D meson predictions from AdS/CFT are oversuppressed compared to data when constrained to RHIC non-photonic electron data, although due to the large experimental uncertainties, the method is not falsified. We also present for the first time theoretically controlled AdS/CFT predictions for light particle suppression, which also suggest that strong-coupling methods leads to a too-large suppression. Finally, we emphasize how the comparison of mass-dependent energy loss calculations to data provides a powerful tool for determining the dominant energy loss mechanism in heavy ion collisions.

Speaker: Horowitz Will (Department of Physics University of Cape Town)

Slides 120528-HP-Horowitz.pptx

14:20 Gauge invariant definition of the jet quenching parameter \hat{q}

The jet quenching parameter \hat{q} describes the transverse momentum broadening of a high-energy parton moving through a thermal medium. In the past, several authors were able to relate this quantity to the expectation value of two light-cone Wilson lines. Recently, the results were reobtained in a systematic way by employing effective field theory methods (Soft-Collinear Effective Theory). However, the calculations were only explicitly performed in a covariant gauge. General arguments were given, motivating the generalization of this result through the addition of transverse gauge links but the explicit derivation of this in effective field theory terms is missing. In this talk I will discuss how the adoption of a general gauge changes the picture and gives rise to a gauge invariant definition of the jet quenching parameter \hat{q}. Furthermore I will comment on practical applications of this result.

Speaker: Michael Benzke (Technische Universität München (TUM))

Slides BenzkeM_P1B.pdf

14:40 Shining a Gluon Beam through Quark-Gluon Plasma

A holographic calculation of the quenching of a beam of gluons with typical momenta q shining through strongly coupled quark-gluon plasma shows that such a beam is attenuated rapidly over a distance of order q^{1/3} (pi T)^{-4/3} as it propagates at the speed of light, shedding trailing sound waves with momenta of order (pi T). At larger and larger q, the trailing sound wave becomes less and less prominent. The outward-going beam of gluon radiation itself shows no tendency to spread in angle or to shift toward larger wavelengths, even as it is completely attenuated. In this regard, the behavior of the beam of gluons that we analyze is reminiscent of the behavior of jets produced in heavy ion collisions at the LHC that lose a significant fraction of their energy without appreciable change in their angular distribution or their momentum distribution as they plow through the strongly coupled quark-gluon plasma produced in these collisions. However, we know that quark-gluon plasma must be weakly coupled at short enough distance scales. This means that even if jet quenching typically occurs as in a strongly coupled plasma, there should be rare events in which a hard parton is scattered by a larger angle, picking up significant transverse momentum.

Speaker: Rajagopal Krishna (MIT Department of physics)

Slides RajagopalK_1B.pdf

15:00 Radiative Energy Loss in the absorptive QGP

A firm knowledge of medium-induced energy loss mechanisms in the deconfined plasma state of QCD matter is essential for our understanding of phenomena such as strong jet quenching and high-pT hadron suppression observed in high energy nuclear collisions at RHIC and LHC. Based on perturbative QCD calculations, it is commonly accepted that the radiative energy loss contribution is dominant, where heavy quarks radiate less than light partons. In these studies, however, possible damping mechanisms on bremsstrahlung gluons have so far been neglected. In this talk it is argued that in an absorptive and polarizable plasma the radiation spectrum becomes significantly reduced, in particular, for charges with high initial energy [1]. This is because damping mechanisms in the absorptive medium may influence the formation of radiation, prominently, in the case of large formation times. Consequently, a suppression of the radiation spectrum has to be attributed to absorption rather than to coherence effects. Being quark mass independent, this new effect might provide a suitable explanation for the observed single electron puzzle. Moreover, as an imaginable damping mechanism, gluon bremsstrahlung from preformed radiation gluons could serve as a source for isotropic, low energy gluon radiation with a significant impact on phenomena studied at RHIC and even more at LHC. [1] M. Bluhm, P. B. Gossiaux, J. Aichelin, Phys. Rev. Lett. 107, 265004 (2011).

Speaker: Marcus Bluhm (Laboratoire SUBATECH)

Slides BluhmM_1B.pdf

15:20 LHC phenomenology of light quark jet quenching in AdS/CFT

In this talk, we explore phenomenological signatures of light quark jet quenching, modeled as falling strings within the AdS/CFT correspondence. In particular, we show that even in the simplest models, it is possible to obtain the correct qualitative behavior of the pion RAA at LHC, and we also present the quantitative predictions. We address the effect of the QCD conformal anomaly on this observable by exploring the behavior of falling strings in a non-conformal gravity dual, whose action is constrained by the lattice QCD data. In addition to this, we will also present results for falling strings in a geometry dual to a system experiencing a Bjorken expansion and investigate the effects of the expansion on the light quark jet quenching observables.

Speaker: Andrej Ficnar (Columbia University)

Slides FicnarA_1B.ppt

15:40 Probe the nuclear matter with jet productions at NLO

Jets physics in heavy ion reactions is an important new area of active research at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) that paves the way for novel tests of QCD multi-parton dynamics in dense nuclear matter. At present, perturbative QCD calculations of hard probes in elementary nucleon-nucleon reactions can be consistently combined with the effects of the nuclear medium up to next-to-leading order (NLO). While such accuracy is desirable but not necessary for leading particle tomography, it is absolutely essential for the new jet observables. With this motivation, We investigate the cold nuclear matter(CNM) effects on jet productions in high-energy nuclear collisions at LHC with the NLO perturbative QCD. The nuclear modifications for dijet angular distributions, dijet invariant mass spectra, dijet transverse momentum spectra and dijet momentum imbalance due to CNM effects are calculated by incorporating EPS, EKS, HKN and DS parametrization sets of parton distributions in nucleus . It is found that dijet angular distributions and dijet momentum imbalance are insensitive to the initial-state CNM effects and thus provide optimal tools to study the final-state hot QGP effects such as jet quenching. Furthermore we present the results and predictions at NLO for productions of the single, the Z0 tagged jet, and double inclusive jet cross sections by including parton energy loss effect in the QGP to be formed in high-energy nucleus-nucleus collisions. We demonstrate how an enhanced di-jet asymmetry in central Pb+Pb reactions at the LHC, recently measured by the ATLAS and CMS experiments, can be derived from these results. We show quantitatively that a significant fraction of this enhancement may be related to the ambiguity in the separation between the jet and soft background medium and point to a suite of measurements that can help build a consistent picture of parton shower modification in heavy ion collisions at the LHC.

Speaker: Ben-Wei Zhang (Central China Normal University)

Slides bwz_HP12.pdf

14:00 → 16:00 Parallel IC: Correlations

14:00 Measurement of charged hadron v_2 at high pT in PbPb collisions at sqrt(s)=2.76TeV with CMS

Measurements of the azimuthal anisotropy of charged hadrons are presented for PbPb collisions at sqrt(s_NN) = 2.76 TeV over an extended transverse momentum range. The data were collected with the CMS detector at the LHC. The anisotropy parameter (v_2) is extracted up to a significantly higher pT region than previous achieved, by correlating charged tracks with respect to the event plane reconstructed using the energy deposited in forward-angle calorimeters. Dihadron angular correlations over wide pseudorapidity gap are also presented for the very high-pt particles. These new data can impose quantitative constraints on the details of in-medium parton energy loss models, particularly the influence of the path length and the shape of the interaction region.

Speaker: Victoria Zhukova (University of Kansas)

Slides VictoriaZhukova_HardProbes.pdf

14:20 Two particle correlation measurements at PHENIX

Measurements of two particle correlations are important tools to dissect the interplay of hard-scattered partons and the hot dense medium created by ultra-relativistic heavy ion collisions. One of important topic of correlation measurements in heavy ion collisions is to evaluate the path-length dependence of parton energy loss and to discern the medium response to it. Another topic is to analyze the contributions of higher harmonic flow, which result from fluctuations within the initial collision geometry, to long range rapidity correlations. In this talk, we will present recent PHENIX measurements in Au+Au 200 GeV collisions of azimuthal correlations with trigger selections relative to the event plane for both high and mid transverse momenta. These produce measurements sensitive to the path-length dependence of parton energy loss and to the influence of medium effects respectively. Azimuthal correlations in which background from higher harmonic flow is removed will also be discussed.

Speaker: Takahito Todoroki (Tsukuba)

Slides 0528_todoroki_hardprobes2012.pdf 0528_todoroki_hardprobes2012.pptx

14:40 Measurement of Fourier components of two-particle correlations in PbPb collisions at sqrt(s)=2.76TeV with CMS

Measurements from the CMS experiment at the LHC of dihadron correlations for charged particles produced in PbPb collisions at a nucleon-nucleon centre-of-mass energy of 2.76 TeV are presented. The results are reported as a function of the particle transverse momenta and collision centrality over a broad range in relative pseudorapidity (delta-eta) and the full range of relative azimuthal angle (delta-phi). A Fourier harmonic decomposition analysis of the long-range azimuthal dihadron correlations is performed. The factorization relation between the extracted Fourier coefficients and a product of single-particle azimuthal anisotropies are studied and discussed in detail. These data provide important insight on the physical origin of the observed long-range dihadron correlations.

Speaker: Rylan Conway (University of California, Davis)

Paper Conway_HardProbes_proceedings_v1.pdf

Slides RylanConway_HardProbes_talk.pdf.pdf

15:00 Production anisotropy of h±, π± and protons at high-pT in Pb-Pb collisions at √sNN=2.76 TeV

The ALICE collaboration has recently reported an observation of an enhanced intra-jet yield of charged particles associated with the high-pT trigger particle in central Pb-Pb collisions at √s_NN=2.76 TeV \cite{Aamodt:2011vg}. There are several possible explanations of the origin of this enhancement: (i) modifications of the fragmentation function, (ii) bias on the p_T distribution of the outgoing partons due to the energy loss and (iii) possible change of the quark/gluon relative abundance due to the different coupling of quarks and gluons to the nuclear medium. An analysis of the transverse jet-fragmentation variation with collisional centrality may help to unravel the origin of the intra-jet charged particle yield enhancement. I will present the measurement of the centrality evolution of the transverse component distribution of the particles associated to the high-pT particle in Pb-Pb collisions at √s_NN=2.76 TeV.

Speaker: Jan Rak (Jyvaskyla Univesity, Finland)

Slides JanRak-HP11_v6.pptx

15:20 Measurement of the coefficients and correlations between higher-order flow harmonics with the ATLAS detector

Recently, the study of harmonic flow coefficients v_n has been the focus of many experimental efforts in heavy-ion collisions at RHIC and the LHC. These coefficients are believed to be associated with the various shape components in the initial geometry, arising from fluctuations of the participating nucleons in the overlap region. The orientation of these harmonic flow (event plane or $\Psi_n$) are generally correlated due to the correlations between the original shape components in the initial geometry. We present detailed measurements of coefficients v_1-v_6, obtained from event plane, two-particle correlation and cumulant methods, as well as measurements of various event plane correlations involving $\Psi_2$ to $\Psi_6$ in Pb-Pb collisions. We discuss the implications of these results on our understanding of the nature of the initial geometry, and on the dynamical evolution of the medium.

Speaker: Jia JIANGYONG (Stony Brook University and BNL)

Slides Jiangyong_v4.ppt

15:40 jet-like near-side peak shapes in Pb-Pb collisions at sqrt s_NN=2.76 TeV with ALICE

In central heavy-ion collisions at the LHC hot and dense matter is formed in which outgoing partons suffer significant energy loss. The quenched energy seems to dominantly reappear at low and intermediate pT, a pT region where collective effects dominate and jet reconstruction is not feasible. To characterize in-medium energy loss in this pT region, we analyse two-particle angular correlations of charged particles using the ALICE detector. Correlations having their origin in collective effects are subtracted using an eta-gap method. The shape and magnitude of the remaining jet-like peak are studied to characterize effects of energy loss occurring in the hot and dense medium. We compare results from central collisions with those from peripheral collisions and pp collisions at the same center-of-mass energy. Further, we compare to Monte Carlo models.

Speaker: Andreas Morsch (CERN)

Paper MorschA_1C.pdf

16:00 → 16:30 Coffee break

16:30 → 18:10 Parallel IIA: Heavy flavour

16:30 D mesons suppression in Pb--Pb collisions at sqrt(sNN)=2.76 TeV measured by ALICE

The production of the prompt charm mesons D0, D+, D∗+, and their antiparticles, in Pb–Pb collisions at the LHC, at a centre-of-mass energy sqrt(sNN) = 2.76 TeV per nucleon–nucleon collision, has been measured with the ALICE detector. The pt-differential production yields in the range 2 < pt < 16 GeV/c at central rapidity, |y| < 0.5, were used to calculate the nuclear modification factor RAA with respect to a proton–proton reference obtained from the cross section measured at sqrt(s) = 7 TeV and scaled to sqrt(s) = 2.76 TeV. For the three meson species, RAA shows a suppression of a factor 3–4, for transverse momenta larger than 5 GeV/c in the 20% most central collisions. The suppression is reduced for peripheral collisions. Prospects for extending these measurements using the Pb–Pb data collected during the 2011 data taking period will also be discussed.

Speaker: Zaida Conesa del Valle (CERN)

Slides ConesadelValleZ_2A.pdf

16:50 Open charm hadron production via hadronic decays at STAR

Heavy quarks are a unique probe to study the medium produced in ultra-relativistic heavy ion collisions. The dominant process of charm quark production at RHIC is believed to be initial gluon fusion which can be calculated in the perturbative QCD. The upper limit of FONLL calculation seems to be in good agreement with charm cross section measurements at mid-rapidity in $p+p$ collisions at $\sqrt{s_{NN}} = 200 GeV provided by STAR. The same measurement in Au+Au collisions at equal energy reveals the number-of-binary-collisions scaling of charm cross section indicating that charm production is dominated by initial hard scatterings. This talk will present the measurements of $D^{0}$, $D^{*}$ in $p+p$ and $D^0$ in Au+Au collisions at 0.6 GeV/$c < p_T < 6$ GeV/$c$ via hadronic decays $D^{0}\rightarrow K^-\pi^+,\ D^{*+}\rightarrow D^0\pi^+\rightarrow K^-\pi^+\pi^+$ in $\sqrt{s_{NN}}=200$ GeV $p+p$ collisions at mid-rapidity $|y|<1. Furthermore, we will present the analysis status on open charm measurement in $\sqrt{s_{NN}}=500$ GeV $p+p$ collisions.

Speaker: David Tlusty (NPI ASCR)

Paper http://arxiv.org/abs/1204.4244

Slides

• Tlusty_HardProbes.pdf
• Tlusty_HardProbes.ppt
• Tlusty_HardProbes.pptx

17:10 Heavy flavor suppression: an interplay of electric and magnetic mass effects

Reliable predictions for jet quenching in ultra-relativistic heavy ion collisions require accurate computation of radiative energy loss. While all available energy loss calculations assume zero magnetic mass – in accordance with the one-loop perturbative calculations – different non-perturbative approaches report a non-zero magnetic mass at RHIC and LHC. We generalized the dynamical energy loss formalism [1], to consistently include a possibility for existence of non-zero magnetic screening [2]. We show that this generalization indicates a fundamental constraint on electric to magnetic mass ratio, which appears to be supported by lattice QCD simulations. Jet suppression patterns, obtained from this newly developed generalization, will be compared with RHIC and LHC measurements. Interestingly, comparison with RHIC data indicates that the generalized dynamical energy loss formalism may provide a reasonable explanation of the "Heavy flavor puzzle at RHIC" [3]. [1] M. Djordjevic, Phys. Rev. C 80, 064909 (2009) (highlighted in: M Gyulassy, Physics 2, 107 (2009)). [2] M. Djordjevic and M. Djordjevic, Physics Letters B 709 (2012) 229 [3] M. Djordjevic, Phys. Rev. C (in press, 2012, arXiv:1105.6082)

Speaker: Magdalena Djordjevic (Institute of Physics, University of Belgrade)

arXiv:1209.0198 Djordjevic_HP2012_proc.zip

Slides MDjordjevic_HP2012.ppt

17:30 Open-charm meson elliptic flow measurement in Pb-Pb collisions at √sNN = 2.76 TeV with ALICE at the LHC.

A Large Ion Collider Experiment (ALICE) is one of the four major experiments at the Large Hadron Collider (LHC), and it is dedicated to the study of ultra-relativistic heavy ion collisions, with the goal of investigating the properties of the high-density state of QCD matter produced in these collisions. The study of D meson production azimuthal anisotropy and the measurement of their elliptic flow (v2) can provide insight on the degree of thermalization of charm quarks in the medium and on the hadronization mechanism. We present the measurement of the D+, D0 and D*+ meson v2 in Pb-Pb collisions at √sNN = 2.76 TeV at the LHC with ALICE. We discuss the details of the analysis and we show the results obtained from data samples collected in 2010 and 2011.

Speaker: Giacomo Ortona (INFN Torino)

Slides OrtonaG_2A.pdf

17:50 Recent results on heavy quark quenching in ultrarelativistic heavy ions collisions: the effect of gluon damping

Recently, we have proposed a microscopic approach for the quenching and thermalisation of heavy quarks (HQ) in URHIC [1, 2, 3], assuming that they interact with light partons through both elastic and radiative processes evaluated by resorting to some parameterization of the running coupling constant, while those partons are spatially distributed along hydrodynamical evolution of the hot medium. This approach is able to explain successfully several observables measured at RHIC, such as the nuclear modification factor and the elliptic flow of non-photonic single electrons. The diffusion coefficient of heavy quarks in the quark gluon plasma – a fundamental property of this state of matter – can thus be extracted and compared with recent lattice calculations. In this contribution at Hard Probes, we would like to discuss the predictions of our model [2] for D and B mesons production in URHIC at LHC energies and confront them with experimental results obtained so far by ALICE and CMS collaborations for Pb-Pb collisions at sqrt s = 2.76 TeV . Jointly, we would like to discuss the question of the influence of a possible gluon damping on the phenomenological consequences of radiative energy loss. In [5], we have indeed studied the effect of an absorptive medium on standard LPM [6] radiation in electrodynamics and have advocated that the large time needed for the photon formation in Bremsstrahlung from ultrarelativistic charges is not affordable if damping is taken into account. Similar effect manifests itself in QCD, as one of us (M. Bluhm) proposes to demonstrate at HP 2012 in another contribution. In our contribution, we would concentrate on the implications of such an effect on the quenching of particles in URHIC, discussing observables such as spectra, elliptic flow, azimutal correlations, as well as the single electron puzzle. References [1] P.B. Gossiaux, J. Aichelin, Phys. Rev. C78, 014904 (2008), [hep-ph/0802.2525]. [2] P.B. Gossiaux, R. Bierkandt, J. Aichelin, Physical Review C79 (2009) 044906 [3] P.B. Gossiaux, V. Guiho, J. Aichelin, J. Phys. G: Nucl. Part. Phys. 37 (2010) 094019 [4] P.B. Gossiaux et al., arXiv:1102.1114 [5] M. Bluhm, P.B. Gossiaux, and J. Aichelin, arXiv:1106.2856, PRL 107 (2011) 265004 [6] L.D. Landau and I. Ya. Pomeranchuk, Dokl. Akad. Nauk SSSR 92 (1953) 535; ibid. 92 (1953) 735.

Speaker: Pol Bernard Gossiaux (Subatech)

Slides GossiauxPB_2A.pptx

16:30 → 18:10 Parallel IIB: Jet quenching and energy loss

16:30 p/pi ratio in jet and bulk region in heavy ion collisions

An important open question regarding hadronization in heavy-ion collisions is to understand the baryon enhancement at intermediate pT observed at RHIC. In this work we analyze the p/π ratio in the associated yield (pT < 5.0 GeV/c) correlated to a high-pT trigger (pT > 5.0 GeV/c) in central Pb–Pb collisions. For this analysis we used Pb–Pb data taken by the ALICE detector at a center of mass energy of √s_NN = 2.76 TeV. We measure the p/π ratio in the near-side jet peak, as well as in the near-side ridge region at large ∆η, to distinguish between hadrons originating from hard partons and hadrons originating from the hot and dense QCD medium. We observe that at intermediate pT the p/π ratio in the jet peak is much lower than in the ridge region.

Speaker: Misha Veldhoen (Utrecht University)

Slides VeldhoenHP2012.pptx

16:50 Proton/pion ratios in relative azimuth with respect to a jet in √sNN = 200 GeV Au+Au collisions at STAR

Inclusive Proton/pion ratios show an enhancement at intermediate transverse momentum (PT ~1.5 - 4.0 GeV/c) in central √sNN = 200 GeV Au+Au collisions compared to peripheral Au+Au and p+p collisions. This effect suggests a production mechanism, different from fragmentation, which is consistent with coalescence and recombination models. A high-ET trigger particle selects a surface-biased jet, which is measured to have a similar PT distribution as a p+p jet. This jet is used to enhance the quenching effects of the recoiling, medium traversing one. We reconstruct the trigger jet using the FASTJET algorithm, with a ET (PT) cut of 3.0 GeV(/c) on the towers(tracks) in order to reduce the heavy-ion collision background. The particle identification of tracks with PT up to ~ 2.8 GeV/c is obtained by taking advantage of STAR TOF and TPC detectors with full azimuth coverage. Correlations in Δϕ between jets and identified hadrons are presented, and the particle ratios in different regions of azimuth are measured. Particle ratios associated with the trigger jet vs. the recoil jet, and comparisons to inclusive particle ratios can help to distinguish between jet-related (vacuum and medium-modified) and bulk-related contributions to the ratios enhancement.

Speaker: Alan Davila (University of Texas at Austin)

Slides DavilaHardProbes2012.pdf

17:10 Data-driven analysis methods for the measurement of reconstructed jets in heavy ion collisions at RHIC and LHC

We present new, data-driven analysis methods for the measurement of reconstructed jets over a broad kinematic range, in the complex environment of heavy ion collisions. We utilize simple model calculations that contain the essential features of jet reconstruction in nuclear collisions at both energies, in order to assess systematic uncertainties of the methods due to large experimental backgrounds. Based on a Bayesian approach to unfolding of background effects, we establish criteria for systematically significant jet measurements over the full jet energy range at both RHIC and LHC. These criteria are satisfied only by specific choices of inclusive and coincidence observables, which have not been considered previously.

Speaker: Gabriel de Barros (Universidade de São Paulo)

Slides hp12v9.pdf

17:30 Measurement of inclusive jet cross section and jet fragmentation in pp collisions with ALICE experiment at the LHC

Jet reconstruction and jet structure variables provide important information to study the interaction between hard scattered partons and the Quark-Gluon Plasma. This talk presents the measurement of the inclusive cross-section for fully reconstructed jets in pp collisions at \sqrt{s}=2.76 TeV, which provides an essential reference for jet measurements in Pb-Pb collisions at the same \sqrt{s_{NN}}. In addition, we report the inclusive cross section and jet structure measurements for charged particle jets in pp collisions at \sqrt{s}=7 TeV. These measurements utilize the ALICE central barrel tracking system to detect charged particles with good efficiency above 150 MeV/c, together with the Electromagnetic Calorimeter (EMCal). The jet cross section and structure measurements are compared to theoretical calculations.

Speaker: Rongrong Ma (Yale University)

Slides 2012HP_ppJets_Final.pdf

17:50 A probabilistic picture for in-medium jet evolution

We study the perturbative evolution of a jet via multiple gluon emissions induced by the interactions between the jet constituents and a dense QCD medium like a quark-gluon plasma. We focus on the typical medium-induced gluons emissions, for which the gluon formation time is much smaller than the overall size of the medium. We show that the typical time between two subsequent emissions is parametrically larger than the formation time for one gluon (in contrast to jet fragmentation in the vacuum, where these two scales get identified with each other). This separation of scales has a remarkable physical consequence: it implies that coherence phenomena are negligible and therefore successive emissions can be treated as independent from each other and ordered in time. This is important as it allows for a probabilistic interpretation of the in-medium jet fragmentation as a classical branching process, which is in particular suitable for implementation as an event generator.

Speaker: Yacine Mehtar-Tani (IPhT (CEA-Saclay))

Slides HP-2012-Mehtar.pdf

16:30 → 18:10 Parallel IIC: High pT suppression, Global Observables

16:30 Measurement of dN/deta and dET/deta in PbPb collisions at sqrt(s)=2.76TeV with CMS

Measurements of the charged hadron multiplicity and transverse energy are presented for minimum bias PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair. The number of charged hadrons was obtained by two different methods based on the inner silicon pixel system of the CMS detector at the LHC. One technique involved counting the number of reconstructed single particle hits in the pixel detector, while the other formed hit pairs from the different detector layers. The two methods are in excellent agreement. For the transverse energy measurement, CMS has almost hermetic calorimetry with fine granularity and excellent resolution allowing to measure the energy over nearly the complete angular range. The measurements are performed as a function of pseudorapidity and centrality. The results are compared with heavy-ion results from earlier experiments, where a smooth dependence on the collision energy is observed. The centrality dependence of transverse energy production becomes steadily weaker as the pseudo-rapidity increases from 0 to 5.

Speaker: Michael Murray (University of Kansas)

Slides MurrayHardProbes2012v4.pdf

16:50 Neutral pion production in Au+Au collisions at RHIC

Neutral pion production in \sqrt{sNN}=130 and 200 GeV central Au+Au collisions at the Relativistic Heavy Ion Collider has been found to be strongly suppressed in comparison to the expectations from properly scaled p+p collisions [1], while data from d+Au collisions showed no suppression [2]. This observation was one of the first convincing signatures of a strongly interacting partonic medium created in high energy heavy ion collisions. A study of the azimuthal anisotropy of pi0 production in the RHIC \sqrt{sNN}=200 GeV Au+Au data from 2004 indicated that the azimuthal dependence of the nuclear modification factor R_AA is strongly correlated with the (approximately elliptical) geometry of the overlap region up to pT=10 GeV/c [3]. The approximately 4 times higher integrated luminosity of 2007 Au+Au collision data, together with a reaction plane detector upgrade, allowed for a significantly improved determination of the dependence of R_AA on the reaction plane up to pT=18 GeV/c. Recent results on the azimuthal anisotropy of pi0 production will be shown and compared to several theoretical models. A 2005 RHIC energy scan with Cu+Cu collisions pointed out that pi0 suppression still exists in this lighter system at 62.4 GeV, while the 22.4 GeV Cu+Cu results showed an enhancement consistent with theoretical predictions [4]. Onset and evolution of the in-medium suppression with collision energy, centrality and system size will be presented, including the new results from Au+Au data collected in 2010 at \sqrt{sNN}=39 and 62.4 GeV c.m.s. energies. [1] K. Adcox et al., Phys. Rev. Lett. 88, 022301 (2002); S. S. Adler et al., Phys. Rev. Lett. 91, 072301 (2003). [2] S. S. Adler et al., Phys. Rev. Lett. 91, 072303 (2003). [3] S. Afanasiev et al., Phys. Rev. C 80, 054907 (2009). [4] A. Adare et al., Phys. Rev. Lett. 101, 162301 (2008).

Speaker: Robert Vertesi (Institute for Particle and Nuclear Physics, Wigner RCP, HAS)

Slides vertesi-hp12.pdf

Slides with overlay vertesi-hp12b.pdf

17:10 Neutral meson production in pp and Pb-Pb collisions at LHC

Identified hadron spectra are considered as a sensitive probe for transport properties of strongly interacting matter produced in heavy ion collisions. The ALICE detector at the LHC studies pi0 and eta meson production via their two-photon decays by two complementary methods, using electromagnetic calorimeters and the central tracking system for photons converted to e+e- pairs on the material of the inner ALICE detectors. Production spectra of pi0 and eta mesons were measured with ALICE in pp collisions at LHC energies at mid-rapidity in a wide transverse omentum range. The spectrum and the nuclear modification factor R_AA of the pi0 production measured in Pb-Pb collisions at different centralities, show a clear pattern of strong suppression in a hot QCD medium with respect to pp collisions. Azimuthal anisotropy v_2 of pi0 production measured with ALICE is consistent with v_2 for other hadron species. Comparison of the ALICE results on neutral mesons with the lower-energy experiments is discussed.

Speaker: Yury Kharlov (Institute for High Energy Physics)

Paper http://arxiv.org/abs/1208.4712

Slides NeutralMesons_HP2012_v11.pdf

17:30 Measurement of charged hadron R_AA at high pT in PbPb collisions at sqrt(s)=2.76TeV with CMS

The nuclear modification factor R_AA is one of the key signatures for the energy loss of fast partons traversing a QCD medium. Charged particle transverse momentum (pT) spectra have been measured by CMS for pp and PbPb collisions at the same collision energy per nucleon pairs, sqrt(s_(NN))=2.76 TeV, corresponding to integrated luminosities of 230 nb-1 and 150 ub-1, respectively. Calorimeter-based high-transverse-energy jet triggers are employed to enhance the statistical reach of the high-pT measurements. The pp results are compared to various generator tunes and also to an empirical scaling of different collision energies with xT=2pT/sqrt(s) over the pT range up to 100 GeV/c. We have obtained R_AA in bins of collision centrality for the PbPb data sample dividing by the measured pp reference spectrum. In the range pT = 5-10 GeV/c, the charged particle yield in the most central PbPb collisions is suppressed by up to a factor of 7. At higher pT, this suppression is significantly reduced, approaching roughly a factor of 2 for particles with pT = 40 - 100 GeV/c.

Speaker: Krisztian Krajczar (MIT)

Slides HP2012_Krajczar.pdf

17:50 Charged particle production in Pb-Pb collisions at the LHC with the ALICE detector

(on behalf of the ALICE Collaboration) The ALICE experiment measured charged particle production in $\sqrt{s_{NN}}=2.76$~TeV Pb-Pb collisions at the LHC. We report on results on charged particle multiplicity and transverse momentum spectra. All the results are presented as a function of centrality, estimated with a Glauber Monte Carlo fit to multiplicity distributions reconstructed in various detectors. The applicability of the Glauber picture at LHC energies, the resolution of the centrality determination and the related systematic uncertainties will be discussed in detail. Particles are tracked in the pseudorapidity window $\left|\eta\right| < 0.8$\ with the silicon Inner Tracking System (ITS) and the Time Projection Chamber (TPC), allowing us to cover the range $0.15 < p_t \lesssim 50$~GeV/$c$. The low-$p_t$ cut-off is further reduced in the multiplicity measurement using ``tracklets'', reconstructed in the 2 innermost layers of the ITS. The charged particle multiplicity is measured to be $\mathrm{d}N_{ch}/\mathrm{d}\eta = 1601 \pm 60$ in 0-5\% most central Pb-Pb collisions, indicating an energy density a factor $\sim 3$ higher than at RHIC. Its evolution with centrality shows a pattern strikingly similar to the one measured at RHIC. High transverse momentum particles are found to be strongly suppressed with respect to pp collisions, consistent with a large energy loss of hard-scattered partons in the hot and dense medium. The results are presented in terms of the nuclear modification factor $R_{\mathrm{AA}}$ and compared to theoretical expectations.

Speaker: Michele Floris (CERN)

Slides FlorisM_2C.pptx

Tuesday 29 May

09:00 → 10:25 Plenary 2A

09:00 Modeling jet quenching effects

High P_T measurements of hard hadrons or jets at RHIC and LHC appear contradictory and in some cases counter-intuitive, but upon closer investigation they represent a coherent picture of jet-medium interaction physics which can be established with systematic comparisons of models against a large body of data. This picture is consistent with a perturbative QCD mechanism and does not require exotic assumptions. I outline how several key measurements each partially constrain shower-medium interaction physics and how from the sum of those the outlines of the mechanism of jet quenching can be deduced. I then explain how current jet results from LHC can be naturally understood in this picture and summarize the remaining open issues.

Speaker: Thorsten Renk (University of Jyväskylä)

Slides trenk-hp2012.pdf

09:30 Jets and Jet-like Correlations in STAR

The propagation and modification of hard-scattered partons in the QGP can be studied using various types of jet and jet-like correlation measurements. The STAR detector with its full azimuthal and large pseudorapidity acceptance, as well as its wide transverse momentum coverage, is well-suited for these measurements. Correlations of neutral pions and jet-like clusters at forward rapidity in d+Au collisions are used to probe low x physics and the possible onset of gluon saturation effects. At mid-rapidity, azimuthal correlations of charged hadrons with the axis of a reconstructed trigger jet are used to study the modification of jet shapes and associated hadron yields from p+p to Au+Au. We also show results that take advantage of STAR's increased particle identification capabilities due to the Time-Of-Flight detector, such as particle ratios associated with reconstructed jets, and dihadron correlations with identified trigger particles. Such measurements will be used to explore the differences between jet-related and bulk-related particle production. The comprehensive set of STAR jet-quenching measurements can be used to further constrain theories of parton energy loss at RHIC.

Speaker: Alice Ohlson (Yale University)

Slides HPtalk_v5.pptx

10:00 Hadron correlations in ALICE

Particle correlations are a powerful tool to study collective effects and in-medium jet modification as well as their interplay in the hot and dense medium produced in central heavy-ion collisions. The talk presents measurements of two-particle correlations of inclusive charged and identified particles with the ALICE detector. We study the short-range correlation region and quantify the shape and particle content of the near-side peak. The results suggest strong modifications of the peak shape and particle ratios in central Pb-Pb collisions, compared to proton-proton or peripheral data. We compare to MC models and discuss the importance of the inclusion of collective effects in these models.

Speaker: Jan Fiete Grosse Oetringhaus (CERN)

Slides JFGO.ppt.ppt

10:25 → 10:55 coffee break

10:55 → 13:00 Plenary 2B

10:55 Hadron correlations in CMS

The measurements of the anisotropic flow of single particles and particle pairs have provided some of the most compelling evidence for the creation of a strongly interacting quark-gluon plasma (sQGP) in relativistic heavy ion collisions, first at RHIC, and more recently at the LHC. Using PbPb collision data taken in the 2010 and 2011 heavy ion runs at the LHC, the CMS experiment has investigated a broad scope of these flow phenomena. The v2 elliptic flow coefficient has been extracted with four different methods to cross-check contributions from initial state fluctuations and non-flow correlations. The measurements of the v2 elliptic anisotropy have been extended to a transverse momentum of 60 GeV/c, which will enable the placement of new quantitative constraints on parton energy loss models as a function of path length in the sQGP medium. Additionally, for the first time at the LHC, the CMS experiment has extracted precise elliptic anisotropy coefficients for the neutral pi meson (pi0) in the centrality range 20-80% and over a transverse momentum range 1.6 to 8 GeV/c. These results will be compared with both the pi0 results reported by the PHENIX detector at RHIC and with the inclusive charged particle anisotropy results reported from the LHC. Finally, the CMS experiment has mounted an extensive study of charged hadron pair azimuthal correlations using a Fourier harmonic decomposition to fit the data. The relationship between these pair coefficients and the single particle harmonic flow coefficients will be explored for its insight in the early dynamics this viscous medium.

Speaker: Charles Maguire (Vanderbilt University)

Slides hadronCorrCMS_HP2012_Maguire.pdf

11:20 Hadron Correlations in ATLAS

Hadron correlations are important tools used to study the properties of the medium produced in relativistic heavy ion collisions. We present detailed measurements of flow harmonics $v_{2}$-$v_{6}$ via di-hadron correlations in broad $p_{T}$, $\Delta\eta$ and centrality ranges using the 2010 Pb+Pb data from ATLAS. These measurements are compared to the corresponding values obtained via event-plane measurements. This result provides new insights on the origin of the long range "ridge" structure over broad $\pT$ ranges. Measurements of the dipolar flow ($v_{1}$) associated with initial dipole asymmetry as function of $p_{T}$ and centrality are also presented. Finally, the results of correlations between harmonic planes of different orders measured via two-plane and three-plane correlations are also discussed.

Speaker: Soumya Mohapatra (Stony Brook University)

Slides SoumyaMohapatra.pdf

11:45 Energy loss and hadron correlations in PHENIX

Speaker: Justin Frantz (Ohio University)

Slides jfrantz_hp12.pptx

12:10 Holography and heavy ion collisions: promises and challenges

I review the application of the AdS/CFT correspondence to heavy ion collisions. The talk will first review and critique available holographic calculations of energy loss, with an emphasis on the parametric similarities and differences between perturbation theory and the strongly coupled theory. Then the talk will summarize the results of current AdS/CFT motivated energy loss models and their domain of applicability. In addition, several other results from the AdS/CFT correspondence of relevance to heavy ion collisions will be discussed. For example, recent results on the (strikingly fast) thermalization of holographic plasmas will be reviewed.

Speaker: Derek Teaney (Stony Brook University)

Slides TeaneyD_2B.pdf

12:35 Future opportunities for jet physics at RHIC (with an eye at recent LHC results)

Speaker: Brian Cole (Columbia University)

Slides sPHENIX_hardprobes_2012_v4.pdf

13:00 → 14:15 Lunch

14:15 → 15:55 Parallel IIIA: Heavy flavour

14:15 Quarkonium production in pp collisions with CMS

Using large data samples of di-muon events, CMS has performed detailed measurements in the field of quarkonium production. Differential cross-sections of J/psi, psi(2S) and Y(nS) states in pT and rapidity will be presented, separated in prompt and non-prompt contributions for the charmonium case: experimental results are compared with recent predictions in the context of NLO nonrelativistic QCD and the FONLL scheme. Studies of P-wave charmonia and bottomonia (chi_c, chi_b), using the decay mode (J/psi, Y) + gamma where the photon converts in an e+e- pair inside the detector, will be also discussed.

Speaker: Carlos Lourenco (CERN)

Slides LourencoC_3A.pdf

14:35 Open Heavy-Flavour and J/psi production in proton-proton collisions measured with the ALICE experiment at LHC

Open heavy-flavour and J/psi production measurements are powerful tools to test pQCD calculations in proton-proton collisions at the new LHC energy regime. In addition, the measurement of open heavy-flavour and J/psi production in proton-proton collisions provides the necessary reference for the ALICE Pb-Pb program. The ALICE experiment at the LHC collected proton-proton collisions at sqrt(s)=7 and 2.76 TeV in 2010 and 2011. In this talk, the latest results of open heavy-flavour and J/psi production in proton- proton collisions measured by the ALICE experiment at both mid- and forward rapidity via their various hadronic and leptonic decay channels will be presented. These include, among other topics, the first LHC result on J/psi polarization, multiplicity dependence of J/psi production, D-meson mea- surement down to low transverse momentum and J/psi production from B-hadron decays. Comparisons with different theoretical models will be discussed as well.

Speaker: Claudio Geuna (CEA Saclay Irfu/SPhN)

Slides GeunaC_3A.pdf

14:55 Heavy quark studies in single-muon measurements from heavy ion collisions in ATLAS

Heavy quarks are very important probes to study the hot, dense medium produced in heavy ion collisions. Heavy quarks are produced at a relatively early stage of the nucleus-nucleus collision and they may have a reduced level of gluon radiation due to a suppression of small angle gluon radiation known as the ‘dead cone effect.’ The first results on the suppression of the J/ψ as a function of the collision centrality were reported by the ATLAS experiment in 2011. Semi-leptonic decay muons from the heavy flavor hadrons can be employed as a proxy for the heavy flavor quarks by making use of the high precision and high efficiency of the ATLAS muon system. In this talk, we present the nuclear modification factor for the muons coming from the semi-leptonic decays of heavy flavor particles. The results are measured separately for the b and c → μ decay channels as a function of the transverse momentum of the muon in the range from 4 to 14 GeV for different centralities of Pb+Pb collisions at √sNN =2.76TeV.

Speaker: Yujiao Chen (Columbia university)

Slides YujiaoChenHB.pdf

15:15 Measurement of heavy-flavour decay muon production at forward rapidity in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV with the ALICE experiment

The main goal of the ALICE experiment is the study of the properties of the strongly-interacting matter at very high energy density which is formed in ultra-relativistic heavy-ion collisions at the LHC. Heavy-flavours (charm and beauty) have an important role in the investigation: being produced in the early stage of the collision, they are sensitive probes of the Quark-Gluon Plasma and allow one to study the parton-medium interaction. The ALICE experiment measured heavy-flavour production in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV in different decay channels and rapidity ranges. After a short description of the ALICE muon spectrometer, the latest results of open heavy-flavour measurements in the semi-muonic decay channels at forward rapidities (2.5<y<4) will be presented. A particular emphasis will be placed on the measurement of the nuclear modification factor as a function of transverse momentum and centrality, with respect to a pp reference at the same center of mass energy.

Speaker: Diego Stocco (Subatech)

Slides StoccoD_3A.pdf

15:35 Open Heavy Flavor Production at Forward Angles in PHENIX

Measurement of the production of heavy quarks in heavy ion collisions can be used to probe the early stages of the created medium, study hot and cold nuclear matter effects, and test theoretical predictions concerning quark energy loss and initial-state effects. This is a current area of active research in the field and it is important to extend such measurements to the forward region. PHENIX can measure the production of open heavy flavor at forward angles with subsequent semi-leptonic decay of heavy flavor mesons into muons. We report the nuclear modification factor for the production of muons from heavy flavor decay in Cu+Cu collisions at root s_NN = 200 GeV for three centrality intervals and compare to theoretical predictions concerning the suppression of heavy quark production which incorporate heavy quark energy loss and in-medium heavy meson disassociation. Additionally, we report the charm production cross section for p+p collisions at root s = 200 GeV and compare results to Fixed Order plus Next-to-Leading Log predictions.

Speaker: Kenneth Read (ORNL and University of Tennessee)

Slides Read-HardProbes-v10.pdf

14:15 → 15:55 Parallel IIIB: Jet quenching and energy loss

14:15 Out of Medium Fragmentation from Long-Lived Jet Showers

We study the time structure of vacuum jet evolution via a simple uncertainty principle estimate in the kinematic range explored by current heavy ion collisions at the LHC. We observe that a large fraction of the partonic splittings occur at large times, of the order of several fm. We compare the time distribution of vacuum splittings with the distribution of path lengths traversed by jets in a heavy ion collision. We find that if no medium induced modification of the jet dynamics were present, a very large fraction (larger than 80% for inclusive jets) of the jet splittings would occur outside of the medium. We confront this observation with current available data on jet properties in heavy ion collisions and discuss its implications for the dynamics of jet-medium interactions.

Speaker: Jorge Casalderrey-Solana (U. de Barcelona)

Slides CasalderreySJ_3B.pdf

14:35 Coherence and broadening effects in medium induced gluon radiation

Radiative interferences in the multi-parton shower is the building block of QCD jet physics in vacuum. The presence of a hot medium made of quarks and gluons is expected to alter this interference pattern. To study such effects, we derive the gluon emission spectrum off an "asymptotic quark" traversing a hot and dense QCD medium at first order in the medium density. The resulting induced gluon distribution gets modified when the new interference terms between the initial and final quark are included. We comment on the possible phenomenological consequences of this new contribution for jet observables in heavy-ion collisions.

Speaker: Mauricio Martinez Guerrero (Universidade de Santiago de Compostela)

Slides MartinezM_3B.pdf

14:55 Transverse Momentum Broadening in Weakly Coupled Quark-Gluon Plasma

We calculate P(k_perp), the probability distribution for an energetic parton propagating for a distance L through a medium to pick up transverse momentum k_perp, for a medium consisting of weakly coupled quark-gluon plasma. We use full or HTL self-energies in appropriate regimes, resumming each in order to find the leading large-L behavior. We estimate the jet quenching parameter and compare to results in the literature. And, we compare P(k_perp) at weak coupling to the P(k_perp) expected from holographic calculations that presume the quark-gluon plasma to be strongly coupled at all length scales. We find that the weak coupling and strong coupling results need not differ greatly at modest k_perp, but we find that P(k_perp) must be parametrically larger in a weakly coupled plasma than in a strongly coupled plasma at large enough k_perp. By looking for rare large-angle deflections of the jet resulting from a parton produced initially back-to-back with a hard photon, experimentalists can find the weakly coupled quark and gluon short-distance constituents of the strongly coupled liquid quark-gluon plasma, much as Rutherford found the nuclei within atoms or Friedman, Kendall and Taylor found the quark within nucleons.

Speaker: Mindaugas Lekaveckas (MIT)

Slides LekaveckasM_3B.pdf

15:15 Calculating the jet quenching parameter $\hat{q}$ in lattice gauge theory

We present a framework where first principles calculations of jet modication may be carried out in a non-perturbative thermal environment. As an example of this approach, we compute the leading order contribution to the transverse momentum broadening of a high energy (near on-shell) quark in a thermal medium. This involves a factorization of a non-perturbative operator product from the perturbative process of scattering of the quark. An operator product expansion of the non-perturbative operator product is carried out and related via dispersion relations to the expectation of local operators. These local operators are then evaluated in quenched SU(2) lattice gauge theory.

Speaker: Abhijit Majumder (Wayne State University)

Paper http://arxiv.org/abs/1210.3352

Slides majumder_HP_2012.pdf

15:35 The role of jet collimation in jet energy loss

The observation of a significant enhancement of the dijet asymmetry in PbPb collisions as compared to the pp reference case, compounded with the absence of modication of the azimuthal dijet distribution, led us to highlight the importance of medium induced transverse transport of soft quanta as a source of jet energy loss. Here, we extend our analysis and explore the consequences of 'jet collimation' for more differential observables such as, but not restricted to, the dependence of jet energy loss on the leading jet energy as recently studied by CMS.

Speaker: Jose Guilherme Milhano (CENTRA-IST (Lisbon) and CERN-PH-TH)

Slides MilhanoG_3B.pdf

14:15 → 15:55 Parallel IIIC: Electroweak Probes

14:15 Measurement of isolated direct photons in lead-lead collisions at sqrt(s_NN)=2.76 TeV with the ATLAS detector

Direct photons are a powerful tool in heavy ion collisions. Their production rates provide access to the initial state parton distribution functions, which are expected to be modified by nuclear effects. They also provide a means to calibrate the expected energy of jets that are produced in the medium, and thus are a tool to probe the physics of jet quenching more precisely both through jet rates and fragmentation properties. The ATLAS detector measures photons with its hermetic, longitudinally segmented calorimeter, which has excellent spatial and energy resolution, providing detailed information about the shower shape of each measured photon. These capabilities provide powerful rejection against the background from neutral pions in jets. Rejection against jet fragmentation products is further enhanced by isolation criteria, which can be based on calorimeter energy or the presence of high pT tracks. First results on the rates of isolated direct photons from approximately 140 µb-1 of lead-lead data will be shown, as a function of transverse momentum, pseudorapidity and centrality, and their rates compared to expectations from perturbative QCD.

Speaker: Peter Steinberg (Brookhaven National Laboratory)

Slides Steinberg_HP2012_final.pdf

14:35 Measurement of isolated photon R_AA at high pT in PbPb collisions at 2.76TeV with CMS

Isolated photon production is measured in pp and PbPb collisions at nucleon-nucleon center-of-mass energies of 2.76 TeV in the pseudorapidity range |eta| < 1.44 and transverse energies ET between 20 and 80 GeV with the CMS detector at the LHC. The measured ET spectra are found to be in good agreement with NLO perturbative QCD predictions. The ratio of PbPb to pp isolated photon ET-differential yields, scaled by the number of incoherent nucleon-nucleon collisions, is consistent with unity for all PbPb reaction centralities.

Speaker: Yen-Jie Lee (CERN)

Slides LeeYJ_3C.pdf

14:55 Measurement of electroweak boson production in PbPb collisions at 2.76TeV with CMS

The unprecedented centre-of-mass energy available at the LHC offers unique opportunities for studying the properties of the strongly-interacting QCD matter created in PbPb collisions at extreme temperatures and very low parton momentum fractions. The Compact Muon Solenoid (CMS) is fully equipped to measure leptonic decays of electroweak probes in the high multiplicity environment of nucleus-nucleus collisions. Electroweak boson production is an important benchmark process at hadron colliders. Precise measurements of W and Z production in heavy-ion collisions can help to constrain nuclear PDFs as well as serve as a standard candle of the initial state in PbPb collisions at the LHC energies. The inclusive and differential measurements of the Z boson yield in the muon decay channel will be presented, establishing that no modification is observed with respect to next-to-leading order pQCD calculations, scaled by the number of incoherent nucleon-nucleon collisions. Measurements of the yield of W to $\mu \nu$ decays as a function of centrality and the W charge asymmetry as a function of rapidity show no modifications beyond isospin effects when compared to pp collisions. The status of the W and Z measurement in the electron decay channel will also be given. Results from the 2010 data taking period are reported and an outlook on the 2011 data analysis will be presented.

Speaker: Lamia Benhabib (LLR-Ecole Polytechnique)

Slides LamiaB_3C.pdf

15:15 Measurements of the Z boson via the two-lepton channels in heavy ion collisions in ATLAS

The color neutral Z boson is an excellent probe of the hot dense matter produced in heavy ion collisions. The products of the Z boson di-lepton decays do not interact with the dense color matter, thus, the Z boson allows us insight into the initial hard scattering that produced it and provides a clean test of our understanding of the collision. The ATLAS experiment has measured Z->ee and Z->mumu in Pb+Pb collisions with sqrt(s_NN)=2.76 TeV in a data sample corresponding to 140 inverse microbarns of integrated luminosity. The measurement of Z boson production and their properties as observed in these interactions will be described.

Speaker: Zvi Citron (Weizmann Institute of Science)

Slides CitronZ_3C.pdf

15:35 Dielectron Production in $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions at STAR

Dilepton production has been proposed to serve as a penetrating probe for the hot and dense medium created in high-energy nuclear collisions. Their small final-state interaction cross sections, let dileptons escape the interaction region undistorted. Since dileptons originate from all stages of a heavy ion reaction, their sources vary with the kinematic phase space under consideration: In the low mass region (LMR: mass<1.1GeV/$c^{2}$), vector mesons and direct photons are the dominating source, while dileptons in the intermediate mass region (IMR: 1.1<mass<3GeV/$c^{2}$) primarily stem from QGP thermal radiation and semileptonic decays of charmed mesons. In the high mass region (HMR: mass>3 GeV/$c^{2}$), heavy quark decays and Drell-Yan processes contribute the most to the dilepton spectrum. Due to the time-energy correlation, the higher the dilepton pair mass, the earlier the production. Therefore the dilepton distributions, especially in the IMR and HMR, provide information on early collision dynamics in heavy ion collisions. In this talk we will present STAR results on dielectron production in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The results will be compared to hadron decay cocktails as well as theoretical calculations on vector meson in-medium modifications and the QGP thermal radiation. A systematic analysis of transverse mass distributions in the IMR region as a function of pair invariant mass will be discussed.

Speaker: Jie Zhao (Shanghai Institute of Applied Physics/ Lawrence Berkeley National Lab)

Slides HP2012_JieZhao.pdf

16:15 → 18:15 Poster session Wine & Cheese

18:15 → 19:30 Round table discussion: uses (and potential abuses) of MC codes in Hard Probes

Wednesday 30 May

09:00 → 10:30 Plenary 3A

09:00 Heavy flavour production in ALICE at the LHC

Heavy quarks, charm and beauty, are excellent probes to investigate QCD processes in hadronic interactions, and to characterize the deconfined medium produced in high-energy heavy-ion collisions, the Quark-Gluon Plasma. They are produced dominantly through hard partonic scattering processes in the earliest stage of the hadronic collisions and thus they experience the whole history of the collision. ALICE at the LHC is the experiment dedicated to study the physics of nucleus-nucleus collisions, and particularly the physics of heavy quarks. High resolution tracking down to low transverse momentum, and good particle identification give access to many hadronic and semileptonic decay channels of heavy-flavour hadrons, both at mid- and at forwardrapidity. A selection of results from pp collisions at √s = 2.76 and 7 TeV, and from Pb-Pb collisions at √s_NN = 2.76 TeV will be presented. In proton-proton collisions, the high precision measurements provide an important test of perturbative QCD predictions. The precise vertex reconstruction together with the particle identification, allows the separation of the charm and the beauty components. Furthermore, the pp results are essential as a reference for the measurements in heavyion collisions. Nuclear modification factors were measured for D mesons, for electrons and for muons from heavy-flavour decays. Elliptic flow of D meson will be presented. The results provide information on the Quark-Gluon Plasma, via the energy loss of the heavy partons in the strongly interacting medium, and hints on the medium thermalization.

Speaker: Silvia Masciocchi (GSI Darmstadt)

Slides sma_HFalice_HP2012.2.pdf

09:30 Quarkonia and heavy flavor production in CMS

The Compact Muon Solenoid (CMS) is fully equipped to measure quarkonia in the di-muon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Quarkonia are especially relevant for studying the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS has measured the nuclear modification factors of non-prompt J/psi (from b-hadron decays), prompt J/psi and Y(1S) in PbPb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. For prompt J/psi with relatively high p_T (6.5 < p_T < 30 GeV/c), a strong, centrality-dependent suppression is observed in PbPb collisions, compared to the yield in pp collisions scaled by the number of inelastic nucleon-nucleon collisions. Such strong suppression at high p_T has previously not been observed at RHIC. In the same kinematic range, a suppression of non-prompt J/psi, which is sensitive to the in-medium b-quark energy loss, is measured for the first time. Also the low-pt Y(1S) mesons are suppressed in PbPb collisions. Furthermore, a suppression of the excited Y-states has been measured with respect to the Y(1S). During the 2011 data taking period the data sample has been increased by a factor twenty, which allows more detailed measurements of the observed quarkonia suppression patterns and opens the door to new observables. Results from the 2010 data taking period are presented and the status of the 2011 data analysis is reported.

Speaker: Torsten Dahms (LLR, Ecole Polytech., IN2P3-CNRS, France)

Slides tdahms_20120530.pdf

10:00 Recent developments in lattice studies for quarkonia

I will give a review on recent developments in lattice QCD studies for quarkonia in the quark gluon plasma medium. I will discuss recent progress in the extraction of spectral properties from lattice QCD calculations of hadronic correlation functions. Besides medium modifications of bound states and their dissociation in the plasma I will focus on transport coefficients, like heavy quark diffusion constants, extracted from different correlation functions on the lattice. Present limitations and future perspectives for studies of quarkonia and related transport coefficients on the lattice will be discussed.

Speaker: Olaf Kaczmarek (Bielefeld University)

Paper http://arxiv.org/abs/1208.4075

Slides Kaczmarek_HP12.pdf

10:30 → 11:00 Coffee break

11:00 → 13:00 Plenary 3B

11:00 Quarkonia production in ALICE

Heavy quarkonium states are expected to provide essential information on the properties of the high-density strongly-interacting system formed in the early stages of heavy-ion collisions. In particular the J/psi suppression, in heavy-ion collisions, via color screening mechanism, can be seen as a direct effect of deconfinement. During 25 years, the J/psi suppression has been extensively studied at the SPS and at RHIC. It was indeed clearly observed but to a level surprisingly similar despite the large difference in the center of mass energy at the two accelerators. At the same time, new mechanisms of J/psi regeneration via recombination of charm and anti-charm quarks were also proposed. The measurement of J/psi suppression is especially promising at the Large Hadron Collider where the high energy density of the medium and the large number of charm quarks pairs produced in central Pb-Pb collisions should help to disentangle between the different suppression and recombination scenarios. ALICE is the LHC experiment mainly dedicated to the study of nucleus-nucleus collisions. At forward rapidity (2.5 < y < 4), the J/psi production is measured in the Muon Spectrometer, via the mu+ mu- decay channel, down to zero transverse momentum. After a brief description of the apparatus, the analysis of the inclusive J/psi production in Pb-Pb collisions at a center of mass energy of sqrt(Snn)= 2.76 TeV will be discussed. Results on the nuclear modification factor (Raa) dependence on the collision centrality will be shown. Thanks to the large statistics collected in 2011, preliminary results on Raa as a function of transverse momentum and rapidity will also be presented.

Speaker: Christophe Suire (CNRS-IN2p3)

Slides HP2012-suire-final.pdf

11:30 Quarkonia production in effective field theory and potentials

Speaker: Alexander Rothkopf (Institute for Theoretical Physics, University of Bern)

Paper http://arxiv.org/abs/arXiv:1108.1579 http://arxiv.org/abs/arXiv:1110.1203

Slides ROTHKOPF_HardProbes2012_FINAL.pptx

12:00 Quarkonia and open heavy-flavor production at STAR

Heavy flavor production is a useful tool to study the medium produced in heavy ion collisions. We will present results on production of charm hadrons via the reconstruction of D0 and D* mesons, and compare them to FONLL calculations. The nuclear modification factor of D mesons can then be used to study medium effects. We will also present results on charmonium and bottomonium production. By studying charmonium v2, one can estimate the amount of coupling between heavy quarks to the hot, flowing medium. The Upsilon nuclear modification factor, an important signature of deconfinement, will also be presented.

Speaker: Manuel Calderon De La Barca Sanchez (UC Davis)

Slides STAR-HeavyQ-HP2012-Calderon-v2.pptx

12:30 Cold nuclear matter effects and heavy-quark production in PHENIX

Measurements of cross section modifications for hard processes that take place in a nuclear target provide information about the modification of parton densities in nuclei at low Bjorken $x$. This is interesting because it illuminates the physics of QCD at high parton density, and also because it provides a baseline against which the modification of cross ssections for hard processes in heavy ion collisions can be measured. PHENIX has now completed analysis of data from the 2008 d+Au RHIC run for production of quarkonia, open heavy flavor, and correlated high $p_{T}$ $\pi^{0}$ production at forward rapidty. In this talk we will discuss these results and what they convey about cold nuclear matter effects, and their implications for hot matter effects in Au+Au collisions.

Speaker: Anthony Frawley (Florida State University)

Slides FrawleyA_3B.pdf

13:30 → 19:00 Excursion

Thursday 31 May

09:00 → 10:30 Plenary 4A

09:00 Measurement of the W,Z and photon production in lead-lead collisions at sqrt(s_NN) = 2.76 TeV with ATLAS detector

The color neutral bosons are an unparalleled probes of the hot and dense matter produced in relativistic heavy ion collisions at LHC. Because the decay products do not carry color charges, the W and Z bosons allows us insight into the initial hard scattering that produced it and provides a clean test of our understanding of the collision. In particular, the yield of bosons in heavy ion collisions is a sensitive test of binary scaling. The ATLAS experiment has measured W and Z boson yield via leptonic decay modes in Pb+Pb collisions with sqrt(s_NN)=2.76 TeV in a data sample corresponding to 148 ub^-1 of integrated luminosity. The measurement of W,Z boson production and their properties will be presented.

Speaker: Alexander Milov (Department of Particle Physics and Astrophysics Weizmann Institute of Science)

Slides hp2012_sasha_12-05-31_v3.pptx

09:30 W,Z and photon production in CMS

Electroweak gauge bosons, $\gamma$, W and Z, do not participate in the strong interaction, and thus constitute clean probes of the initial state of nucleus-nucleus collisions. The comparison of their production cross-sections in pp and in nuclear collisions provides an estimate of the nuclear parton distribution functions. Despite the low production cross section of weak bosons compared to other nuclear processes, the relatively clean signal of their leptonic decay channel allows their detection and reconstruction. The measurement of prompt photon production is challenging because of the presence of a large background coming from electromagnetic decays of neutral mesons, which is suppressed by photon isolation criteria. This talk will report measurements of isolated photons, as well as Z and W bosons, produced in PbPb and pp collisions at nucleon-nucleon sqrt(s) = 2.76 TeV with the CMS detector.

Speaker: Begona De La Cruz (CIEMAT, Spain)

Slides HP_EWK_bosons.pdf

10:00 Electromagnetic radiation in heavy ion collisions: Progress and puzzles

I review the theory behind the emission of electromagnetic radiation in relativistic collisions. The rates for photon and lepton pair production are examined, both for a quark-gluon plasma and for a hadronic gas at finite temperatures and densities. A special emphasis is placed on the connection between electromagnetic spectra and the details of modern 3D hydrodynamic simulations of high-energy nuclear collisions. More specifically, the effects of a finite shear viscosity coefficient are examined, together with those of fluctuating initial states. I will summarize our current understanding of the relevant experimental data.

Speaker: Charles Gale (McGill University)

Slides Gale_HP2012.pdf

10:30 → 11:00 Coffee break

11:00 → 12:24 Plenary 4B

11:00 How can we prove/disprove the relevance of CGC/saturation physics at LHC?

The forthcoming LHC run on p+Pb collisions will offer a unique possibility to explore the small-x regime of QCD. I shall review the physics prospects for different observables within the Color Glass Condensate framework: from multiplicity distributions, and single inclusive spectra to two-particle correlations. I will discuss in detail the quantitative challenges for a precise discrimination of the CGC/saturation studies to other phenomenological approaches in Heavy ion collisions.

Speaker: Javier L Albacete (IPN Orsay)

Slides HP2012Plenary-Albacete.pdf

11:25 Global analysis of nuclear PDFs – latest developments

I will present an overview of the nuclear parton distribution functions (nPDFs). I will first discuss the developments in the DGLAP-based global analysis during the last years. In the latter part of the talk, I will focus on the extraction of the spatial dependence of the globally analysed nPDFs, which has led to the new spatially dependent nPDFs called EPS09s and EKS98s (“s” for spatial). With these, one may now compute the nuclear hard-process cross sections in different centrality classes for the first time more consistently with the global analysis, and including also the spatially dependent versions of the error sets of EPS09. As an application, I will discuss the centrality dependence of the nuclear modification factor R_dAu for neutral pions in d+Au collisions at RHIC, compare with the data, and also present predictions for p+Pb collisions at the LHC.

Speaker: Kari J. Eskola (Department of Physics, University of Jyväskylä)

Slides EskolaK_4B.pdf

11:55 Light vector meson production at the LHC with the ALICE detector

Speaker: Elisa Incani (CA)

Slides IncaniE_flash.pdf

12:00 Azimuthal angular correlations between heavy-flavour electrons and charged hadrons in pp collisions at sqrt(s) = 2.76 TeV in ALICE

Speaker: Ms Deepa Thomas (Utrecht University)

Slides DeepaT_flash.pptx

12:05 Collision centrality and tau_zero dependence of the emission of thermal photons from fluctuating initial conditions

Speaker: Dr Rupa Chatterjee (University of Jyväskylä, Finland)

Slides ChatterjeeR_flash.pdf

12:10 Direct photon measurements at PHENIX

Speaker: Dr Baldo Sahlmueller (Stony Brook University)

Slides SahlmuellerB_flash.pdf

12:25 → 13:00 Discussion on pA physics at the LHC

12:25 Discussion on pA physics at the LHC

Speaker: Carlos Salgado (Roma Univ. La Sapienza)

Slides HP12-pAdiscussion.pdf

13:00 → 14:00 Lunch

14:00 → 16:00 Parallel IVA: heavy flavour

14:00 The measurement of non-photonic electrons in STAR

Due to their large masses, charm and bottom quarks, are mostly produced during the initial phase of collisions via gluon fusion. Non-photonic electrons (NPE) are mainly produced by semileptonic decays of D and B mesons, hence the study of NPE in hadron-hadron and ion-ion collisions provides the information about heavy quarks production as well as the medium properties. In order to interpret NPE measurements it is important to determine the relative charm and bottom contribution to NPE spectrum. The measurement of azimuthal correlation between NPE and hadrons in Au+Au collisions can shed light on heavy flavor jet-medium interactions. NPE elliptic flow can be a good proxy to reveal heavy flavor collectivity, which can significantly improve our understanding of the medium thermalization. This talk presents the recent STAR measurement of non-photonic electrons from p+p collisions at $\sqrt{s}=200$ GeV, Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. We will also report the analysis status in d+Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Speaker: Olga Hajkova (Czech Technical University in Prague)

Slides Olga_Hajkova_HP2012.pdf

14:20 Measurement of electrons from Heavy Quarks at PHENIX (for PHENIX collaboration)

Heavy quarks are one of the most valuable probes for the matter produced in relativistic heavy ion collisions at RHIC. PHENIX experiment being designed specifically to study leptons, so electrons from the decay of heavy quarks acts as one of the most important tools in PHENIX for measurement of heavy quarks. Measurements of electron spectra at mid rapidity region has been done by PHENIX in broad $p_{T}$ regions. Recent measurement in $p+p$ by PHENIX extends upto $pT = 15$ GeV. After subtraction of photonic background electrons from the measured electron spectra, electrons from semileptonic decay of hadrons containing heavy quarks are left, which provides the measurement of heavy quarks. Measurement of electrons from heavy quarks by PHENIX has been done for various collision species like $p+p$, $d+Au$, $Cu+Cu$ and $Au+Au$. In $Au+Au$ strong suppression of single electron compared with scaled $p+p$ is observed which challenges radiative energy loss models because heavy quarks are expected to radiate less. It is important to understand what happens when the collision species is asymmetric and has size in between $Au+Au$ and $p+p$. Measurements done in $d+Au$ collisions by PHENIX answer this question along with disentangling cold nuclear matter effect in heavy ion collisions. Also it is needed to know the situation when the collision species is symmetric and has size between $p+p$ and $Au+Au$. Measurements done in $Cu+Cu$ collisions by PHENIX provide the answer. Measurement of azimuthal anisotropy by using electrons from heavy quark decay has been done recently in PHENIX for $Au+Au$ at $sqrt{s_{NN}} = 62.4$ GeV. The techniques used by PHENIX for measuring electrons from heavy quarks and the latest results in collision species $p+p$, $d+Au$, $Cu+Cu$ and $Au+Au$ will be discussed in this talk.

Speaker: Sourav Tarafdar (Banaras Hindu University, Varanasi, India)

Paper Sourav_hardprobes_proceedings.pdf

Slides Sourav_hardprobes.pptx

14:40 Heavy-flavour production in pp and AA collisions at the LHC

A refined version of a multi-step calculation of heavy-flavour observables in pp and AA collisions has been developed, based on pQCD at NLO accuracy followed by parton shower evolution to describe heavy-quark production and on the relativistic Langevin equation to describe their stochastic evolution in the QCD plasma. Then, hadronization is modeled through an implementation of the available experimental data on heavy-quark fragmentation fractions and a set of pQCD fragmentation functions into pseudo-scalar and vector mesons. Results of our calculations can be compared with some recent measurements performed at the LHC in Pb-Pb collisions at sqrt(s_NN)=2.76 TeV: pT-differential spectra at mid-rapidity of heavy-flavour decay electrons and of exclusively reconstructed open-charm mesons at different centralities, as well as their nuclear modification factor RAA with respect to the pT spectra observed in pp collisions. In addition, predictions are given for the pT-differential elliptic-flow v2(pT) of open-charm mesons measured at mid-rapidity. To test the validity of our setup for such studies, its predictions are also checked against the pT spectra measured in pp collisions at sqrt(s)=2.76 TeV and 7 TeV, considered as benchmark data samples.

Speaker: Marco Monteno (INFN Torino)

Slides MontenoM_4A.pdf

15:00 Open heavy flavor and J/psi at RHIC and LHC

Heavy flavors are important probes to study the early phase of ultra-relativistic heavy ion collisions at RHIC and LHC. We present results on the production and space-time evolution of heavy quarks and J/psi in the quark gluon plasma within the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). Heavy quarks interact with the medium via binary and radiative scatterings with running coupling and a more precise Debye screening which is derived from hard thermal loop calculations. We compare our results on the elliptic flow and nuclear modification factor not only to experimental data of heavy flavor electrons at RHIC, but also to LHC data of heavy flavor electrons, muons, D mesons and non-prompt J/psi. The latter two are in particular sensitive on the mass difference of charm and bottom quarks. Furthermore, results on J/psi suppression are reported for central and non-central collisions, taking cold nuclear matter effects and the dissociation as well as regeneration of J/psi in the quark-gluon plasma into account.

Speaker: Jan Uphoff (Goethe University Frankfurt)

Paper http://arxiv.org/abs/1205.4945

Slides jan_uphoff_hardprobes12.pdf

15:20 Measurement of the nuclear modification factor of electrons from heavy-flavour decays in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV with ALICE

The medium-induced parton energy loss is expected to depend on its mass and colour charge. At the LHC, heavy quarks are copiously produced. This allows us to provide new constraints on partonic energy loss mechanisms in the medium produced in the heavy-ion collisions. The yield of electrons from semileptonic heavy-flavour decays has been measured at mid-rapidity \mbox{($|y| < 0.8$)} in Pb-Pb collisions at \mbox{$\sqrt{s_{\rm NN}}$ = 2.76 TeV} by the ALICE experiment at the LHC in the transverse momentum range \mbox{1.5~$< p_{\rm t} <$~6~GeV/$c$}. The $p_{\rm t}$ dependence of the nuclear modification factor $R_{\rm AA}$ at central rapidity has been calculated with respect to a pp reference obtained from the cross section measured at \mbox{$\sqrt{s}$ = 7 TeV} and scaled to \mbox{$\sqrt{s}$ = 2.76 TeV}. We present the spectra and the nuclear modification factor of electrons from heavy-flavour decays for different centrality classes. The production of electrons from beauty decays can be addressed using their displacement from the primary vertex. We discuss the status of this analysis in Pb-Pb collisions, towards the measurement of the nuclear modification factor of electrons from beauty decays.

Speaker: MinJung Kweon (University of Heidelberg)

Slides KweonM_4A.pdf

15:40 Toward a Complete Description of Heavy Flavor Transport in Medium

We evaluate heavy-flavor (HF) transport in relativistic heavy ion collisions within a nonperturbative (strong-coupling) framework. In the Quark-Gluon Plasma, heavy-quark diffusion coefficients are obtained from a potential-based T-matrix approach, which builds up resonance correlations toward the phase transition region. Those resonance correlations are also utilized for hadronizing heavy quarks via resonance recombination with light quarks from the medium. In the hadron-resonance gas, the diffusion coefficients of HF mesons are calculated in terms of empirical scattering amplitudes obtained from effective hadronic theory. A carefully constrained hydrodynamic model is used for the evolution of the background medium to implement the diffusion and hadronization components via relativistic Langevin simulations. Our calculations thus represent a conceptually self-consistent strongly-coupled framework for HF probes at both micro- and macroscopic levels. Pertinent HF observables (electron and D-meson spectra andv2) are computed and show good agreement with available RHIC and LHC data.

Speaker: Mr Min He (Cyclotron Institute, Texas A&amp;M University)

Slides Min_He_HP2012.pdf

14:00 → 16:00 Parallel IVB: Jet quenching and energy loss

14:00 Dijet transverse momentum imbalance, fragmentation functions and jet-track correlations in PbPb collisions at sqrt(s)=2.76TeV with CMS

Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV was studied with the CMS detector at the LHC. Jets are reconstructed using the energy deposited in the CMS calorimeters and studied as a function of collision centrality. With increasing collision centrality, a striking imbalance in dijet transverse momentum is observed, consistent with jet quenching. Correlations of charged particle tracks with jets indicate that the momentum imbalance is accompanied by an excess of low transverse momentum particles over a large range in the hemisphere of the second most energetic, away-side jet. Jet fragmentation functions measured in PbPb collisions will also be presented, constructed with charged particle tracks with transverse momenta pT > 4 GeV/c in a sample of dijet events. Both the leading and subleading jets show a fragmentation pattern similar to that of a jet in vacuum, as seen by comparisons to pp data at 2.76 TeV.

Speaker: Yetkin Yilmaz (MIT)

Slides hardprobes_yetkin_v4.pdf

14:20 Measurements of Jets and Jet Properties in sqrt(s_NN)=2.76 TeV PbPb Collisions with the ATLAS Detector at the LHC

Jet quenching in the hot and dense medium created in ultra-relativistic heavy ion collisions is a well-established experimental phenomenon at RHIC. It has long been anticipated that the LHC heavy ion program would substantially advance the study of jet quenching by providing access to highly energetic jets and by measuring fully-reconstructed jets. Immediately following turn-on of the LHC in November, 2010, that expectation was fulfilled through the observation of large di-jet asymmetries that may indicate substantial jet quenching. In this talk we will present recent results from ATLAS aimed to provide further understanding of this phenomenon. Measurements of single jet production, di-jet correlations and jet fragmentation in Pb+Pb collisions at $\sqrt{s_{NN} = 2.76$~TeV will be presented. In addition to measurements from the 2010 data, results using the full 2011 run will also be presented, benefiting from a factor of 20 improvement in statistics.

Speaker: MArtin Rybar (CERN)

Slides RybarM_4B.pdf

14:40 Measurement of jet spectra in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV with the ALICE detector at the LHC

We report a measurement of transverse momentum spectra of ets detected with the ALICE detector in Pb-Pb collisions at $\sqrt{s_NN}$=2.76 TeV. Jets are reconstructed from charged particles using the anti-$k_{T}$ jet algorithm. The background from soft particle production is determined for each event and subtracted. The remaining influence of underlying event fluctuations is quantified by embedding different probes into heavy-ion data. The reconstructed transverse momentum spectrum is corrected for background fluctuations by unfolding. We compare the inclusive jet spectra reconstructed with R=0.2 and R=0.3 for different centrality classes and compare the jet yield in Pb-Pb and pp events.

Speaker: Marta Verweij (Utrecht University)

Slides MVerweij_HP2012_PbPbChargedJetsv4.pdf

15:00 Reconstructed Jet Results in p+p, d+Au and Cu+Cu collisions at 200 GeV from PHENIX

Jet reconstruction in heavy ion collisions at RHIC and the LHC is becoming a popular tool to explore medium effects including the energy loss and modified fragmentation of hard-scattered partons. In p+A and d+A collisions, reconstructed jets are important for evaluating cold nuclear matter effects such as the impact parameter dependence of nuclear parton distribution functions and initial state energy loss. We present current PHENIX results from p+p, d+Au, and Cu+Cu collisions at 200 GeV using the Gaussian filter and anti-kT algorithms. The systematic study of direct jet reconstruction across a variety of collisions systems at PHENIX will help to tell a coherent story of jet physics at RHIC.

Speaker: Dennis Perepelitsa (Columbia University)

Slides dvp-HP12-plenary4B-talk.pdf

15:20 What can we learn from the Dijets? A systematic study at RHIC and the LHC with VNI/BMS

We present a systematic study of dijet suppression at RHIC and the LHC using the VNI/BMS parton cascade. We examine the medium modification of the dijet asymmetry Aj and the energy distribution within the dijets (jet-shape). Understanding the sensitivity of these observables to properties of the deconfined medium and to experimental factors is vital if dijets are to be useful for QCD tomography. VNI/BMS provides a controllable testbed with sufficient complexity to model jet modification without confounding results with fluctuations from hydrodynamics and hadronization. Dijets are examined under the modification of: the jet transport coefficient qhat; the path length of leading and sub-leading jets; cuts on the jet energy distributions; jet cone angle and the jet-medium interaction mechanism. We find that Aj is very sensitive to the distance traveled by the secondary jet and cuts applied to the leading jet energy, the jet shape is dominated by qhat and the nature of the interaction mechanism.

Speaker: Christopher Coleman-Smith (Department of Physics, Duke University)

Paper http://arxiv.org/abs/1210.3377

Slides MuellerB_ColemanC_4B.pdf

15:40 Background subtraction and jet quenching on jet reconstruction

In order to asses the ability of jet observables to constrain the characteristics of the medium produced in heavy-ion collisions at the LHC, we investigate the influence of background subtraction and jet quenching on jet reconstruction, with focus on dijet asymmetries as currently studied by ATLAS and CMS. Using two models for the background, a full simulation and a toy model, we examine the influence of different background subtraction methods on dijet momentum imbalance and azimuthal distributions. We compare the usual jet-area based background subtraction technique and a variant of the noise-pedestal subtraction method used by CMS. The purpose is to understand what are the differences between the two techniques, given the same event configuration. Also, we analyze the influence of the quenching effect with the aim of understanding quantitatively how these methods are able to handle these modifications of the hard part of the collision. It turns out that the two background methods show different sensitivity to background fluctuations and quenching.

Speaker: Liliana Apolinário (Universidade de Santiago de Compostela)

Slides LilianaApolinario.pdf

14:00 → 16:00 Parallel IVC: Initial state and pA

14:00 Initial state and thermalization in the glasma

In the color glass condensate framework, the initial state of the system formed immediately after the collision of two heavy ions at high energy can be represented as a superposition of a classical color field and gaussian quantum fluctuations. This approximation is substanciated by results that show that all the leading logarithms of the collision energy are universal and can be factorized in distributions that represent each incoming nucleus. The fluctuating part of the initial color field results from a resummation of the modes that grow in the subsequent time evolution due to the Weibel instability. We have carried out this resummation program fully for a simpler toy model that also exhibits similar instabilities, and we show that the instabilities drive the system towards local isotropy and thermal equilibrium. References: [1] Role of quantum fluctuations in a system with strong fields: Onset of hydrodynamical flow, K. Dusling, T. Epelbaum, F. Gelis, R. Venugopalan, 1009.4363 [hep-ph], Nucl. Phys. A 850, 69 (2011). [2] Role of quantum fluctuations in a system with strong fields: Spectral properties and Thermalization, T. Epelbaum, F. Gelis, 1107.0668 [hep-ph], Nucl. Phys. A 872, 210 (2011).

Speaker: Francois Gelis (Institut de Physique Theorique)

Slides Gelis.pdf

14:20 Di-hadron angular correlation as a probe of saturation dynamics

Di-hadron angular correlations in the forward rapidity region of proton-nucleus collisions probe multi-gluon correlators (n-point functions of Wilson lines) in the wave function of target nucleus at small x and thus, provide a more detailed picture of QCD dynamics at high energy (CGC). The Renormalization Group equations that govern the energy dependence of these n-point functions will be derived and their approximate solutions motivated. A connection to an alternative approach to high energy QCD, based on BJKP equation involving pomeron and Reggeon exchanges, will be made.

Speaker: Jamal Jalilian-Marian (Baruch College)

Slides JalilianMarianJ_4C.pdf

14:40 Parton Energy Loss and Modified Beam Quark Distribution Functions in Drell-Yan Process in p+A Collisions

Within the framework of generalized collinear factorization in perturbative QCD (pQCD), we study the effect of initial multiple parton scattering and induced parton energy loss in Drell-Yan (DY) process in proton-nucleus collisions. We express the contribution from multiple parton scattering and induced gluon radiation to the DY dilepton spectra in terms of nuclear modified effective beam quark distribution functions. The modification depends on the quark transport parameter in nuclear medium. This is similar to the final-state multiple parton scattering in deeply inelastic scattering (DIS) off large nuclei and leads to the suppression of the Drell-Yan cross section in $p+A$ relative to $p+p$ collisions. With the value of quark transport parameter determined from the nuclear modification of single-inclusive DIS hadron spectra as measured by the HERMES experiment, we calculate DY spectra in $p+A$ collisions and find the nuclear suppression due to beam parton energy loss negligible at the Fermilab energy $E_{\rm lab}$=800 GeV in the kinematic region as covered by the E866 experiment. Most of the observed nuclear suppression of DY spectra in E866 experiment can be described well by parton shadowing in target nuclei as given by the EPS08 parameterization. The effect of beam parton energy loss, however, becomes significant for DY lepton pairs with large beam parton momentum fraction $x^{\prime}$ or small target parton momentum fraction $x$. We also predict the DY cross section in $p+A$ collisions at lower beam proton energy $E_{\rm lab}$=120 GeV and show significant suppression due to initial state parton energy loss at moderately large $x^{\prime}$ where the effect of parton shadowing is very small.

Speaker: Xin-Nian Wang (Central China Normal University and Lawrence Berkeley National Laboratory)

Slides WangX_XingH_4C.ppt

15:00 Linear vs non-linear QCD evolution: from HERA data to LHC phenomenology

The forthcoming p+Pb run at the LHC will provide crucial in formation on the initial state effects of heavy ion collisions and on the gluon saturation phenomena. In turn, most of the saturation inspired phenomenology in heavy ion collisions borrows substantial empiric information from the analysis of e+p data, where abundant high quality data on the small-x kinematic region is available. Indeed, the very precise combined HERA data provides a testing ground in which the relevance of novel QCD regimes, other than the successful linear DGLAP evolution, in small-x inclusive DIS data can be ascertained. We present a study of the dependence of the AAMQS fits, based on the running coupling BK non-linear evolution equations (rcBK), on the fitted dataset. This allows for the identification of the kinematical region where rcBK accurately describes the data, and thus for the determination of its applicability boundary. It also set important constraints to the saturation models used to model the early stages of heavy ion collisions. Finally we compare the rcBK results with NNLO DGLAP fits, obtained with the NNPDF methodology with analogous kinematical cuts. Further, we explore the impact on LHC phenomenology of applying stringent kinematical cuts to the low-x HERA data in a DGLAP fit.

Speaker: Paloma Quiroga-Arias (LPTHE, Univ. Paris 6 and CNRS)

Slides QuirogaAriasP_4C.pdf

15:20 Hadron and prompt photon production in pA collisions at the LHC from the Color Glass Condensate

I will show the results of our recent investigation of the inclusive hadron and prompt photon production in proton (deuteron)-nucleus (pA) collisions at RHIC and the LHC within the Color Glass Condensate (CGC) framework. We investigate the contribution of inelastic and elastic processes to single inclusive hadron production in proton-proton and pA collisions at RHIC and the LHC. Using the hybrid formulation which includes both elastic and inelastic contributions, supplemented with the running-coupling Balitsky-Kovchegov equation, we get a good description of RHIC data. It is shown that inclusion of the inelastic terms makes the transverse momentum dependence of the production cross section steeper in the mid-rapidity region but does not affect the cross section in the very forward region. The inelastic processes also lead to a sharper increase of the nuclear modification factor R_{pA} with increasing p_T. For the inclusive (and semi-inclusive) prompt photon production, we divide the k_t factorized cross-section obtained in the CGC approach to the direct and fragmented part and investigate which part is more sensitive to the gluon saturation. We also investigate azimuthal photon-hadron correlations at the LHC and RHIC at various rapidities. We show predictions for the nuclear modification factor for both inclusive hadron and prompt (and isolated) photon production in pA collisions at the LHC at various rapidities. We will compare the predictions for R_{pA} for both inclusive hadron and prompt photon production coming from the collinear and the k_t factorization approach at the LHC. The forthcoming day-one LHC measurement of hadron multiplicity in pA collisions is a crucial test of the k_t factorization and gluon saturation based models. We provide quantitative predictions for the pseudorapidity distribution of charged particles produced in minimum bias pA collisions at the LHC based on the idea of gluon saturation in the CGC framework. My talk is based on the following papers: 1: J. Jalilian-Marian and A. H. Rezaeian, "Hadron production in pA collisions at the LHC from the Color Glass Condensate", Phys. Rev. D85, 014017 (2012) [arXiv:1110.2810]. 2: A. H. Rezaeian, "Charged particle multiplicities in pA interactions at the LHC from the Color Glass Condensate", Phys. Rev. D85, 014028 (2012)[ arXiv:1111.2312]. 3: J. Jalilian-Marian and A. H. Rezaeian, "Prompt photon production and photon-hadron correlations at RHIC and the LHC", To be published.

Speaker: Amir Rezaeian (Universidad Tecnica Federico Santa Maria)

Slides RezaeianA_4C.pdf

15:40 Small-x physics at the Large Hadron-electron Collider at CERN

The Large Hadron-electron Collider LHeC is a proposed facility at CERN to collide electrons and positrons against the LHC beams at center-of-mass energies around 1 TeV per nucleon, with the aim of studying previously unexplored kinematical regions of the hadron and nuclear wave functions. After a brief physical motivation, I will present the project. Then I will focus on the opportunities for small-x studies, with special attention to the case of lepton-nucleus collisions. I will show the different opportunities for inclusive studies, exclusive and inclusive diffraction, and final states. For each observable, I will discuss the possibilities that it offers to establish the linear or non-linear behavior in small-x evolution, to determine the partonic structure of protons and nuclei at small x, and to investigate the dynamics of QCD branching and of hadronization. Ref.: cern/ch/lhec.

Speaker: Brian Cole (Columbia University)

Slides ColeB_4C.pdf

16:00 → 16:30 Coffee break

16:30 → 18:30 Parallel VA: Quarkonia

16:30 Measurement of bottomonium production in PbPb collisions at 2.76TeV with CMS

The Compact Muon Solenoid (CMS) is fully equipped to measure hard probes in the di-muon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Such probes are especially relevant for studying the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS has measured the nuclear modification factors of Y(1S) in PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV. A suppression of Y(1S) mesons is observed in PbPb collisions, compared to the yield in pp collisions scaled by the number of inelastic nucleon-nucleon collisions. Furthermore, a suppression of the excited Y-states has been measured with respect to the Y(1S) state, expressed in a double ratio [Y(2S+3S)/Y(1S)]$_{PbPb}$/[Y(2S+3S)/Y(1S)]$_{pp}$ which is found to be $0.31^{+0.19}_{-0.15}(stat.)\pm0.03(syst.)$. Results from the 2010 data taking period are reported and an outlook on the 2011 data analysis will be given.

Speaker: camelia mironov (LLR/Ecole polytechnique)

Slides MironovC_5A.pdf

16:50 Upsilon Suppression in PbPb Collisions at sqrt(s_NN) = 2.76 TeV

We suggest that gluon-induced dissociation and screening of the Y(nS) states together with feed-down explain the suppression of the Y(2S+3S) states relative to the Y(1S) ground state that has been observed by CMS in PbPb collisions at sqrt(s_NN) = 2.76 TeV at the CERN LHC. The minimum-bias gluodissociation cross sections of the 1S-3S states are calculated using a screened Cornell potential and a thermal gluon distribution. The 3S state dissolves due to screening before sizeable gluodissociation occurs, but for the 2S and 1S states there is an interplay between screening, gluodissociation, and feed-down from the chi_b(2P) and chi_b(1P) states. The calculated suppression of the Y(2S) and Y(3S) states relative to Y(1S) is consistent with the CMS result, but allows for additional suppression mechanisms. The Y(1S) suppression through gluodissociation is in excellent agreement with the CMS data.

Speaker: Georg Wolschin (Heidelberg University)

Slides hp12_wolschin.pdf

17:10 Upsilon Measurements at PHENIX

The suppression of quarkonia in heavy ion collisions has long been thought to provide an indication of the temperature dependent Debye screening length of color charge in the quark gluon plasma. A large sample of d+Au and Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV has been collected by PHENIX in 2008 and 2010. The large statistical sample allows for both the observation of Upsilon in the hot dense medium created by Au+Au collisions, but also a baseline measurement in d+Au. These baseline measurements aid the interpretation of the heavy ion suppression by separating the cold nuclear matter effects that result from using a nuclear target, but also add to our fundamental understanding of quarkonia production. In this talk, we will present measurements of Upsilon yields in hot and cold nuclear matter as observed by PHENIX at mid-rapidity in the di-electron decay channel and at forward rapidity in the di-muon decay channel.

Speaker: Shawn Whitaker (Iowa State University)

Slides whitaker-hard_probes_2012.pdf

17:30 Measurements of Upsilon Production and Nuclear Modification Factor at STAR

Thermal suppression of quarkonium production in heavy-ion collisions, due to Debye screening of the quark-antiquark potential, has been proposed as a clear signature of quark-gluon plasma (QGP) formation. At RHIC energies, the \$Upsilon meson is a clean probe of the early system thanks to negligible levels of enhancement from b-bbar recombination and non-thermal suppression from co-mover absorption. We report on our measurement of the $\Upsilon \to e^+ e^-$ cross section for Au+Au collisions at $\sqrt{S_{NN}}$=200 GeV. We compute a Nuclear Modification Factor by comparing these results to newly analyzed p+p collisions from 2009 (21 pb^-1 compared to 7.9 pb^-1 in 2006). In order to have a complete assessment of both hot and cold nuclear matter effects on Upsilon production we also report on results from d+Au collisions.

Speaker: Anthony Kesich (University of California, Davis)

Paper http://arxiv.org/abs/1207.7166

Slides HardProbes2012-akesich.pdf.pdf HardProbes2012-akesich.pptx.pptx

17:50 Upsilon production in pp and pA collisions from RHIC to the LHC

In the first part, I will discuss the impact of QCD corrections on the P_T differential cross section for Upsilon production in pp collisions at RHIC, Tevatron and LHC energies, as well as the behaviour of the differential cross section in rapidity. I will discuss the very good agreement between the parameter-free predictions of the Colour-Singlet Model and the first LHC data, especially in the region of low transverse momenta, which is the most relevant one for heavy-ion studies. I will also discuss predictions for the polarisation to be compared with the forthcoming LHC results. In the second part, I will discuss the nuclear-matter effects on Upsilon production at RHIC and the LHC in proton-nucleus and, by extension, in nucleus-nucleus collisions. We will argue that (i) the Upsilon break-up probability can be neglected in a first approximation, (ii) the gluon shadowing and antishadowing are not strong enough to describe forward RHIC data, (iii) the backward data hints at a gluon EMC effect, likely stronger than the quark one, (iv) fractional energy loss provides a very convincing explanation for the Upsilon suppression seen by PHENIX in the forward region. Following these discussions, outlooks for the LHC pPb run will be presented.

Speaker: Jean-Philippe Lansberg (IPNO - Paris-Sud U. - CNRS/IN2P3)

Slides HP-2012-300512.pdf

18:10 Quarkonia production in relativistic heavy ion collisions

Using the hydrodynamic model to describe the dynamics of heavy ion collisions, we have studied quarkonia production in these collisions by including both their dissociation in initial cold nuclear matter and subsequently produced quark-gluon plasma [1-4]. For the latter, we used the screened Cornell potential and the next-to-leading order perturbative QCD to determine, respectively, their in-medium properties and dissociation cross sections. Our results for the dependence of the quarkonia nuclear modification factors on the collision centrality as well as the quarkonia transverse momentum indicate that including medium modifications of the quarkonia properties in the quark-gluon plasma gives a better description of the experimentally data measured at SPS, RHIC and LHC. References 1) T. Song, C. M. Ko, S. H. Lee, and J. Xu, ``J/psi production and elliptic flow in relativistic heavy ion Collisions”, Phys. Rev. C 83, 014914 (2011). 2) T. Song, K. Han, and C. M. Ko, ``Charmonium production in relativistic heavy-ion collisions", Phys. Rev. C 84, 034907 (2011). 3) T. Song, K. C. Han, and C. M. Ko, ``Bottomonia suppression in heavy ion collisions at RHIC and LHC", Phys. Rev. C 85, 014902 (2012). 4) T. Song, K. C. Han, and C. M. Ko, ``The effect of initial fluctuations on bottomonia suppression in relativistic heavy ion collisions”, arXiv:1112.0313 [nucl-th].

Speaker: Che-Ming Ko (Texas A&amp;M University)

Slides ko_hp2012.pdf

16:30 → 18:30 Parallel VB: Jet quenching and energy loss

16:30 Measurement of isolated photon-jet correlations in PbPb collisions at sqrt(s)=2.76TeV with CMS

Modification of jets transitioning through the quark–gluon plasma (QGP), or quenching, has been well established by the RHIC and LHC data, using particle correlation and recently, reconstructed jets. At LHC, the first measurement of the jet quenching employed dijets. However, the advantage of the large yield of dijets is offset by loss of the perturbative final state information, as both jets interact with the medium. This also causes the observed high p_T dijet to be predominantly emitted from the surface. These shortcomings can be addressed by measuring isolated photon–jet correlation, where the directly produced photons carries the kinematic information before interaction with the QGP. We describe the measurement of the momentum inbalance and azimuthal correlation of isolated photon–jet pairs from sqrt(s_NN) = 2.76 TeV PbPb collisions at the LHC, and compare it to both the sqrt(s) = 2.76 TeV pp collision data and PYTHIA calculations. We discuss the characterization of the in-medium parton energy loss using the results.

Speaker: Lai Yue Shi (MIT)

Slides hp-20120531.pdf

16:50 Jet-Tagged Back-Scattering Photons For Quark Gluon Plasma Tomography

We investigate the correlations of photons produced by Compton back scattering and annihilation of fast quarks in quark gluon plasma with their away-side jets. Compton back scattering and quark annihilation into photons effectively act as jet-photon conversion mechanisms and were originally proposed as novel electromagnetic sources in heavy ion collisions in Phys. Rev. Lett. 90, 132301 (2003). The unique appeal of these processes lies in the fact that their photons carry information about both the medium via a T^2 log 1/T dependence of the yield, and about the energy loss of partons before the back scattering occurs. Attempts to identify these photons in experiment through inclusive direct photon spectra or direct photon v_2 measurements at intermediate PT at RHIC have been inconclusive so far. We show that the capability to measure jets in coincidence with photons at the upgraded STAR and SPHENIX experiments, or at one of the LHC experiments, offers a unique opportunity to identify back scattering photons at large photon momenta. Jet-triggered back-scattering photons can be distinguished from bremsstrahlung through their strong correlation with the given trigger ET, and they are set apart from prompt hard photons through the energy loss of their parent parton. We demonstrate with leading and next-to-leading order calculations that jet-triggered direct photon spectra and nuclear modification factors in nuclear collisions as a function of photon PT show a distinct feature around the trigger ET due to back-scattering photons. The height and width of this structure are correlated with the medium temperature and parton energy loss spectrum, respectively.

Speaker: Rainer Fries (Texas A&amp;M University)

Slides HP2012_Fries.pptx

17:10 Realistic medium-averaging in radiative energy loss

There has been a lot of interest in testing radiative energy loss calculations against data from RHIC and the LHC. It is customary, as in the Gyulassy-Levai-Vitev (GLV) approach, to formulate the energy loss of a jet parton as a line integral from the production point along a straight-line trajectory. Calculations then account for variations in pathlength with jet origin and direction, and most recently also for fluctuations in the medium density (lumpiness), which give rise to fluctuations in energy loss that significantly affect high-pT observables such as the nuclear suppression R_AA or the momentum anisotropies v_n. These studies, however, ignore that the line integrals themselves represent an average over the location of possible interaction points (color sources in GLV) along the jet path. We will present results from a calculation with stochastically chosen interaction points for each jet, which gives additional fluctuations in energy loss. The influence of these fluctuations on RHIC and LHC observables will be discussed utilizing both medium density parameterizations and realistic medium evolution from bulk dynamics models, such as the parton transport MPC. A special advantage of using parton transport is that it conveniently provides the evolution of an ensemble of scattering centers.

Speaker: Molnar Denes (Purdue University)

Slides DenesMolnar_5B.pdf

17:30 Jet Flavor Tomography at RHIC and LHC

Jet Flavor Tomography is a powerful tool used to probe the properties of Quark Gluon Plasma formed in heavy ion collisions at RHIC and LHC. A new Monte Carlo model of jet quenching developed at Columbia University, CUJET1.0, was applied to predict the jet flavor and centrality dependence of some of the main phenomenological observables, the nuclear modification factor $R_{AA}$ and the elliptic flow $v_2$. The predictions for fragments $f=\pi,D,B,e$, derived from quenched jet flavors $g,u,c,b$ in central and peripheral collisions at RHIC and LHC, exhibit novel features such as a level crossing pattern in $R_{AA}$ over a broad transverse momentum range which can test jet-medium dynamics in quark gluon plasmas and help discriminating between current energy loss models.

Speaker: Alessandro Buzzatti (Columbia University)

Slides Alessandro_Buzzatti_-_HP2012.pdf

17:50 Realistic 3+1-dimensional modelling of QCD jets in heavy-ion collisions

The reconstruction of jets in heavy-ion collisions provides insight into the dynamics of hard partons in media. Unlike the spectrum of single hadrons, the spectrum of jets is highly sensitive to $\hat{q}_{\perp}$, as well as being sensitive to partonic energy loss and radiative processes. We use MARTINI, an event generator, to study how finite-temperature processes at leading order affect single jet, dijet, and photon-jet observables. While these observables are studied at the LHC, detector upgrades at RHIC may allow measurements of these observables at lower collision energies.

Speaker: Clint Young (McGill University)

Slides YoungC_5B.pdf

16:30 → 18:30 Parallel VC: Initial state and pA

16:30 Azimuthal angle correlations in forward dihadron production in pA collisions

Single inclusive hadron production in the forward rapidiy region in deuteron-gold collisions is well understood in the Color Glass Condensate (CGC) framework. As a complement to single inclusive spectra, detailed information is obtained with two-particle correlations. Recent measurements of the azimuthal angle correlations at RHIC have shown that there is a strong suppression of the away side peak at forward rapidities. This is easily understood in the CGC framework: the produced partons are initially back-to-back in the transverse plane, but the interaction with the nucleus causes a momentum transfer of the order of the saturation scale. In forward dihadron production the small-$x$ structure of the nucleus is probed, implying a large saturation scale. We present on going work on calculating the dihadron correlation using the running coupling BK equation. We include the inelastic terms neglected in some of the previous literature and show that they naturally contain the double parton scattering part that has so far been treated as a separate contribution. We also use, for the first time in a phenomenological application, a Gaussian approximation of JIMWLK to go beyond the large-Nc limit.

Speaker: Heikki Mäntysaari (University of Jyväskylä)

Slides hard_probes_2012_mantysaari.pdf

16:50 The Future of Relativistic Heavy Ion Physics in the Foward Region in PHENIX

The field of Relativistic Heavy Ion Physics has made great strides in the past decade with the establishment of the strongly interacting Quark Gluon Plasma (sQGP) in high energy collisions of heavy ions at the Relativistic Heavy Ion Collider (RHIC) and now at the LHC. Experience has taught us that large rapidity coverage, as well as careful measurements of cold nuclear matter will be important factors in giving us a quantitative understanding of the sQGP. I will outline plans for a upgrade aimed at covering a region 1<eta<4 and the contributions that such as upgrade can make to our understanding of the initial state, entropy production, parton energy loss, and long range correlations in the sQGP. I will also briefly indicate some of the spin measurements which such an upgrade will enable as well as its capabilities in an early phase of a possible electron-ion program. Inherent in this plan, is the capability of an sPHENIX detector, which together with the central region upgrade will cover a rapidity region between -1 and 4 units of rapidity with a capability to make the most critical measurements towards a comprehensive understanding of the sQGP.

Speaker: Richard Seto (University of California, Riverside)

Slides SetoR_5C.pptx

17:10 Multigluon correlations in the Color Glass Condensate

At high energy and/or for large atomic numbers the wave-function of a generic hadron is dominated by gluon modes with soft longitudinal momenta and high occupation numbers, which can reach the state of saturation. This state, known as the Color Glass Condensate (CGC), is universal and can manifest itself in any hadronic process involving small-x partons, like deep inelastic scattering and proton-proton, proton-nucleus and nucleus-nucleus collisions at the LHC. The CGC effective theory describes the ensemble of multigluon correlations at small x, which evolve with energy according to a renormalization group equation, the JIMWLK equation. After a quick review of the theory of the CGC, we focus on recent progress in solving the JIMWLK equation and thus obtaining explicit, analytic solutions for the multigluon correlations at high energy. Although approximate, these solutions are in fact very accurate, as demonstrated by their comparison to numerical solutions to the JIMWLK equation in all the cases where the latter are known.

Speaker: DIONYSIS TRIANTAFYLLOPOULOS (ECT*, Trento)

Slides Triantafyllopoulos.pdf

17:30 Modeling the Impact Parameter Dependence of the nPDFs With EKS98 and EPS09 Global Fits

The knowledge of the parton distribution functions (PDFs) is essential for interpreting any hard-process results from hadronic and nuclear collisions. The nuclear modifications of PDFs have been successfully determined through a global DGLAP analysis e.g. in the sets EKS98 and more recently EPS09. So far the nuclear PDFs (nPDFs) in the global fits have been taken to be spatially independent. However, it can be expected that the nuclear modifications vary when going from the dense center of a nucleus to its more dilute edge. In this work, using the $A$-dependence of the globally fitted nPDFs, we have been able to determine the spatial dependence of the nPDFs in terms of powers of the nuclear thickness functions. A routine for public use is released. For applications, we will discuss how one can then compute hard-process cross sections in different centrality classes of nuclear collisions. In particular, we consider the nuclear modification factor $R_{\rm dAu}$ for neutral pion production in deuteron-gold collisions at RHIC. Comparison with the PHENIX data in different centrality classes is also shown. In addition, predictions for the corresponding nuclear modification factor $R_{\rm pPb}$ in proton-lead collisions at the LHC are discussed. Both leading-order and next-to-leading order results are considered.

Speaker: Ilkka Helenius (University of Jyväskylä, Department of Physics)

Slides helenius_HP2012.pdf

17:50 Suppression of high $p_{T}$ hadron spectra in $p+A$ collisions

Multiple hard and semi-hard parton scatterings in high-energy $p+A$ collisions involve multi-parton correlation in both momentum and flavor inside the projectile proton which will lead to modification of the final hadron spectra relative to that in $p+p$ collisions. Such modification of final hadron transverse momentum spectra in $p+A$ collisions is studied within HIJING 2.1 Monte Carlo model which includes nuclear shadowing of the initial parton distributions and transverse momentum broadening. Multi-parton flavor and momentum correlation inside the projectile are incorporated through flavor and momentum conservation which are shown to modify the flavor content and momentum spectra of final partons and most importantly lead to suppression of large $p_{T}$ hadron spectra in $pA$ collisions at both RHIC and LHC energies.

Speaker: Wei-Tian Deng (Frankfurt Institute for Advanced Studies)

Slides DengW_5C.ppt

18:10 Multigluon correlations in JIMWLK

Multiparticle correlations, such as the "ridge" effect in pp and AA collisions and forward dihadron correlations in pA collisions, are an important probe of the strong color fields that dominate the initial stages of a heavy ion collision. We argue that the Color Glass Condensate framework provides the most natural way to understand them. We describe recent progress in understanding two-particle correlations in the dilute-dense system, e.g. in forward dihadron production in deuteron-gold collisions. This requires computing the energy dependence of higher point Wilson line correlators from the JIMWLK renormalization group equation. We find that the large Nc approximation used so far in the phenomenological literature is not very accurate. On the other hand a Gaussian finite Nc approximation is a surprisingly close to the full result.

Speaker: Tuomas Lappi (University of Jyväskylä)

Slides hp12_lappi.pdf.pdf

Friday 1 June

09:00 → 10:30 Plenary 5A

09:00 The di-lepton physics program at STAR

Di-leptons are ideal probes of the strongly interacting hot, dense medium created at RHIC. They are not affected by the strong interaction once produced, therefore they can probe the whole evolution of the collision. In different mass regions, di-leptons can be used to probe vector meson in-medium modifications, Quark Gluon Plasma (QGP) thermal radiations, and color-screening features of QGP. In year 2010, the barrel time-of-flight detector was completed, which enables clean electron identification with full azimuthal coverage at mid-rapidity. In addition, the Muon Telescope Detector and Heavy Flavor Tracker Upgrades, to be completed in year 2014, providing clean muon identification and precise pointing resolution, will enable the precise measurements of correlated charm contribution to di-leptons, therefore, make it possible to measure QGP thermal radiation using di-leptons. In this talk, I will review recent di-electron results at STAR in p+p and Au+Au collisions at $\sqrt{s_{_{NN}}} = 200$ GeV and discuss di-muon and e-muon physics capabilities with future detector upgrades.

Speaker: Lijuan Ruan (Brookhaven National Laboratory)

Slides RuanL_5A.ppt

09:30 Electromagnetic Radiation probing the Space-Time Evolution of Heavy ion collisions

Over the past years PHENIX has published multiple results on electron-positron pairs and direct photons from Au+Au collisions, which reveal a number of puzzling and not understood features. First measurements of direct photons at momenta below 3 GeV point towards a significant thermal yield consistent with initial temperatures well above the transition temperature. However, this thermal source shows large azimuthal momentum anisotropy, seemingly inconsistent with early emission. In addition, dilepton production below a mass of 1 GeV is significantly larger than can be attributed to the thermal source. These excess dileptons exhibit a soft momentum spectrum with an inverse slope of less than 100 MeV. In this talk we will present and discuss the latest results from PHENIX.

Speaker: Axel Drees (Stony Brook University)

Slides DreesA5A.pptx

10:00 Characterizing energy loss in ALICE

This talk will present a global view of what has been learned to date from ALICE data about the interaction of jets with the hot medium generated in high energy nuclear collisions at the LHC. The discussion will incorporate measurements of inclusive charged and identified hadrons, hadron correlations, heavy flavor measurements, and first ALICE measurements of fully reconstructed jets, together with comparison to model calculations. I will also give a perspective on open questions in this area.

Speaker: Peter Jacobs (Lawrence Berkeley National Laboratory)

Slides JacobsP5A.pdf

10:30 → 11:00 Coffee break

11:00 → 12:00 Plenary 5B

11:00 Theory summary

Slides CarlosSalgado.pdf

11:30 Experiment summary

Speaker: Jurgen Schukraft (CERN)

Slides HP2012ExpSummary.pdf

12:00 → 12:15 closing

12:30 → 13:30 Lunch