Scope of the meeting

Hot and dense nuclear matter plays an important role in the quark-hadron transition shortly after the big bang, in the element production in stars and the interior of neutron stars. The properties of such matter under extreme condi-tions can be studied in relativistic heavy-ion collisions. Several countries operate facilities or have plans to build new accelerator systems, such as FAIR at Darmstadt and NICA at Dubna, to investigate hot and dense nuclear matter in heavy-ion collisions. Current experiments are conducted at GSI in Darmstadt, the RHIC (Relativistic Heavy Ion Collider) in Brookhaven and, since the end of last year, at the LHC (Large Hadron Collider) in the ALICE experiment at CERN in Geneva. The primary aim of the experiments at the high-energy frontier is to study a new state of matter, the 'quark-gluon-plasma' (QGP) and to infer its equation of state and transport properties. The interpretation of the data involves substantial theoretical efforts in non-perturbative and perturbative QCD, as well as in large-scale hydrodynamical and transport simulations.

Topics to be discussed:

Physics case:

1. The equation of state and transport properties of strong-interaction matter

As matter gets heated and compressed it ultimately undergoes a transition from confined hadrons to quasi-free quarks and gluons. The current under-standing is, that the resulting change in degrees of freedom leads to a smooth cross-over transition at small quark chemical potential in which spontaneously broken chiral symmetry is restored and quarks and gluons are liberated. Although detailed lattice studies of the equation of state (EoS) in this regime seem to converge to realistic answers, the computation of transport properties of the QGP from first principles remains elusive. At high chemical potential, it is expected that the QCD phase diagram becomes very rich, featuring phases familiar from condensed matter physics. Here the hadron-to-quark transition is much less understood. Because of the fermion sign problem, lattice simulations fail at present to make ab-initio predictions in this region of the phase diagram and one has to resort to QCD inspired models.

2. Simulation of heavy-ion reactions

To interpret heavy-ion data, complex numerical simulations of the collision dynamics are necessary. These include relativistic hydrodynamics and sophisticated transport theory. Of particular interest are hydrodynamical descriptions with non-ideal fluid components in the energy-momentum tensor. At present, a detailed understanding of the transport coefficients of strong-interaction matter is still lacking. Other uncertainties are fluctuations in the initial conditions of the hydro-evolution and the consequences on collective flow variables. The latter can be measured very precisely in the ALICE experiment.

3. Hot and dense matter at small baryo-chemical potential

eavy-ion experiments at very high center-of-mass energies, presently conducted conducted at RHIC and most recently also at the LHC probe strong-interaction matter at high temperatures but small net-baryon densities. The results are thus of direct relevance for the state of the early universe after a few microseconds. Here questions of the high-temperature EoS, the propagation of partons through hot and dense quark -gluon matter, charm production and the radiation of photons and di-leptons are at the focus to research program. Another field of intense research is the characterization of the very early, far-from-equilibrium conditions of the collisions and the rapid thermalization that is required for a hydrodynamical description of the fireball evolution.

4. Reaching high baryon densities

The RHIC low-energy run and planned heavy-ion experiments at NICA and FAIR are designed to explore regions of high baryo-chemical potential in the QCD phase diagram. Here a chiral critical endpoint, the occurrence of inhomo-geneous chiral phases and color superconducting states of quark matter are expected. One of the interesting questions is the interplay between the restoration of spontaneously broken chiral symmetry and the transition to deconfined quark matter, which may lead the a state of so-called 'quarkyonic matter'. It remains a challenging task to come up with signals in the collision of heavy-ions for these exciting predictions from theory.

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