Extreme QCD 2017 - The 15th international workshop on QCD in eXtreme conditions

26 Jun 2017, 01:50 → 28 Jun 2017, 20:00 Europe/Rome

Massimo D'Elia (PI)

The 15^th International Conference on QCD in Extreme Conditions (XQCD 2017) will be held in Pisa 26 to 28 June 2017. XQCD is a series of international workshop-style conferences, held annually, which aims to cover recent advances in the theory and phenomenology of QCD under extreme conditions of temperature and/or baryon density, together with related topics.

Topics

• QCD at Finite Temperature and Density
• Heavy Ion Collisions Phenomenology
• Phase Diagram of Strongly Interacting Matter
• Properties of the Quark-Gluon Plasma
• Properties of Strongly Interacting Gauge Theories
• The Sign Problem in Lattice QCD and Other Theories
• QCD in External Fields

 

                                        
                  

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Poster xqcd2017_poster.jpg

Slides program.pdf

Monday, 26 June

08:30 Registration

1 Welcome and Introduction

09:20 Session Chair: Adriano Di Giacomo

2 The struggle against the sign problem

The 'sign problem', i.e. the presence of configurations with negative or complex weight in the partition sum, has been a technical obstacle of growing importance, which prevents the Monte Carlo study of many systems of physical interest, among them QCD in 'extreme' parameter regimes. I will recall some basic notions about the sign problem, and review current attempts to overcome it.

Speaker: Philippe de Forcrand (ETH Zurich & CERN)

Slides XQCD17_deforcrand.pdf

Video http://mediateca.unipi.it/category/video/Ph-de-Forcrand-The-struggle-against-the-sign-problem/e87c96c860b74991cf34a217d3b7d7f5/108

3 Thimble regularization 5 years later

The proposal of thimble regularization as a solution to the sign problem is by now 5 years old. A report on recent work done in Parma is presented. This includes progress in our algorithmic solutions, in particular with respect to cases in which multiple thimbles are to be taken into account. Very much related to this, the role of symmetries is discussed. Finally, an update on thimble simulations of gauge theories is presented.

Speaker: Francesco Di Renzo (PR)

Slides xqcd2017fdr.pdf

Video http://mediateca.unipi.it/category/video/F-Di-Renzo-Thimble-regularization-5-years-later/a2e261ba20617f740807ec0df27668db/108

4 Fermion bag approach to Hamiltonian lattice field theories

Hamiltonian lattice field theories provide an alternate approach to study traditional Lagrangian lattice field theories in the strongly interacting regime. These formulations are more commonly used in condensed matter physics and may be more natural in theories at finite densities. They can be formulated both in discrete and continuous time. In the continuous time formulation they can help in reducing the fermion doubling problem. Some new sign problems are also solvable within this approach. In the discrete time formulation they are very similar to traditional Lagrangian lattice field theories but with unconventional actions. Unfortunately, traditional Monte Carlo methods for Hamiltonian lattice field theories seem to scale rather poorly with system size. Here we show that by using ideas of fermion bags we can design algorithms that help speed up these calculations and allow us to go to large lattices for the first time. Using this approach we compute critical exponents of the 2+1d Gross-Neveu model with Nf=1, which was impossible so far in Lagrangian lattice field theory.

Speaker: Shailesh Chandrasekharan (Duke University)

Slides Chandrasekharan.pdf

Video http://mediateca.unipi.it/category/video/S-Chandrasekharan-Fermion-bag-approach-to-Hamiltonian-lattice-field-theories/f6362515f2df544c6bc511bc7a31808a/108

5 Canonical simulations of heavy-dense QCD without a sign problem

In this talk I show how the canonical formulation of QCD can be obtained from transfer matrices defined directly in the canonical sectors of QCD. These transfer matrices are closely related to the dimensionally reduced Wilson fermion determinant and provide a complete temporal factorization of the fermion determinant. In the heavy-dense limit, the fermionic contributions to the canonical partition functions can be calculated analytically and I show that the sign problem is absent at infinitely strong coupling. Finally, I construct a cluster algorithm which solves the sign problem away from the strong coupling limit.

Speaker: Urs Wenger (Institute for Theoretical Physics, University of Bern)

Slides xqcd17_wenger.pdf

Video http://mediateca.unipi.it/category/video/U-Wenger-Canonical-simulations-of-heavy-dense-QCD-without-a-sign-problem/006e13b3940fef918a55e9c4f238d267/108

11:15 Coffee Break

6 Canonical Approach for Exploring Finite Density QCD

Canonical approach is a way to map QCD at imaginary chemical potential regions to the real ones. The essential idea is simple: Z(¥mu,T) = ¥sum_n Z_n (exp(¥mu/T))^n, where Z_n, the canonical partition functions, do not depend on ¥mu. We construct Zn in the imaginary ¥mu regions, and we can calculate Z(¥mu,T) at any real ¥mu. Since Z_n drops very fastly as |n| increases, and we must fight against the overlap problem, we need several algorithmic and computational tricks, such as a multi-precision calculation. In Vladivostok, we have investigated the sources of unstable Z_n, and developed methods to overcome them. I will report these studies and the outcomes, especially those for comparison of heavy ion collision experiment.

Speaker: Atsushi Nakamura (FEFU)

Slides Nakamura_XQCD2017.pdf

Video http://mediateca.unipi.it/category/video/A-Nakamura-Canonical-Approach-for-Exploring-Finite-Density-QCD/8910066c4880134017afbc8dea2a44b8/108

11:45 Session Chair: Mariapaola Lombardo

7 The QCD sign problem and the effective Polyakov line action

We suggest to solve the sign problem by mapping full QCD to an effective Polyakov line model. The method of relative weights, coupled with mean field theory, is applied to derive an effective action for an underlying SU(3) gauge theory with dynamical staggered fermions at finite densities. We present first results and discuss issues so far encountered.

Speaker: Roman Höllwieser (VUT)

Slides XQCD_hoellwieser.pdf

Video http://mediateca.unipi.it/category/video/R-Hoellwieser-The-QCD-sign-problem-and-the-effective-Polyakov-line-action/fdb94ff2ebefafc51abf24872f5e5894/108

8 Simulation of theories with a topological term

We discuss two methods for simulating systems with a "theta-like" term in the action, and that therefore suffer from a severe sign problem. Both methods are based on simulations at imaginary values of the "theta-like" parameter, where the sign problem is absent. We show the results of implementing the methods in a variety of physical systems, and we analyze their applicability and shortcomings. We also present preliminary results on the Schwinger model with a theta term, and we discuss the prospects for simulating QCD with a non-zero theta angle.

Speaker: Eduardo Follana (Universidad de Zaragoza)

Slides talk_follana_XQCD17.pdf

Video http://mediateca.unipi.it/category/video/E-Follana-Simulation-of-theories-with-a-topological-term/f6c4a3ed01a26f1d42ebfeb0660bbe88/108

9 Patterns and Partners for chiral symmetry restoration

The nature of chiral symmetry restoration and the identification of its correct pattern in terms of $O(4)$ and $U(1)_A$ restoration are central problems for our present understanding of the QCD phase diagram, which are not fully settled in lattice simulations. We will present a theoretical analysis based on Ward Identities which sheds light on this issue and where partner degeneration is systematically studied and connected with physical processes for the full scalar/pseudoscalar $U(3)$ meson nonets. Model-independent results are derived, which in addition allow to understand the temperature dependence of lattice screening masses through particular quark condensate combinations. Their realization in $U(3)$ Chiral perturbation Theory will also be described. Special attention will be paid to the role of the thermal $f_0(500)$ state to describe lattice data for the scalar susceptibility through thermal corrections to its pole parameters generated from unitarized pion scattering.

Speaker: Ángel Gómez Nicola (Universidad Complutense Madrid)

Slides GomezNicolaExtremeQCD2017.pdf

Video http://mediateca.unipi.it/category/video/A-Gomez-Nicola-Patterns-and-Partners-for-chiral-symmetry-restoration/bf6b041cb1d46242f22956b9f6a5d82e/108

13:30 Lunch break

10 Holographic approaches to the study of strongly interacting matter in extreme conditions

The strong correlations observed in particle production in heavy ion collisions suggest strong interaction between the quarks and gluons that form the Quark Gluon Plasma. These interactions are so large that indicate that the QGP close to the deconfining transition may no be well approximated by a gas of quasiparticles. This has highlighted the need to develop new tools able to describe real-time processes in non-abelian gauge theory plasmas without quasiparticles. Holography provides such a tool. In this talk I will review recent theoretical and phenomenological results arising from the gauge/gravity duality that shed light on how strongly coupled plasma is produced, interacts and quenches energetic probes.

Speaker: Jorge Casalderrey Solana (University of Oxford)

Slides jcsXQCD17.pdf

Video http://mediateca.unipi.it/category/video/J-Casalderrey-Solana-Holographic-approaches-to-the-study-of-strongly-interacting-matter-in-extreme-conditions/48d5075d53f4c89e478712a881102b74/108

15:00 Session Chair: Mikhail Stephanov

11 Inverse Magnetic Catalysis in Holographic QCD

Lattice simulations have shown recently that, contrary to earlier expectations, the chiral condensate is suppressed with growing magnetic field near the chiral and deconfinement transition temperatures of QCD, a phenomenon now known as “Inverse Magnetic Catalysis”. V-QCD is a holographic model for QCD, which fully includes the backreaction of the quarks to the gluonic degrees of freedom. I demonstrate that this holographic model reproduces the inverse magnetic catalysis, and provides a good qualitative match with lattice results for the condensate and related observables. In particular, the inverse catalysis is enhanced with increasing number of flavors, in agreement with lattice data. I will also comment on the predictions of the model at finite chemical potential and magnetic field.

Speaker: Matti Jarvinen (Ecole Normale Superieure, Paris)

Slides 170626_jarvinen_xqcd.pdf

Video http://mediateca.unipi.it/category/video/M-Jarvinen-Inverse-Magnetic-Catalysis-in-Holographic-QCD/1b19aabb9a90370682149d7233824008/108

12 Dense nuclear and quark matter in holographic QCD

Dense matter in the core of neutron stars is strongly coupled and presents an enormous theoretical challenge. First-principle methods from QCD are currently known only for vanishing or asymptotically large baryon densities, while phenomenological models are usually restricted to either nuclear or quark matter and/or contain many unknown parameters. I will discuss whether and how holographic methods can help. In particular, I will present latest work on nuclear matter and the chiral phase transition to quark matter in the Sakai-Sugimoto model, potentially leading to a strongly coupled equation of state with only 3 parameters that is applicable over a wide density regime.

Speaker: Andreas Schmitt (University of Southampton)

Slides schmitt.pdf

Video http://mediateca.unipi.it/category/video/A-Schmitt-Dense-nuclear-and-quark-matter-in-holographic-QCD/4fd21d0b795f8942957bff71e36fbd6d/108

13 Fast Hydrodynamization of Non-conformal Holographic Shockwaves

Ever since fast hydrodynamization has been observed in heavy ion collisions the understanding of the early out-of-equilibrium stage of such collisions has been a topic of intense research. We use the gauge/gravity duality to model the creation of a strongly coupled Quark-Gluon plasma in a non-conformal gauge theory. This numerical relativity study is the first non-conformal holographic simulation of a heavy ion collision and reveals the existence of new relaxation channels due to the presence of non-vanishing bulk viscosity. We study collisions at different energies in gauge theories with different degrees of non-conformality and compare three relaxation times which can occur in different orderings: the hydrodynamization time (when hydrodynamics becomes applicable), the EoSization time (when the average pressure approaches its equilibrium value) and the condensate relaxation time (when the expectation value of a scalar operator approaches its equilibrium value). We find that these processes can occur in several different orderings. Finally, I will discuss a new example of the applicability of hydrodynamics to systems with large gradients. We show that the time evolution and saturation of the spinodal instability (corresponding to black branes afflicted by the Gregory-Laflamme instability in the gravity dual) are accurately described by second-order hydrodynamics, where a set of locally unstable states with a first-order thermal phase transition settle down to a static, inhomogeneous configuration.

Speaker: Maximilian Attems (University of Barcelona)

Slides Attems_XQCD2017.pdf

Video http://mediateca.unipi.it/category/video/M-Attems-Fast-Hydrodynamization-of-Non-conformal-Holographic-Shockwaves/3b565b0dc8e610da01349a5cd3b09260/108

14 Thermal entropic destruction of quark-antiquark pair from dynamical holographic QCD

Lattice QCD results indicate a large amount of entropy associated with the quark-antiquark pair near the deconfinement temperature. The lattice data show a sharp peak in the quark-antiquark entropy near the transition temperature. Further, this entropy increases with inter quark distances. In this work, we used the gauge/gravity duality to reproduce these lattice results. For this purpose, we consider a phenomenological bottom-up Einstein-Maxwell-dilaton gravity model and analytically construct the black hole solutions whose dual boundary theories satisfy the properties of confinement as well as deconfinment. We study the entropy of the quark-antiquark pair in confinement/deconfiment phases and find that our holographic model qualitatively reproduces the corresponding lattice results for the entropy. We further provide holographic results with chemical potential

Speaker: Subhash Mahapatra (KU Leuven)

Slides Subhash_eCQD2017.pdf

Video http://mediateca.unipi.it/category/video/S-Mahapatra-Thermal-entropic-destruction-of-quark-antiquark-pair-from-dynamical-holographic-QCD/270ba1d67f53f08f2a7375d982f043de/108

Poster session

Wine and cheese tasting

15 $SU(2N_F)$-Ward identities for QCD with restored chiral symmetry

Motivated by the recent discovery of the $SU(2N_F)$ symmetric degeneracy in the spectrum of QCD with truncated low modes of the Dirac operator, we derive Ward identities associated to $SU(2N_F)$-transformations in the limit of vanishing quark masses. Low temperature QCD is not invariant under the $SU(2N_F)$-transformations due to the non-invariance of the classical action and the path integral measure, its part related to chiral symmetry is broken spontaneously. We show that, when the quark condensate vanishes at sufficiently high temperature or when it is removed artificially, the restoration of chiral symmetry leads to the simplification of the $SU(2N_F)$-Ward identities. The simplified Ward identities look like they would be if $SU(2N_F)$ is preserved by the classical action and path integral measure.

Speaker: Vasily Sazonov (LPT Orsay, Université Paris-Sud)

16 A way to avoid the global sign problem by modifying the Lefschetz thimble method

The numerical simulation for the lattice QCD at finite density is difficult to perform due to the sign problem. The Lefschetz thimble method is expected to give us a possible solution of the sign problem. However, this method has the global sign problem, which is cancellation between thimbles, and this problem has not been solved yet. We develop a new method to avoid the global sign problem by modifying the Lefschetz thimble method. In this talk, I introduce our method and show that it works well in toy models.

Speaker: Takahiro Doi (Quantum Hadron Physics Laboratory Theoretical Research Division, Nishina Center, RIKEN)

17 Accurate Simulation of the Finite Density Thirring Model

We present a study of the finite density lattice Thirring model in 2 dimensions using the world-line/fermion-bag algorithm. The model has features similar to QCD and provides a test case to explore the accuracy of various methods to solve sign problems. In the massless limit and with open boundary conditions we show that the sign problem is an artifact of the auxiliary field approach and is completely eliminated in the world-line approach. With anti-periodic boundary conditions the sign problem is mild on square lattices in the fermion bag method. We present accurate results for various quantities in the model that can be used as a benchmark for comparison with other methods of solving sign problems.

Speaker: Jarno Rantaharju (Duke University)

Slides poster.pdf

18 Alternatives to the stochastic "noise vector" approach

Several important observables, like the quark condensate and the Taylor coefficients of the QCD pressure with respect to the chemical potential, are based on the trace of the inverse Dirac operator or its powers. Such traces are traditionally estimated with "noise vectors". We explore alternative approaches based on polynomial approximations of the inverse Dirac operator.

Speaker: Benjamin Jaeger (ETH Zurich)

19 Calculation of $B_{K}$ with Wilson fermion using gradient flow

$B_{K}$ is a parameter which is related with the Kaon mixing and is one of the targets of Lattice QCD. Naive calculation of $B_{K}$ with the Wilson fermion is difficult because the chiral symmetry is explicitly broken. In this study we shall present on our calculation of $B_{K}$ with the Wilson fermion by applying the gradient flow method both to the gauge and quark fields. Our study is based on a fact that the chiral symmetry is restored if we apply the gradient flow to the Wilson fermion and take the continuum limit without bothering the UV divergence. This property is already observed for practical numerical simulation in Phys. Rev. D 95, 054502 (2017), where the chiral Ward-Takahashi relation is effectively restored for the topological susceptibility. In our study we expect the same good property even for the four fermi operators used for $B_{K}$.

Speaker: Asobu Suzuki (University of Tsukuba)

20 Charged pion condensation in chiral asymmetric dense quark matter in the framework of a (1+1) NJL_2 model

We investigate the phase structure of a (1+1)-dimensional schematic quark model with four-quark interaction and in the presence of baryon (μB), isospin (μI) and chiral isospin (μI5) chemical potentials. It is established that in the large-Nc limit (Nc is the number of colored quarks) there exists a duality correspondence between the chiral symmetry breaking phase and the charged pion condensation (PC) one. The role and influence of this property on the phase structure of the model are studied. It is also shown that the chemical potential μI5 promotes the appearance of the charged PC phase with nonzero baryon density. Spatially inhomogeneous chiral density wave (for chiral condensate) and single wave (for charged pion condensate) approaches are also used.

Speaker: Tamaz Khunjua (Moscow State University M.V. Lomonosov)

21 Color screening masses under the influence of strong background fields

I will present recent results obtained in the study of the color magnetic and electric screening masses in the QCD plasma. I will focus on how the masses get modified by strong magnetic background fields which are expected to be created in particular physical contexts such as heavy-ion collisions.

Speaker: Andrea Rucci (PI)

22 Compactified gauge theories under extreme conditions

TBD

Speaker: Mohamed Anber (Lewis & Clark College)

23 Complex Langevin simulations of a finite density model for QCD

We study an RMT model for QCD at finite density using the Complex Langevin algorithm. Naive implementation of the algorithm shows convergence towards the phase quenched or quenched theory. A detailed analysis of this issue and a potential resolution of the failure of this algorithm are discussed. Among others we study the behavior of the real and imaginary parts of the action under Langevin evolution, the distribution of the eigenvalues of the Dirac operator as well as the evolution of the fermion determinant. We find that the correct result can be obtained by a reweighting procedure. This indicates that there is a "residual phase" which is not taken into account by the Langevin algorithm, which is probabilistic, but when it is included explicitly, the correct result is obtained.

Speaker: Savvas Zafeiropoulos (College of William & Mary and Jefferson Lab)

24 Correlation function of energy-momentum tensor in SU(3) gauge theory from gradient flow

We study the temporal correlators of energy-momentum tensor in various channels in SU(3) gauge theory for two values of temperature T = 1.68T c and 2.24T c on the lattice. The correlators are measured using energy-momentum tensor operators constructed with the gradient flow, which is found to be quite effective to reduce the statistical error. We numerically confirm that temporal correlators including a conserved charge (energy or momentum) are constants as is consistent with the energy-momentum conservation. It is also checked that these constants satisfy the linear response relations. A novel measurement of specific heat from the energy-energy correlator is performed.

Speaker: Masakiyo Kitazawa (Osaka University)

Slides 170626XQCD-EMTcor.pdf

25 Cumulants of the quark number fluctuations from LQCD at imaginary potentials

I present a determination of the cumulants of the quark number fluctuations, obtained via analytic continuation of Nf=2+1 physical quark mass lattice QCD simulations at imaginary chemical potentials. We employ stout improved staggered fermions and tree level Symanzik gauge action, exploring temperatures ranging from 135 up to 350 MeV, adopting mostly lattices with Nt=8 sites in the temporal direction. We show that below Tc the method can be strongly advantageous, with respect to a direct Montecarlo sampling at μ=0. We discuss the radius of convergence of the Taylor expansion and the possible location of the second order critical point at real potential. No evidence for such a point is found in the explored range of temperature, within present determinations of the pseudo-critical line.

Speaker: Francesco Sanfilippo (INFN - Sezione Roma III)

26 Curvature of the pseudo-critical line in QCD: comparison of analytic continuation and Taylor expansion methods

We study the curvature of the pseudo-critical line in N_f=2+1 QCD through numerical simulations performed using the tree-level Symanzik gauge action and the stout-smeared staggered action. The location of the phase transition is determined from the inflection point of the renormalized chiral condensate and the curvature coefficient is calculated using the Taylor expansion adopting various definitions. We also compare our findings with previous results available in the literature.

Speaker: Kevin Zambello (P)

27 Dispersion relations of charged and uncharged pions in presence of weak magnetic field

In this work, the self energies of $\pi_0$ and $\pi_{\pm}$ up to one loop order have been calculated in the limit of weak external magnetic field . The effective masses get an explicit magnetic field dependence which are modified significantly for the pseudo-scalar coupling due to weak field approximation of the external field. However, for the pseudo-vector coupling, only a modest reduction in the effective masses are noted. These theoretical developments are relevant for the study of the phenomenological aspect of mesons in the context of neutron stars as well as heavy ion collisions.

Speaker: Souvik Priyam Adhya (Post doctoral fellow)

Poster xqcd17_spa_pions.pdf

28 Fate of quarkonium in a quark gluon plasma via a Lindblad equation

What is the dynamics of heavy quarks and antiquarks in a quark gluon plasma? Can heavy-quark bound states (quarkonia) dissociate? Can they (re)combine? These are the questions that this paper aims to address by investigating a Lindblad equation that describes the quantum dynamics of the heavy particles in the medium. The Lindblad equations for a heavy quark and a heavy quark-antiquark pair are derived from the gauge theory after a chain of well defined approximations. In this exploratory work the attention is restricted to the case of an abelian plasma, but the extension to the non-abelian case is feasable. A one-dimensional simulation of the Lindblad equation for a heavy quark-antiquark pair is performed to extract information about bound-state dissociation, recombination and quantum decoherence. All these phenomena strongly depend on the imaginary part of the inter-quark potential appearing in the Lindblad formulation.

Speaker: Davide De Boni (Swansea University)

29 Flux tubes in Nf=2+1 QCD with magnetic fields

We study the confining flux tube between two static color sources and its scaling towards the continuum limit in Nf=2+1 QCD at the physical point. To this aim, we discretize the theory with the tree level Symanzik gauge action and stout-smearing improved staggered quarks. We discuss the effect of a uniform external magnetic field on the flux tube and we show, in particular, that it displays anisotropies with respect to the magnetic field direction.

Speaker: Francesco Negro (PI)

Poster Negro_xqcd2017.pdf

30 Flux Tubes in QCD across deconfinement

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Speaker: Dr Francesca Cuteri (Goethe Universität - Frankfurt am Main)

Slides Poster_XQCD.pdf

31 General equilibrium second-order hydrodynamic coefficients for quantum fields

The quark-gluon plasma created in heavy ion collisions can be described as a relativistic fluid with high values of acceleration and vorticity and, in such extreme local equilibrium conditions, the standard hydrodynamic equations are expected to receive corrections from the non-ideal non-dissipative terms of the stress-energy tensor. We show how these corrections can be obtained in a systematical way by performing a perturbative expansion around the homogeneous global equilibrium condition for small values of acceleration and vorticity. The final outcome is that the thermodynamic coefficients associated with these corrections can be expressed in term of euclidean correlators of the stress-energy tensor operator and the generators of the Lorentz group. These correlators can in principle be estimated nonperturbatively by lattice QCD techniques and we will present the analytic results that can be obtained in two cases: a free scalar charged field and a free Dirac field, both massive and massless.

Speaker: Matteo Buzzegoli (FI)

Poster PosterEquilibriumCoeff.pdf

32 Gluon and ghost correlation functions of 2-color QCD at finite density

SU(2) is the simplest non-abelian gauge theory with fermions without sign problem. Therefore its study on the lattice is a benchmark for other non-perturbative approaches at finite density. We study the Landau-gauge 2-point and 3-point correlation functions of the gauge sector and the running gauge coupling at finite density, and compare them to the vacuum case. We observed no significant effect of the finite density, except for some screening of the gluons. Moreover, no strong signature of the phase change (if not a lattice artifact) in the properties of quarks are observed in the gauge sector, in contrast to the finite-temperature case. This indicates that the finite-density physics is essentially driven by the quarks, rather than by the gluons.

Speaker: Ouraman Hajizadeh (University of Graz)

33 Investigating Columbia plot with clover fermions

We investigate the critical endpoints of the finite temperature phase transition of QCD at zero chemical potential. We employ the renormalization-group improved Iwasaki gauge action and non-perturbatively $O(a)$-improved Wilson-clover fermion action. The critical endpoints are determined by using the intersection point of kurtosis, employing the multi-parameter, multi-ensemble reweighting method. We present results for the critical endline at $N_t=6$ and the continuum extrapolation for the critical endpoint of the SU(3)-flavor symmetric point.

Speaker: Yoshifumi Nakamura (RIKEN)

34 Lattice study of continuity and finite-temperature transition in $2d$ $SU(N) \times SU(N)$ Principal Chiral Model

We present first-principle lattice study of continuity conjecture in $2d$ $SU(N) \times SU(N)$ Principal Chiral Model (PCM) on $\mathbb{R} \times S^1$ with respect to circumference $L$ of $S^1$ in the presence of $Z(N)$-preserving twist. The twist can be considered as analogous to Twisted Eguchi-Kawai reduction in lattice gauge theory. We study static correlation length and find that it exhibits a peak at finite value of $\rho \equiv N L$, the shape of which shows no dependence on $N$ if considered as a function of $\rho$. The peak separates two regions: $\rho \rightarrow \infty$ where static correlation length matches zero temperature value with periodic boundary conditions and $\rho \rightarrow 0$ where it significantly decreases. Without twist we find a signature for large $N$ finite-temperature transition where correlation length demonstrates a peak enhancing with $N$. Using Gradient flow we study non-perturbative content of the theory and find that this transition sets up at the point where typical size of uniton, unstable saddle point of PCM, becomes comparable to $L$. After imposing the twist saddle points become stable and effectively $1d$ in the region $\rho \rightarrow 0$, whereas in the opposite limit they resemble to $2d$ profile of unitons with periodic boundary conditions. The position of the peak in correlation length with twisted boundary conditions seems to coincide with the moment when $2d$ saddle points transform into effectively $1d$. Our findings suggest possible crossover at finite value of $\rho$ which might have impact on continuity conjecture in twisted PCM.

Speaker: Semen Valgushev (Regensburg University)

35 Reweighted complex Langevin approach to chiral fermion models at finite density

We apply complex Langevin equation (CLE) to simulate the 0+1 dimensional Thirring model at finite density. In the crossover region the simulation fails because of the zero of the fermion determinant. To simulate the crossover retion, we then apply the reweighting method with the ensembles generated with CLE at high enough chemical potential. We study the effectiveness of the reweighting method by changing the model parameters, the coupling constant, mass, lattice size, from the viewpoint of the Lefschetz thimble structure of the model. We also like to include results on chiral random matrix model at finite temperature and density, applying the same strategy.

Speaker: Hirotsugu Fujii (U Tokyo)

36 Topological susceptibility and Gribov copies

The topological susceptibility, $\chi^4$, plays an important role in explaining the $\eta^{\prime}$ mass, the so-called $U(1)_{A}$ problem. For $\chi^4 \neq 0$, we must have the Veneziano ghost, an unphysical massless pole in the correlation function of the topological current $K_{\mu}$ correlator. There was a recent attempt in http://inspirehep.net/record/1340323?ln= en to connect the dynamics of the Veneziano ghost, and thus topological susceptibility, with Gribov copies. However, we will discuss that this proposal is incompatible with BRST symmetry, following http://inspirehep.net/record/1402613?ln=en. We will also analyze the topological susceptibility in $SU(2)$ and $SU(3)$ Euclidean Yang-Mills theory in a generic linear covariant gauge taking into account the Gribov ambiguity, while keeping the BRST symmetry. During this analysis, we make use of a Pad{\' e} approximation based on the K\"all\'en-Lehmann spectral integral representation of the topological current correlation function.

Speaker: Caroline Felix (KU Leuven)

37 Сonfinement-deconfinement transition in dense two-color QCD

In this report we study the properties of the dense SU(2) QCD. The lattice simulations are carried out with improved gauge action and smaller lattice spacing as compared to our previous work. This allowed us to approach closer to the continuum limit and reach larger densities without lattice artifacts. We measured string tension and Polyakov loop as functions of chemical potential and temperature. At sufficiently large baryon density and zero temperature we observe confinement/deconfinement transition which manifests itself as a vanishing of the string tension and rising of the Polyakov loop.

Speaker: Aleksandr Nikolaev (ITEP & FEFU)

Tuesday, 27 June

38 Recent results on QCD thermodynamics from Lattice

Fluctuations of conserved charges like baryon number and strangeness are important observables to understand the nature and interactions between the degrees of freedom in different phases of QCD. The higher order fluctuations are particularly important to get information on the location of possible QCD critical end-point and to understand the interplay between fluctuations and non-trivial topological properties in QCD. In this talk I will highlight the recent theoretical and algorithmic developments made in lattice gauge theory in calculating the higher moments of conserved charges. I will also discuss how the lattice data on fluctuations and correlations of different conserved charges could be used for determining the QCD equation of state at finite density, to constraint the possible location of critical end-point in the QCD phase diagram and understand the nature and interactions among quasi-particles across the chiral crossover transition in QCD.

Speaker: Sayantan Sharma (Tata Institute of Fundamental Research)

Slides ssharma_xqcd17.pdf

Video http://mediateca.unipi.it/category/video/S-Sharma-Recent-results-on-QCD-thermodynamics-from-Lattice/7be4e5341ce621c3a56d05102742ac15/108

09:00 Session Chair: Michael Ilgenfritz

39 Strange baryons below and above the deconfinement transition

We investigate the fate of strange baryons in the hadronic gas and the quark-gluon plasma. In the confined phase a strong temperature dependence is seen in the masses of the negative-parity groundstates, while at high temperature parity doubling emerges. We study baryons with different strangeness and find a noticeable effect of the heavier s quark. This study uses nonperturbative lattice simulations, employing the FASTSUM anisotropic Nf = 2 + 1 ensembles.

Speaker: Gert Aarts (Swansea University)

Slides talk-xqcd-Aarts.pdf

Video http://mediateca.unipi.it/category/video/G-Aarts-Strange-baryons-below-and-above-the-deconfinement-transition/d0fe111b98c486e33e3188e73706901c/108

40 Determination of deconfinement transition from Roberge-Weiss periodicity

In this talk, I will report recent progress of our study on the confinement-deconfinement transition in QCD from topological view points. We have proposed a new determination of the confinement-deconfinement transition by using the imaginary chemical potential. The imaginary chemical potential can be interpreted as the Aharonov-Bohm phase and then an analogy of the topological order suggests that the Roberge-Weiss endpoint would define the deconfinement temperature. Based on the topological property, we can construct a new quantity which describes the confinement-deconfinement transition. This quantity is defined as the integral of the quark number susceptibility along the closed loop of the dimensionless imaginary chemical potential.

Speaker: Kouji Kashiwa (Yukawa Institute for Theoretical Physics, Kyoto University)

Slides 20170627-XQCD.pdf

Video http://mediateca.unipi.it/category/video/K-Kashiwa-Determination-of-deconfinement-transition-from-Roberge-Weiss-periodicity/bf8389110c72100bc95b23be5f43cc33/108

41 The QCD phase transition at large N as BKT type transition.

We conjecture that the phase transitions in QCD at large N is triggered by the drastic changes in the instanton density, and corresponding drastic modification of the \theta behaviour in the system. We advocate the picture that these very sharp changes is a result of complete reconstruction of the relevant Euclidean configurations when the instantons at large T ``dissociate" into N fractionally charged constituents (the instanton quarks, the instanton -dyons, the instanton-monopoles, you name it) at small T. This picture is quite universal and describes the corresponding transitions in different circumstances. In particular, the transition to colour superconducting phase at \mu>\mu_c, or to the conformal phase (for sufficiently large N_f/N) can be described using the same universal framework. The talk is based on few recent papers (see [1] and references therein) and some recent development. [1]. A. Zhitnitsky ``Conformal window in QCD for large numbers of colours and flavours,'' Nucl.\ Phys.\ A {\bf 921}, 1 (2014), arXiv:1308.0020 [hep-ph].

Speaker: Ariel Zhitnitsky (University of British Columbia)

Slides PISA-2017.pdf

Video http://mediateca.unipi.it/video/A-Zhitnitsky-The-QCD-phase-transition-at-large-N-as-BKT-type-transition/e2664bb7554cb4462527af79b39b4f46

10:55 Coffee break

11:25 Session Chair: Francesco Di Renzo

42 Taylor series expansions for higher order cumulants

We present results on Taylor series expansions for up to fourth order cumulants of conserved charge fluctuations, construct expansions for ratios of these cumulants and discuss their application to the analysis of freeze-out conditions in heavy ion experiments. Our results are based on lattice QCD calculations performed with the HISQ action and quark masses tuned to their physical values.

Speaker: Frithjof Karsch (Brookhaven National Laboratory)

Slides Karsch_xQCD2017.pdf

Video http://mediateca.unipi.it/video/F-Karsch-Taylor-series-expansions-for-higher-order-cumulants/7b9d0b9d0e2b7034b8d84246485ab106

43 QCD thermodynamics. Hard-thermal-loop perturbation theory vs lattice

The perturbative series for finite-temperature field theories has very poor convergence properties and one needs a way to reorganize it. In this talk, I review one way of reorganizing the perturbative series thermal QCD, namely hard-thermal-loop perturbation theory (HTLpt). I will present results for the pressure, trace anomaly, speed of sound and the quark susceptibilities from a 3-loop HTLpt calculation. A careful comparison with available lattice data shows good agreement for a number of physical quantities.

Speaker: Jens Oluf Andersen (NTNU)

Slides AndersenPisa17.pdf

Video http://mediateca.unipi.it/video/JO-Andersen-QCD-thermodynamics-Hard-thermal-loop-perturbation-theory-vs-lattice/2491249cc4c4d551737add7c2ca9e4d9

44 Scale invariant resummed perturbation at finite temperature

We will illustrate how our recently developed renormalization group optimized perturbation (RGOPT) efficiently resums perturbative expansions in thermal field theories. The resulting convergence and scale dependence of optimized thermodynamical quantities are drastically improved as compared to standard perturbative expansions, as well as compared to other related methods such as the screened perturbation or (resummed) hard-thermal-loop perturbation. Our general method will be illustrated for the nonlinear sigma model, as a toy model for thermal QCD, and we will also discuss application of RGOPT to hard thermal loop resummation for QCD thermodynamical quantities.

Speaker: Jean-Loic Kneur (Lab. Charles Coulomb (L2C) Montpellier)

Slides kneur_XQCD2017.pdf

Video http://mediateca.unipi.it/video/J-Kneur-Scale-invariant-resummed-perturbation-at-finite-temperature/702becbfd2034e092b6aaf11af2421ae

45 Diagrammatic Monte-Carlo for large-N non-Abelian lattice field theories based on the convergent weak-coupling expansion

We demonstrate that non-Abelian lattice field theories such as principal chiral model and pure lattice gauge theory in the large-N limit admit infrared-finite weak-coupling expansion in powers of coupling and logs of coupling, reminiscent of re-summed series in thermal field theory and resurgent trans-series without exponential terms. Such a double-series structure arises due to the bare mass term proportional to the coupling constant, which stems from the Jacobian in the path integral measure and is absent in the scale-invariant classical action. This term renders the perturbative expansion infrared-finite even for an infinite lattice size, which allows to sample it using Diagrammatic Monte-Carlo. On the exactly solvable example we demonstrate that this expansion incorporates the non-perturbative mass gap. We then develop a DiagMC algorithm for sampling planar diagrams in the principal chiral model and numerically demonstrate the convergence of our expansion for up to 12 leading orders, which is the practical limit set by the increasingly strong sign problem at high orders. We find reasonably good agreement with conventional Monte-Carlo, extrapolated to infinite N. Finally, we comment on the applicability of our approach to planar QCD at zero and finite density.

Speaker: Pavel Buividovich (Regensburg University)

Slides XQCD2017.pdf

Video http://mediateca.unipi.it/video/P-Buividovich-Diagrammatic-Monte-Carlo-for-large-N-non-Abelian-lattice-field-theories-based-on-the-convergent/da75a5c58c800ca5cc2fa168e688ce58

13:05 Lunch break

46 Chiral effects in strong magnetic backgrounds: from QCD to condensed matter physics

Chiral symmetry and its breaking play a profound role in the theory of strong interactions. Over the years, numerous advances have been made in understanding the dynamics responsible for chiral symmetry breaking in vacuum and its restoration at high temperatures. One of the instructive tools in studying the underlying physics is a strong background magnetic field. On the one hand, such a field acts as a catalyst by helping the symmetry breaking via the dimensional reduction and an increased density of low-energy states. On the other, the confinement is eroded by an enhanced screening from the low-energy states in the strong magnetic field. The outcome is a subtle interplay between the two competing dynamics that I will attempt to review. I will also discuss the recent advances in pseudo-relativistic condensed matter systems, in which chirality plays an equally important role and a number of interesting phenomena could be realized in the presence of strong magnetic fields.

Speaker: Igor Shovkovy (Arizona State University)

Slides shovkovy_2017_06_27_Pisa.pdf

Video http://mediateca.unipi.it/category/video/I-Shovkovy-Chiral-effects-in-strong-magnetic-backgrounds-from-QCD-to-condensed-matter-physics/a87fd2932c2e0f35d196a792699b6df0/108

14:50 Session Chair: Andreas Schmitt

47 Lattice simulation of Chiral Magnetic Effect in Dirac Semimetals

Recently discovered Dirac Semimetals Na3Bi and Cd3As2 provide perfect opportunity for investigation of phenomena which were usually attibuted to high energy physics. The reason for this is the existence of two massless Dirac fermions in the quasi-particle dispersion relation for these materials. One of the manifestations of the chiral anomaly, Chiral Magnetic Effect, can be be observed in these materials as a large magnetoconductivity. We study the conductivity of these materials in external magnetic field within lattice effective field theory approach. Our results confirm the existence of Chiral Magnetic Effect in Dirac Semimetals.

Speaker: Andrey Kotov (Institute for Theoretical and Experimental Physics)

Slides xqcd_kotov.pdf

Video http://mediateca.unipi.it/category/video/A-Kotov-Lattice-simulation-of-Chiral-Magnetic-Effect-in-Dirac-Semimetals/a5a8e4780725dd62d4f6cbab3ce53ac0/108

48 Landau levels in lattice QCD in an external magnetic field

I will discuss the issue of Landau levels of quarks in lattice QCD in an external magnetic field. In two dimensions the lowest Landau level can be identified unambiguously even if the strong interactions are turned on. This allows to define a "lowest Landau level" also in the four-dimensional case. It is then possible to study to what extent the effect of a magnetic field on observables can be explained in terms of the lowest Landau level, and test the validity of low-energy models of QCD that make use of the lowest-Landau-level approximation.

Speaker: Matteo Giordano (Eotvos University)

Slides giordano_landau_lev.pdf

Video http://mediateca.unipi.it/category/video/M-Giordano-Landau-levels-in-lattice-QCD-in-an-external-magnetic-field/cbe82f9455798b2c4ae53172a4814b47/108

49 Chiral phase transition of three flavor QCD with nonzero magnetic field

Lattice simulations for (2+1)-flavor QCD demonstrated that the quark mass is one of the important parameters responsible for the (inverse)-magnetic catalysis. In this talk we will discuss the dependence of chiral condensates and susceptibilities on the magnetic field in three flavor QCD in the regime of the first order phase transition. The lattice simulations were performed using standard staggered fermions and the plaquette action with spatial sizes $N_s$ = 16 and 24 and a fixed temporal size $N_t$ = 4. The value of the quark mass was chosen such that the system undergoes a first order chiral phase transition with zero magnetic field. We find that the quark chiral condensate undergoes magnetic catalysis in the whole temperature region, and the first order phase transition becomes stronger as magnetic field increases. The underlying mechanism will also be discussed.

Speaker: Akio Tomiya (CCNU)

Slides XQCD2017_akio_final.pdf

Video http://mediateca.unipi.it/category/video/A-Tomiya-Chiral-phase-transition-of-three-flavor-QCD-with-nonzero-magnetic-field/f82574963af9a978e6e0ff546bf6ab1a/108

16:45 Coffee Break

50 Effects of magnetic field on the plasma evolution in relativistic heavy­ ion collisions

Very strong magnetic fields can arise in non­central heavy­ion collisions at ultra­ relativistic energies, which may not decay quickly in a conducting plasma. We carry out relativistic magneto­hydrodynamics (RMHD) simulations to study the effects of this magnetic field on the evolution of the plasma and on resulting flow fluctuations in the ideal RMHD limit. Our results show that magnetic field leads to enhancement in elliptic flow, though in general effects of magnetic field on elliptic flow are very complex. Interestingly, we find that magnetic field in localized regions can temporarily increase in time as evolving plasma energy density fluctuations lead to reorganization of magnetic flux. This can have important effects on chiral magnetic effect. Magnetic field has non­trivial effects on the power spectrum of flow fluctuations. For very strong magnetic field case one sees a pattern of even­odd difference in the power spectrum of flow coefficients arising from reflection symmetry about the magnetic field direction if initial state fluctuations are not dominant. We discuss the situation of nontrivial magnetic field configurations arising from collision of deformed nuclei and show that it can lead to anomalous elliptic flow. Special (crossed body­body) configurations of deformed nuclei collision can lead to presence of quadrupolar magnetic field which can have very important effects on the rapidity dependence of transverse expansion (similar to beam focusing from quadrupole fields in accelerators). We also show the possibility of the dynamo like effects in the presence of (CFL) superfluid vortex (which may arise in low energy collisions experiments e.g. FAIR and NICA) in our RMHD simulations.

Speaker: Shreyansh Shankar Dave (Institute of Physics)

Slides dave_magnhydro.pdf

Video http://mediateca.unipi.it/video/S-Dave-Effects-of-magnetic-field-on-the-plasma-evolution-in-relativistic-heavy-ion-collisions/35bec1ce56e08f8feaf123b13805ed88

17:15 Session Chair: Ariel Zhitnitsky

51 QED and QCD with massless fermions in three dimensions

Recent numerical results on QED and QCD with massless fermions in three dimensions will be presented. The main focus will be classifying theories as conformal or non-conformal. Results for the absence or presence of bilinear condensates will be presented using the behavior of low lying eigenvalues. Correlation functions of fermion bilinears will also be presented.

Speaker: Rajamani Narayanan (Florida International University)

Slides narayanan-xqcd2017.pdf

Video http://mediateca.unipi.it/category/video/R-Narayanan-QED-and-QCD-with-massless-fermions-in-three-dimensions/34689de494a277431b32fcd7e915a5b0/108

52 Phase structure of large-N gauge theory at finite temperature

We use a semiclassical method to analyze the phase structure of SU(N) gauge theory at infinite N in the presence of the external field. The effective potential can be written in terms of a Landau free energy for Polyakov loops, and we construct it using the perturbative contribution and a double trace deformation as the unknown confining potential. We show that there is a surface of a continuous phase transition analogous to the Gross-Witten-Wadia transition, whose boundary terminates at a tricritical point of a critical first-order phase transition. Depending on the confining potential we have considered, it gives rise to a third, fourth, or fifth-order phase transition in the Ehrenfest classification. Because the specific heat and the eigenvalue distribution of the Polyakov loop are sensitive to the confining potential, we argue that lattice simulations for large N could probe the order of phase transition as well as the form of the confining potential in any 1+d dimensions.

Speaker: Hiromichi Nishimura (BNL)

Slides XQCD_Nishimura_2017.pdf

Video http://mediateca.unipi.it/category/video/H-Nishimura-Phase-structure-of-large-N-gauge-theory-at-finite-temperature/e18a7ec85e7152224ab9050cc8132a00/108

20:00 Social Dinner

Wednesday, 28 June

09:00 Session Chair: Frithjof Karsch

53 Strongly interacting matter in extreme conditions: insights from hydrodynamic modeling of heavy ion collisions

I the first part of the talk I will review several physics features of strongly interacting matter (such as, for example, equation of state and kinetic coefficients) that have been established by comparing heavy-ion data with hydrodynamic models. Then I will turn to the general problems of applicability of relativistic viscous hydrodynamics, in particular, for description of the early stages of heavy-ion collisions. Finally, I will turn to observables connected with polarization of the observed particles and comment on recent developments of relativistic fluid hydrodynamics with spin.

Speaker: Wojciech Florkowski (UJK Kielce / IFJ PAN Krakow)

Slides Pisa-Florkowski.pdf

Video http://mediateca.unipi.it/category/video/W-Florkowski-Strongly-interacting-matter-in-extreme-conditions-insights-from-hydrodynamic-modeling-of-heavy-ion-collisions/54c168b3dbe843b8ea1abaa51f217474/108

54 QCD Critical Point and Hydrodynamics

I shall discuss the physics of fluctuations near the QCD critical point and application of hydrodynamics in the critical regime.

Speaker: Mikhail Stephanov (UIC)

Slides stephanov-xqcd-2017-slides.pdf

Video http://mediateca.unipi.it/category/video/M-Stephanov-QCD-Critical-Point-and-Hydrodynamics/b0d18deb45c24ff275f87072e06ab76b/108

55 Quasiparticle anisotropic hydrodynamics for ultrarelativistic heavy-ion collisions

We present the first comparisons of experimental data with phenomenological results from 3+1d quasiparticle anisotropic hydrodynamics (aHydroQP). We compare charged-hadron multiplicity, identified-particle spectra, identified-particle average transverse momentum, charged-particle elliptic flow, and identified-particle elliptic flow produced in LHC 2.76 TeV Pb+Pb collisions. The dynamical equations used for the hydrodynamic stage utilize non-conformal aHydroQP. The resulting aHydroQP framework naturally includes both shear and bulk viscous effects in addition to higher-order non-linear transport coefficients. The 3+1d aHydroQP evolution obtained is self-consistently converted to hadrons using anisotropic Cooper-Frye freezeout performed on a fixed-energy-density hypersurface. The final production and decays of the primordial hadrons are modeled using a customized version of THERMINATOR 2. In this first study, we utilized smooth Glauber-type initial conditions and a single effective freeze-out temperature $T_{\rm FO} = 130$ MeV with all hadronic species in full chemical equilibrium. With this rather simple setup, we find a very good description of many heavy-ion observables.

Speaker: Mubarak Alqahtani (Kent State University)

Slides XQCD-Mubarak.pdf

Video http://mediateca.unipi.it/category/video/M-Alqahtani-Quasiparticle-anisotropic-hydrodynamics-for-ultrarelativistic-heavy-ion-collisions/7608c8cbf1748c6e0a9c39d147fc1606/108

56 Collective Excitations in QCD Plasma

We study the long wavelength excitations in rotating QCD fluid coupled to an external magnetic field at finite vector and axial charge densities. We first find the generalization of the both well-known Chiral Magnetic Wave (CMW) and Chiral Vortical Wave (CVW), separately. It turns out that at µ5 = 0 and in the absence of rotation, there exist two CMWs which propagate in the same and in the opposite directions of the magnetic field with the same velocities. However when µ5 6= 0, one of the CMW modes propagates faster than the other and additionally, they do not necessarily propagate in the opposite directions. The similar situation happens for the two CVWs in the rotating fluid at finite axial chemical potential. We then show that in general, when the fluid is either rotating and is coupled to a magnetic field, the CMW and the CVW mix with each other and make the Chiral Magnetic-Vortical Wave (CMVW). The resultant coupled waves have generally different velocities compared to the sum of velocities of the individual waves. We also find another excitation in the QCD plasma; the so-called Chiral Alfv´en Wave (CAW), an analogue of what as recently found in a chiral fluid with single chirality. We specifically show that in contrast to the latter case, the CAWs in QCD fluid may propagate only when both the vector and axial charge densities are non-vanishing. Furthermore, while the velocity of CAWs in a chiral fluid with single chirality depends on the coefficient of a gravitational anomaly, we show that in QCD fluid, it depends on the coefficients of both chiral and gravitational anomalies.

Speaker: Ali Davody (Institute of Theoretical Physics, Regensburg University, 93040 Regensburg, Germany)

Slides Pisa-2017-davody.pdf Pisa-2017-davody.pptx

Video http://mediateca.unipi.it/category/video/A-Davody-Collective-Excitations-in-QCD-Plasma/e3f214f17c9a4dc091011937c9c4e4bc/108

10:55 Coffee break

57 Finite temperature gluon spectral functions from twisted mass lattice QCD

I will report on an attempt to fix the gluon spectral functions at finite temperature in Landau gauge. Our study used a novel Bayesiann approach for the extraction of non-positive definite spectral functions. The spectral functions are extracted at three different lattice spacing. For each of them, a scan of temperatures around the crossover transition is carried out. There are indications for the existence of a well defined quasi-particle peak. Due to a relatively small number of imaginary frequencies available, we focus on the momentum and temperature dependence of the peak position while the width is beyond our present possibilities. The dispersion relation reveals different in-medium masses for longitudinal and transversal gluons at high temperatures, in agreement with weak coupling expectations.

Speaker: Ernst-Michael Ilgenfritz (JINR, BLTP)

Slides XQCD-2017-ilgenfritz.pdf

Video http://mediateca.unipi.it/category/video/M-Ilgenfritz-Finite-temperature-gluon-spectral-functions-from-twisted-mass-lattice-QCD/f896b64fef8dc323443d3deaac2761dd/108

11:25 Session Chair: Gert Aarts

58 Momentum anisotropy effects for quarkonium in a weakly-coupled quark-gluon plasma below the melting temperature

In the early stages of heavy-ion collisions, the hot QCD matter expands more longitudinally than transversely. This imbalance causes the system to become rapidly colder in the longitudinal direction and a local momentum anisotropy appears. In this talk we study the heavy-quarkonium spectrum in the presence of a small plasma anisotropy. We work in the framework of potential non-relativistic QCD at finite temperature. We inspect arrangements of non-relativistic and thermal scales complementary to those considered in the literature. In particular, we consider temperatures larger and Debye masses smaller than the binding energy, which is a temperature range relevant for presently running LHC experiments. In this setting we compute the leading thermal corrections to the binding energy and the thermal width induced by quarkonium gluo-dissociation

Speaker: Simone Biondini (Albert Einstein Center, Institute for Theoretical Physics - Uni Bern)

Slides Biondini.pdf

Video http://mediateca.unipi.it/category/video/S-Biondini-Momentum-anisotropy-effects-for-quarkonium-in-a-weaklycoupled-quark-gluon-plasma-below-the-melting-temperature/a0261cd087512945c9dd7a7575542664/108

59 Thermal Sommerfeld effect for P-wave quarkonium in lattice NRQCD

Annihilation of heavy particles in thermal environment (e.g., quarkonium decay in Quark-Gluon Plasma and heavy dark matter particle annihilation in early universe) is influenced by long distance non-perturbative effect because multple exchanges of light particle between slow-moving incoming heavy particle is possible. Previously, we found that the existence of bound states in thermal QCD can lead to large enhancement for S-wave pair annihilation of heavy quark and heavy anti-quark in QGP by lattice NRQCD measurement. We continue our study of thermally averaged Sommerfeld factor, and here we report on our recent lattice calculation of the Sommerfeld factor for P-wave channel.

Speaker: Seyong Kim (Sejong University)

Slides xQCD2017skim.pdf

Video http://mediateca.unipi.it/category/video/S-Kim-Thermal-Sommerfeld-effect-for-P-wave-quarkonium-in-lattice-NRQCD/e9383007d5217f08fc150955471979c3/108

60 Quarkonium spectral functions at finite temperature on large quenched lattices and towards the continuum limit

We present our recent studies about quarkonium spectral functions in lattice QCD at finite temperature. We performed lattice QCD simulations with the quenched approximation on very large and fine lattices. We calculated quarkonium correlation functions in a quark mass range between charm and bottom quarks at temperatures between 0.75$T_c$ and 2.2$T_c$. Using the correlation functions for our finest lattice with lattice spacing $a^-1$ = 22 GeV, we reconstructed spectral functions, where we employed the conventional maximum entropy method as well as the stochastic methods to check systematic uncertainties. In this talk we discuss dissociation temperatures for charmonia and bottomonia and also show some estimates of the heavy quark diffusion coefficient. In addition, we show some results on a continuum extrapolation of the quarkonium correlation functions, which has been done for the first time, towards the continuum limit of the spectral functions.

Speaker: Hiroshi Ohno (University of Tsukuba)

Slides XQCD2017_Ohno.pdf

Video http://mediateca.unipi.it/category/video/H-Ohno-Quarkonium-spectral-functions-at-finite-temperature-on-large-quenched-lattices-and-towards-the-continuum-limit/a8e782070bb21ec9a6bb9feb56664aa1/108

13:05 Lunch Break

61 Constraints on extreme density matter from GW observa- tion of neutron star mergers

Neutron stars in binary systems are among the strongest sources of gravitational waves and among the main targets for ground-based gravitational-wave interferometers Advanced LIGO and Virgo. The observation of these events in the gravitational-wave window can provide us with unique information on neutron stars' masses, radii, and spins, including the possibility to set strong constraints on the unknown equation of state of matter at supranuclear densities. A crucial and necessary step for the development of gravitational-wave astronomy with these sources is the precise knowledge of the dynamics of the sources and of the emitted waveforms. I will talk about recent developments on the modeling of gravitational waves from neutron star mergers using numerical simulations in general relativity, focusing on how these observations can constrain extreme density matter.

Speaker: Sebastiano Bernuzzi (PR)

Slides Bernuzzi.pdf

Video http://mediateca.unipi.it/category/video/S-Bernuzzi-Constraints-on-extreme-density-matter-from-GW-observation-of-neutron-star-mergers/6bdab7a7efa197b413317d80326b4c4a/108

14:30 Session Chair: Ignazio Bombaci

62 New dynamic critical phenomena in nuclear and quark superfluids

We study the static and dynamic critical phenomena near the possible high-density QCD critical point in the superfluid phase of nuclear and quark matter. In particular, we find that its dynamic universality class is different from those studied in QCD and condensed matter systems so far. We argue that this novelty stems from the interplay between the chiral criticality and the presence of the superfluid phonon---a feature specific for high-density QCD critical point.

Speaker: Noriyuki Sogabe (Keio University)

Slides Sogabe.pdf

Video http://mediateca.unipi.it/category/video/N-Sogabe-New-dynamic-critical-phenomena-in-nuclear-and-quark-superfluids/380b2aa516378c7bc4a1e12dc065968c/108

63 Neutral pion matter in strong magnetic fields

The ground state of QCD in sufficiently strong external magnetic fields and at moderate baryon chemical potential is a chiral soliton lattice (CSL) of neutral pions. This is a model-independent result based on low-energy effective field theory, and the magnetic fields and baryon densities required may occur in the cores of neutron stars. The spectrum of excitations above the CSL ground state contains a soft, nonrelativistic mode that gives an anomalous contribution to pressure, scaling with temperature and magnetic field as $T^{5/2}B^{3/2}$. Finally, I will argue that in stronger but still achievable magnetic fields, the neutral pion CSL background may catalyze Bose-Einstein condensation of charged pions.

Speaker: Tomas Brauner (University of Stavanger)

Slides talk_XQCD_Brauner.pdf

Video http://mediateca.unipi.it/category/video/T-Brauner-Neutral-pion-matter-in-strong-magnetic-fields/5201bd2c5d295e4186cb59e07a3f2d90/108

16:00 Coffee Break

16:30 Session Chair: Atsushi Nakamura

64 Temperature dependence of bulk viscosity in SU(3)-gluodynamics

This report is devoted to the study of temperature dependence of bulk viscosity in SU(3)-gluodynamics. To calculate bulk viscosity we measured the correlation function of the trace anomaly for a set of temperatures in the region T/T_c \in (0.9, 1.5). We used multilevel algorithm which allowed us to improve the accuracy of the data. To extract the values of bulk viscosity we used two approaches: fitting of the data by physically motivated ansatz and the Backus-Gilbert method.

Speaker: Victor Braguta (ITEP)

Slides report_braguta.pdf

Video http://mediateca.unipi.it/category/video/V-Braguta-Temperature-dependence-of-bulk-viscosity-in-SU3-gluodynamics/4ab30fb1dd63da47f46832be30001a6e/108

65 Energy-momentum tensor correlation function in Nf=2+1 full QCD at finite temperature

We measure correlation functions of energy-momentum tensor in Nf=2+1 full QCD at finite temperature by applying the gradient flow method both to the gauge and quark fields. Our main interest is to study the conservation law of the energy-momentum tensor and to extract thermodynamical quantities from the correlation function. We adopt a fine lattice spacing a=0.07 (fm) and cover a wide range of temperature region $174\le T\le697$ MeV. The ud quark mass is rather heavy with $m_{\pi}/m_{\rho}\simeq0.63$ while the s quark mass is set to approximately its physical value.

Speaker: Yusuke Taniguchi (University of Tsukuba)

Slides tanigchi_20170628_XQCD2017_handout.pdf

Video http://mediateca.unipi.it/category/video/Y-Taniguchi-Energy-momentum-tensor-correlation-function-in-Nf21-full-QCD-at-finite-temperature/2bcc561c64b292ef9fc95b7b233d118d/108

66 Thermodynamics of QCD at physical point with (2+1)-flavors of improved Wilson quarks using gradient flow

The energy-momentum tensor and the chiral condensate are studied in (2+1)-flavor QCD with improved Wilson quarks at the physical point, applying the method of Makino and Suzuki based on the gradient flow. Following a strategy of our previous study at a heavier quark mass, we adopt a nonperturbatively O(a)-improved Wilson quark action and the renormalization group-improved Iwasaki gauge action and perform finite-temperature simulations in the range $T \simeq 155$-544 MeV ($N_t = 4$-14 including odd numbers) at $a \simeq 0.09$ fm based on the fixed-scale approach using zero-temperature physical point configurations generated by the PACS-CS Collaboration. We present preliminary results on the equation of state and the chiral condensate obtained so far.

Speaker: Kazuyuki Kanaya (University of Tsukuba)

Slides 170628_kanaya_xqcd.pdf

Video http://mediateca.unipi.it/category/video/K-Kanaya-Thermodynamics-of-QCD-at-physical-point-with-21-flavors-of-improved-Wilson-quarks-using-gradient-flow/e4a7e0f7b697c1d83aa8b69fcb8d573d/108

18:00 Pisa Raining Hour