Reheating after Supercooling in the Chiral Phase Transition

The chirally symmetric quark-gluon plasma produced in energetic heavy-ion collisions is predicted to supercool at the late stages of its evolution. The thermal energy is then transformed into the potential energy associated with an energetically unfavorable field configuration. Since the system is in an unstable state it eventually rolls down to the true minimum of the effective chiral potential. When this motion is described in terms of the sigma-model, we find that the energy of the coherent $\sigma-$field is very efficiently converted into pionic excitations due to anharmonic oscillations around this minimum. The system is expected to partially thermalize before its disintegration.Comment: Final version accepted for publication, 8 pages, REVTe

Momentum Broadening in an Anisotropic Plasma

The rates governing momentum broadening in a quark-gluon plasma with a momentum anisotropy are calculated to leading-log order for a heavy quark using kinetic theory. It is shown how the problematic singularity for these rates at leading-oder is lifted by next-to-leading order gluon self-energy corrections to give a finite contribution to the leading-log result. The resulting rates are shown to lead to larger momentum broadening along the beam axis than in the transverse plane, which is consistent with recent STAR results. This might indicate that the quark-gluon-plasma at RHIC is not in equilibrium.Comment: 14 pages, 2 figures, uses revtex4; see source for numerics; v2: typos corrected, note added in appendix, matches published versio

Photons from anisotropic Quark-Gluon-Plasma

We calculate medium photons due to Compton and annihilation processes in an anisotropic media. The effects of time-dependent momentum-space anisotropy of {\em Quark-Gluon-Plasma} (QGP) on the medium photon production are discussed. Such an anisotropy can results from the initial rapid longitudinal expansion of the matter, created in relativistic heavy ion collisions. A phenomenological model for the time-dependence of the parton hard momentum scale, $p_{hard}(\tau)$, and anisotropy parameter, $\xi(\tau)$, has been used to describe the plasma space-time evolution. We find significant dependency of photon yield on the isotropization time ($\tau_{iso}$). It is shown that the introduction of early time momentum-space anisotropy can enhance the photon production by a factor of $10 (1.5)$ (in the central rapidity region) for {\em free streaming} ({\em collisionally-broadened}) {\em interpolating} model if we assume fixed initial condition. On the other hand, enforcing the fixed final multiplicity significantly reduces the enhancement of medium photon production.Comment: 15 pages, 19 figures, few refs added, one new paragraph is added in introduction, published in Physical Rev.

Overview of event-by-event analysis of high energy nuclear collisions

The event-by-event analysis of high energy nuclear collisions aims at revealing the richness of the underlying event structures and provide unique measures of dynamical fluctuations associated with QGP phase transition. The major challenge in these studies is to separate the dynamical fluctuations from the many other sources which contribute to the measured values. We present the fluctuations in terms of event multiplicity, mean transverse momentum, elliptic flow, source sizes, particle ratios and net charge distributions. In addition, we discuss the effect of long range correlations, disoriented chiral condensates and presence of jets. A brief review of various probes used for fluctuation studies and available experimental results are presented.Comment: Invited talk at the "XIth International Workshop on Correlation and Fluctuation in Multiparticle Production", Nov 21-24, 2006, Hangzhou, China (19 pages

Elliptic Flow Analysis at RHIC: Fluctuations vs. Non-Flow Effects

The cumulant method is applied to study elliptic flow ($v_2$) in Au+Au collisions at $\sqrt{s}=200$AGeV, with the UrQMD model. In this approach, the true event plane is known and both the non-flow effects and event-by-event spatial ($\epsilon$) and $v_2$ fluctuations exist. Qualitatively, the hierarchy of $v_2$'s from two, four and six-particle cumulants is consistent with the STAR data, however, the magnitude of $v_2$ in the UrQMD model is only 60% of the data. We find that the four and six-particle cumulants are good measures of the real elliptic flow over a wide range of centralities except for the most central and very peripheral events. There the cumulant method is affected by the $v_2$ fluctuations. In mid-central collisions, the four and six-particle cumulants are shown to give a good estimation of the true differential $v_2$, especially at large transverse momentum, where the two-particle cumulant method is heavily affected by the non-flow effects.Comment: 7 pages, 7 figures, revtex 4; The discussion on the non-flow effects is extended, a new figure (Fig.3) on v2-eccentricity correlation is added, accepted for publication in Phys. Rev.

Hydrodynamic Models for Heavy Ion Collisions

Application of hydrodynamics for modeling of heavy-ion collisions is reviewed. We consider several physical observables that can be calculated in this approach and compare them to the experimental measurements.Comment: 42 pages, 15 figures, An invited review for Nov. 2006 edition of Annual Review of Nuclear and Particle Physic

Strangeness Chemical Equilibration in QGP at RHIC and LHC

We study, in the dynamically evolving QGP fireball formed in relativistic heavy ion collisions at RHIC and LHC, the growth of strangeness yield toward and beyond the chemical equilibrium. We account for the contribution of the direct strangeness production and evaluate the thermal-QCD strangeness production mechanisms. The specific yield of strangeness per entropy, s/S, is the primary target variable. We explore the effect of collision impact parameter, i.e., fireball size, on kinetic strangeness chemical equilibration in QGP. Insights gained in study the RHIC data with regard to the dynamics of the fireball are applied to the study strangeness production at the LHC. We use these results and consider the strange hadron relative particle yields at RHIC and LHC in a systematic fashion. We consider both the dependence on s/S and directly participant number dependence.Comment: 21 pages, 13 figures, PRC in press. Strangeness production recomputed with K-factor K=1.7. Particle yields recomputed with SHARE 2.

Jet broadening in unstable non-Abelian plasmas

We perform numerical simulations of the QCD Boltzmann-Vlasov equation including both hard elastic particle collisions and soft interactions mediated by classical Yang-Mills fields. We provide an estimate of the coupling of jets to a hot plasma which is independent of infrared cutoffs. For weakly-coupled anisotropic plasmas the local rotational symmetry in momentum space is broken. The fields develop unstable modes, forming configurations where B_t>E_t and E_z>B_z. This provides a possible explanation for the experimental observation that high-energy jets traversing the plasma perpendicular to the beam axis experience much stronger broadening in rapidity than in azimuth.Comment: 6 pages, 7 figures, version accepted for publication in Phys.Rev.C, typos fixed, more detailed discussion of q-ha

Jet energy loss in the quark-gluon plasma by stream instabilities

We study the evolution of the plasma instabilities induced by two jets of particles propagating in opposite directions and crossing a thermally equilibrated non-Abelian plasma. In order to simplify the analysis we assume that the two jets of partons can be described with uniform distribution functions in coordinate space and by Gaussian distribution functions in momentum space. We find that while crossing the quark-gluon plasma, the jets of particles excite unstable chromomagnetic and chromoelectric modes. These fields interact with the particles (or hard modes) of the plasma inducing the production of currents; thus, the energy lost by the jets is absorbed by both the gauge fields and the hard modes of the plasma. We compare the outcome of the numerical simulations with the analytical calculation performed assuming that the jets of particles can be described by a tsunami-like distribution function. We find qualitative and semi-quantitative agreement between the results obtained with the two methods.Comment: 10 pages, 3 figure

Elliptic flow fluctuations in 200 GeV Au+Au collisions at RHIC

We present first results on event-by-event elliptic flow fluctuations in Au+Au collisions at 200 GeV obtained with the PHOBOS detector. Over the measured range in centrality, large relative fluctuations of 40--50% are found. The elliptic flow fluctuations are well described as being proportional to fluctuations in the shape of the initial collision region, as estimated event-by-event with the participant eccentricity using Glauber Monte Carlo.Comment: 5 pages, 2 figures, QM 2006 proceedings; v2: Corrected a few typo