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Register a new userComments on the chemical and kinetic equilibration in heavy ion collisions
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E.Shuryak
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I argue that perturbative scattering of quarks and gluons are incompatible with lattice and heavy ion data on QGP properties. The non-perturbative mechanisms for quasiparticle rescattering and quark production are briefly discussed, as well as experiments needed to measure matter anisotropy and quark density at early stages of the collisions.
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Kinetic equilibration of the matter and baryon densities attained in central region of colliding Au+Au nuclei in the energy range of $sqrt{s_{NN}}=$ 3.3--39 GeV are examined within the model of the three-fluid dynamics. It is found that the kinetic equilibration is faster at higher collision energies: the equilibration time (in the c.m. frame of colliding nuclei) rises from $sim$5 fm/c at $sqrt{s_{NN}}=$ 3.3 GeV to $sim$1 fm/c at 39 GeV. The chemical equilibration, and thus thermalization, takes longer. We argue that the presented time evolution of the net-baryon and energy densities in the central region is a necessary prerequisite of proper reproduction of bulk observables in midrapidity. We suggest that for informative comparison of predictions of different models it is useful to calculate an invariant 4-volume ($V_4$), where the proper density the equilibrated matter exceeds certain value. The advantage of this 4-volume is that it does not depend on specific choice of the 3-volume in different studies and takes into account the lifetime of the high-density region, which also matters. The 4-volume $V_4=$ 100 fm$^4$/c is chosen to compare the baryon densities attainable at different different energies. It is found that the highest proper baryon density increases with the collision energy rise, from $n_B/n_0approx$ 4 at 3.3 GeV to $n_B/n_0approx$ 30 at 39 GeV. These highest densities are achieved in the central region of colliding system.
We develop a combined hydro-kinetic approach which incorporates a hydrodynamical expansion of the systems formed in textit{A}+textit{A} collisions and their dynamical decoupling described by escape probabilities. The method corresponds to a generalized relaxation time ($tau_{text{rel}}$) approximation for the Boltzmann equation applied to inhomogeneous expanding systems; at small $tau_{text{rel}}$ it also allows one to catch the viscous effects in hadronic component - hadron-resonance gas. We demonstrate how the approximation of sudden freeze-out can be obtained within this dynamical picture of continuous emission and find that hypersurfaces, corresponding to a sharp freeze-out limit, are momentum dependent. The pion $m_{T}$ spectra are computed in the developed hydro-kinetic model, and compared with those obtained from ideal hydrodynamics with the Cooper-Frye isothermal prescription. Our results indicate that there does not exist a universal freeze-out temperature for pions with different momenta, and support an earlier decoupling of higher $p_{T}$ particles. By performing numerical simulations for various initial conditions and equations of state we identify several characteristic features of the bulk QCD matter evolution preferred in view of the current analysis of heavy ion collisions at RHIC energies.
The study of high energy collisions between heavy nuclei is a field unto itself, distinct from nuclear and particle physics. A defining aspect of heavy ion physics is the importance of a bulk, self-interacting system with a rich space-time substructure. I focus on the issue of timescales in heavy ion collisions, starting with proof from low-energy collisions that femtoscopy can, indeed, measure very long timescales. I then discuss the relativistic case, where detailed measurements over three orders of magnitude in energy reveal a timescale increase that might be due to a first-order phase transition. I discuss also consistency in evolution timescales as determined from traditional longitudinal sizes and a novel analysis using shape information.
We study charm production in ultra-relativistic heavy-ion collisions by using the Parton-Hadron-String Dynamics (PHSD) transport approach. The initial charm quarks are produced by the PYTHIA event generator tuned to fit the transverse momentum spectrum and rapidity distribution of charm quarks from Fixed-Order Next-to-Leading Logarithm (FONLL) calculations. The produced charm quarks scatter in the quark-gluon plasma (QGP) with the off-shell partons whose masses and widths are given by the Dynamical Quasi-Particle Model (DQPM), which reproduces the lattice QCD equation-of-state in thermal equilibrium. The relevant cross sections are calculated in a consistent way by employing the effective propagators and couplings from the DQPM. Close to the critical energy density of the phase transition, the charm quarks are hadronized into $D$ mesons through coalescence and/or fragmentation. The hadronized $D$ mesons then interact with the various hadrons in the hadronic phase with cross sections calculated in an effective lagrangian approach with heavy-quark spin symmetry. The nuclear modification factor $R_{AA}$ and the elliptic flow $v_2$ of $D^0$ mesons from PHSD are compared with the experimental data from the STAR Collaboration for Au+Au collisions at $sqrt{s_{NN}}$ =200 GeV and to the ALICE data for Pb+Pb collisions at $sqrt{s_{NN}}$ =2.76 TeV. We find that in the PHSD the energy loss of $D$ mesons at high $p_T$ can be dominantly attributed to partonic scattering while the actual shape of $R_{AA}$ versus $p_T$ reflects the heavy-quark hadronization scenario, i.e. coalescence versus fragmentation. Also the hadronic rescattering is important for the $R_{AA}$ at low $p_T$ and enhances the $D$-meson elliptic flow $v_2$.
We present a simple description of the energy density profile created in a nucleus-nucleus collision, motivated by high-energy QCD. The energy density is modeled as the sum of contributions coming from elementary collisions between localized charges and a smooth nucleus. Each of these interactions creates a sharply-peaked source of energy density falling off at large distances like $1/r^2$, corresponding to the two-dimensional Coulomb field of a point charge. Our model reproduces the one-point and two-point functions of the energy density field calculated in the framework of the color glass condensate effective theory, to leading logarithmic accuracy. We apply it to the description of eccentricity fluctuations. Unlike other existing models of initial conditions for heavy-ion collisions, it allows us to reproduce simultaneously the centrality dependence of elliptic and triangular flow.
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