300 research outputs found
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 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,
, and anisotropy parameter, , has been used to
describe the plasma space-time evolution. We find significant dependency of
photon yield on the isotropization time (). It is shown that the
introduction of early time momentum-space anisotropy can enhance the photon
production by a factor of (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 () in Au+Au
collisions at 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 () and fluctuations exist. Qualitatively, the hierarchy
of 's from two, four and six-particle cumulants is consistent with the
STAR data, however, the magnitude of 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 fluctuations. In mid-central collisions, the four and six-particle
cumulants are shown to give a good estimation of the true differential ,
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
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