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Register a new userBulk viscosity of strongly interacting matter in the relaxation time approximation
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This paper presents how thermal mean field effects are incorporated consistently in the hydrodynamical modelling of heavy-ion collisions. The nonequilibrium correction to the distribution function resulting from a temperature-dependent mass is obtained in a procedure which automatically satisfies the Landau matching condition and is thermodynamically consistent. The physics of the bulk viscosity is studied here for Boltzmann and Bose-Einstein gases within the Chapman-Enskog and 14-moment approaches in the relaxation time approximation. Constant and temperature-dependent masses are considered in turn. It is shown that, in the small mass limit, both methods lead to the same value of the ratio of the bulk viscosity over its relaxation time. The inclusion of a temperature-dependent mass leads to the emergence of the $beta_lambda$-function in that ratio, and it is of the expected parametric form for the Boltzmann gas, while for the Bose-Einstein case it is affected by the infrared cut-off. This suggests that the relaxation time approximation may be too crude to obtain a reliable form of $zeta/tau_R$ for gases obeying Bose-Einstein statistics.
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The microscopic formulae of the bulk viscosity $zeta $ and the corresponding relaxation time $tau_{Pi}$ in causal dissipative relativistic fluid dynamics are derived by using the projection operator method. In applying these formulae to the pionic fluid, we find that the renormalizable energy-momentum tensor should be employed to obtain consistent results. In the leading order approximation in the chiral perturbation theory, the relaxation time is enhanced near the QCD phase transition and $tau_{Pi}$ and $zeta $ are related as $tau_{Pi}=zeta /[beta {(1/3-c_{s}^{2})(epsilon +P)-2(epsilon -3P)/9}]$, where $epsilon $, $P$ and $c_{s}$ are the energy density, pressure and velocity of sound, respectively. The predicted $zeta $ and $% tau_{Pi}$ should satisfy the so-called causality condition. We compare our result with the results of the kinetic calculation by Israel and Stewart and the string theory, and confirm that all the three approaches are consistent with the causality condition.
This talk is devoted to review the field of strangeness production in (ultra-)relativistic heavy ion collisions within our present theoretical understanding. Historically there have been (at least) three major ideas for the interest in the production of strange hadronic particles: (1) mass modification of the kaons in a (baryon-)dense environment; (2) (early) K+ - production probes the nuclear equation of state (EoS); (3) enhanced strangeness production especially in the (multi-)strange (anti-)baryon channels as a signal of quark gluon plasma (QGP) formation. As a guideline for the discussion I employ the extensive experience with microscopic hadronic transport models. In addition, I elaborate on the recent idea of antihyperon production solely by means of multi-mesonic fusion-type reactions.
The thermodynamic geometry formalism is applied to strongly interacting matter to estimate the deconfinement temperature. The curved thermodynamic metric for Quantum Chromodynamics (QCD) is evaluated on the basis of lattice data, whereas the hadron resonance gas model is used for the hadronic sector. Since the deconfinement transition is a crossover, the geometric criterion used to define the mbox{(pseudo-)critical} temperature, as a function of the baryonchemical potential $mu_B$, is $R(T,mu_B)=0$, where $R$ is the scalar curvature. The (pseudo-)critical temperature, $T_c$, resulting from QCD thermodynamic geometry is in good agreement with lattice and phenomenological freeze-out temperature estimates. The crossing temperature, $T_h$, evaluated by the hadron resonance gas, which suffers of some model dependence, is larger than $T_c$ (about $20%$) signaling remnants of confinement above the transition.
In this study we investigate the dynamics of strongly interacting parton-hadron matter by calculating the centrality dependence of direct photons produced in Au+Au collisions at $sqrt{s_{NN}}=200$ GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. As sources for direct photons, we incorporate the interactions of quarks and gluons as well as hadronic interactions ($pi+pitorho+gamma$, $rho+pitopi+gamma$, meson-meson bremsstrahlung $m+mto m+m+gamma$, meson-baryon bremsstrahlung $m+Bto m+B+gamma$), the decays of $phi$ and $a_1$ mesons and the photons produced in the initial hard collisions (pQCD). Our calculations suggest that the channel decomposition of the observed spectrum changes with centrality with an increasing (dominant) contribution of hadronic sources for more peripheral reactions. Furthermore, the thermal photon yield is found to scale roughly with the number of participant nucleons as $N_{part}^alpha$ with $alpha approx$ 1.5, whereas the partonic contribution scales with an exponent $alpha_p approx1.75$. Additionally, we provide predictions for the centrality dependence of the direct photon elliptic flow $v_2(p_T)$. The direct photon $v_2$ is seen to be larger in peripheral collisions compared to the most central ones since the photons from the hot deconfined matter in the early stages of the collision carry a much smaller elliptic flow than those from the final hadronic interactions.
We study the kinetic and chemical equilibration in infinite parton-hadron matter within the Parton-Hadron-String Dynamics transport approach, which is based on a dynamical quasiparticle model for partons matched to reproduce lattice-QCD results - including the partonic equation of state - in thermodynamic equilibrium. The infinite matter is simulated within a cubic box with periodic boundary conditions initialized at different baryon density (or chemical potential) and energy density. The transition from initially pure partonic matter to hadronic degrees of freedom (or vice versa) occurs dynamically by interactions. Different thermodynamical distributions of the strongly-interacting quark-gluon plasma (sQGP) are addressed and discussed.
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