955 research outputs found
Gluon Distributions and Color Charge Correlations in a Saturated Light-cone Wavefunction
We describe the light-cone wavefunction in the saturation regime in terms of
the density of gluons per unit of transverse phase space, the occupation
number, and in terms of the color charge correlator. The simple McLerran-
Venugopalan model gives what are claimed to be general results for the phase
space gluon density, but it does not well describe the general case for the
charge correlator. We derive the general momentum dependence and rapidity
dependence of the color charge correlator which exhibits strong color
shielding. A simplel physical picture which leads to these general results is
described.Comment: 17 pages, Latex, 7 figure
Isotropization and Thermalization in Heavy Ion Collisions
We argue that isotropization and, consequently, thermalization of the system
of gluons and quarks produced in an ultrarelativistic heavy ion collision does
not follow from Feynman diagram analysis to any order in the coupling constant.
We conclude that the apparent thermalization of quarks and gluons, leading to
success of perfect fluid hydrodynamics in describing heavy ion collisions at
RHIC, can only be attributed to the non-perturbative QCD effects not captured
by Feynman diagrams.
We proceed by modeling these non-pertrubative thermalization effects using
viscous hydrodynamics. We point out that matching Color Glass Condensate inital
conditions with viscous hydrodynamics leads to continuous evolution of all
components of energy-momentum tensor and, unlike the case of ideal
hydrodynamics, does not give a discontinuity in the longitudinal pressure. An
important consequence of such a matching is a relationship between the
thermalization time and shear viscosity: we observe that small viscosity leads
to short thermalization time.Comment: Includes new results. 8 pages, 1 figure, based on proceedings
contributions at "Quark Matter 2005" conference in Budapest, Hungary, August
4-9, 2005 and at the workshop on "Quark-Gluon Plasma Thermalization" in
Vienna, Austria, August 10-12, 200
Correlation Functions and Cumulants in Elliptic Flow Analysis
We consider various methods of flow analysis in heavy ion collisions and
compare experimental data on corresponding observables to the predictions of
our saturation model proposed earlier. We demonstrate that, due to the nature
of the standard flow analysis, azimuthal distribution of particles with respect
to reaction plane determined from the second order harmonics should always be
proportional to independent of the physical origin of
particle correlations (flow or non-flow). The amplitude of this distribution is
always physical and proportional to . Two-particle correlations analysis
is therefore a more reliable way of extracting the shape of physical azimuthal
anisotropy. We demonstrate that two-particle correlation functions generated in
our minijet model of particle production are in good agreement with the data
reported by PHENIX. We discuss the role of non-flow correlations in the
cumulant flow analysis and demonstrate using a simple example that if the flow
is weak, higher order cumulants analysis does not significantly reduce the
contribution of non-flow correlations to elliptic flow observable in RHIC
data.Comment: 18 pages, 1 figure; v2: IR safety and v2(pT) from 4th order cumulant
discussions adde
Collinear Singularities and Running Coupling Corrections to Gluon Production in CGC
We analyze the structure of running coupling corrections to the gluon
production cross section in the projectile-nucleus collisions calculated in the
Color Glass Condensate (CGC) framework. We argue that for the gluon production
cross section (and for gluon transverse momentum spectra and multiplicity) the
inclusion of running coupling corrections brings in collinear singularities due
to final state splittings completely unaffected by CGC resummations. Hence,
despite the saturation/CGC dynamics, the gluon production cross section is not
infrared-safe. As usual, regularizing the singularities requires an infrared
cutoff Lambda_coll that defines a resolution scale for gluons. We specifically
show that the cutoff enters the gluon production cross section in the argument
of the strong coupling constant alpha_s(Lambda_coll^2). We argue that for
hadron production calculations one should be able to absorb the collinear
divergence into a fragmentation function. The singular collinear terms in the
gluon production cross section are shown not to contribute to the energy
density of the produced matter, which is indeed an infrared-finite quantity.Comment: 35 pages, 16 figures; v2: minor corrections, references and figures
adde
Toward Equilibration in the Early Stages After a High Energy Heavy Ion Collision
The early stages in the evolution of the gluons produced in the central
region of a head-on high-energy heavy ion collision is studied. An equation is
given for the rate of change of transverse momentum into longitudinal momentum
where the longitudinal direction is along the collision axis. We are able to
follow the system up to the time where equilibration seems to be setting in,
but we are unable to actually follow the system as it reaches equilibrium.Comment: 18 pages, Late
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