Open issues in heavy-ion theory and hot QCD

• Urs Wiedemann (CERN PH TH)

Room: High Energy Building, Seminar Room Over the last year, the notion “QGP-like phenomena in small systems” has been used to describe a set of newly established soft physics phenomena in proton-proton and proton-nucleus collisions, that share important commonalities with classical signatures of collectivity in nucleus-nucleus collisions. These include flow-like long-range rapidity correlations and higher-order cumulants of charged particle distributions in p-p and p-A, as well as characteristic multiplicity-dependences of the hadrochemical compositions. These data represent both, a formidable novel challenge to the “standard model of heavy ion collisions” according to which everything sufficiently soft flows, and a formidable novel opportunity for understanding the dynamical mechanisms that underlie the observed signatures of collectivity. I shall review the theoretical concepts and toolbox (fluid dynamic simulations, transport, parton saturation, underlying event models developed for pp collisions, calculations related to multi-parton interactions, color coherence etc etc) that is at our disposal for analyzing these phenomena, I shall show some own exploratory calculations, and I shall share my view of what the technical challenges are and how progress can be made in the coming years.

State of the art of heavy-ion measurements

• Enrico Scomparin (TO)

Room: High Energy Building, Seminar Room Experiments with ultrarelativistic heavy ions started thirty years ago, first at fixed target facilities (AGS, SPS) and then at colliders (RHIC, LHC). With the advent of the LHC, the range of observables and the quality of the results have significantly increased, leading to an accurate characterization of the properties of the Quark-Gluon Plasma. In this talk, after a general introduction I will review recent experimental results obtained at the LHC and discuss their implications also in relation to previous achievements at lower energy experiments. Finally I will discuss the future prospects for experimental work at existing and forthcoming accelerators.

High-gluon densities and the early stages of nucleus-nucleus collisions

• Jean-Paul Blaizot

Room: High Energy Building, Seminar Room Most of the particles that are produced in a nucleus-nucleus collision at high energy involve partons that carry a very small fraction of the momentum of the colliding nucleons. These "small-x" partons, mostly gluons, form high density systems in which new phenomena are expected to occur, such as "gluon saturation". This talk will present a pedagogical introduction to the various facets of this phenomenon, and review the progress that has been achieved in its study over the last two decades. The possible role of high gluon densities in the early stages of heavy ion collisions will be also discussed.

Coherence phenomena in high-energy nuclear collisions: from initial to final state

• Néstor Armesto (Universidade de Santiago de Compostela)

Room: High Energy Building, Seminar Room In this talk I will review how coherence phenomena affect most aspects of the description of high-energy hadronic collisions. Concerning their initial stage, I will consider correlations in the wave function of the colliding hadrons, nucleons or nuclei, that are reflected in the final state. I will argue that such correlations offer an explanation for the description of observed phenomena in proton-proton and proton-nucleus collisions at the LHC such as the ridge, i.e. two-particle correlations along several rapidity units collimated in azimuthal angle, so they are enhanced at zero and 180 degrees. Then, turning to the final stage of the collisions, I will address how coherence in the QCD branching process is involved in the phenomena of energy loss of fast partons traversing the partonic medium produced in heavy-ion collisions, used as hard probes of the medium.

Challenges in experimental jet physics in heavy ion collisions

• Leticia Cunqueiro Mendez

Room: High Energy Building, Seminar Room In this talk I will review recent experimental progress in jet physics in heavy ion collisions, from first generation of measurements like jet cross sections to the current focus of the field, which is jet substructure. Experimental techniques, observables and challenges in the interface with the theory will be discussed.

Thermodynamics of QCD on the lattice: temperature, magnetic fields and chemical potentials

• Massimo D'Elia (PI)

Room: High Energy Building, Seminar Room In this talk I will review the progress achieved by lattice QCD simulations on our understanding of the equilibrium properties of strongly interacting matter under extreme conditions. I will focus on the QCD phase diagram for high T and small chemical potentials, considering also the effects of strong magnetic background fields on the hot medium.

Constraining the equation of state of nuclear matter with gravitational-wave observations

• Valeria Ferrari (ROMA1)

Room: High Energy Building, Seminar Room Gravitational waves emitted by neutron stars in a variety of astrophysical processes carry information on the equation of state (EoS) of nuclear matter prevailing in their inner core. For instance, the EoS imprint is encoded in the frequencies at which a neutron star oscillates emitting gravitational waves, or in the tidal deformability parameter which affects the waveform emitted during the latest phases of a binary coalescence. I will discuss our current understanding of the most interesting processes, and the challenges that need to be met if we want to use gravitational waves to probe neutron star physics.

Pulsar observations and constraints on the equation of state of nuclear matter in neutron stars

• Delphine Perrodin

Room: High Energy Building, Seminar Room Compact stars such as neutron star interiors are ideal laboratories for studying nuclear matter at extremely high densities.  Pulsars are highly-magnetized, fast-rotating neutron stars that emit beams of electromagnetic radiation (ranging from radio to gamma rays) which we observe as periodic "pulses" with extraordinary regularity. Pulsar timing consists in the regular monitoring of the times-of-arrival of these pulses, and allows us to determine many pulsar properties with high precision, including orbital properties of pulsars in binary systems. In particular, radio observations of pulsars have led to precise measurements of neutron star masses, while X-ray observations of pulsars have helped determine neutron star radii. Since each proposed equation of state (EOS) of superdense nuclear matter in neutron stars leads to a unique neutron star mass-to-radius relation, the EOS can be constrained by neutron star mass and radius measurements from pulsar observations. We give an introduction to pulsar timing, constraints on the EOS of neutron stars and on theories of gravity. We also give an introduction on the planned Square Kilometre Array, which will allow us to improve the constraint on the EOS of neutron stars by one order of magnitude.

The nuclear matter equation of state: from nuclei to neutron stars

• Giuseppina Fiorella Burgio (CT)

Room: High Energy Building, Seminar Room I'll present a general overview of the current theoretical models adopted for the derivation of the equation of state (EoS) for nuclear matter over a large density range, i.e. from finite nuclei to the inner core of neutron stars (NS), spanning about 14 orders of magnitude. In particular, I'll compare a set of EoS derived within microscopic many-body approaches, and study their predictions as far as phenomenological data on nuclei from heavy ion collisions, and astrophysical observations on neutron stars are concerned. It is found that all the data, taken together, put strong constraints not easy to be fulfilled accurately. Besides a conventional description where nucleons and leptons are taken into account, I'll discuss the appearance of strange baryonic matter in NS, as well as the consequences of a hadron-quark phase transition. A survey of the currently existing quark matter (QM) models shows that the predicted maximum mass of NS is not larger than 2 solar masses, and that the observation of a more massive NS would require additional repulsion, thus giving access to the QM equation of state.