Photon Production from a Quark-Gluon-Plasma at Finite Baryon Chemical Potential

We compute the photon production of a QCD plasma at leading order in the strong coupling with a finite baryon chemical potential. Our approach starts from the real time formalism of finite temperature field theory. We identify the class of diagrams contributing at leading order when a finite chemical potential is added and resum them to perform a full treatment of the LPM effect similar to the one performed by Arnold, Moore, and Yaffe at zero chemical potential. Our results show that the contribution of $2\mapsto 3$ and $3\mapsto 2$ processes grows as the chemical potential grows.Comment: 28 pages, 14 figure

3+1D hydrodynamic simulation of relativistic heavy-ion collisions

We present MUSIC, an implementation of the Kurganov-Tadmor algorithm for relativistic 3+1 dimensional fluid dynamics in heavy-ion collision scenarios. This Riemann-solver-free, second-order, high-resolution scheme is characterized by a very small numerical viscosity and its ability to treat shocks and discontinuities very well. We also incorporate a sophisticated algorithm for the determination of the freeze-out surface using a three dimensional triangulation of the hyper-surface. Implementing a recent lattice based equation of state, we compute p_T-spectra and pseudorapidity distributions for Au+Au collisions at root s = 200 GeV and present results for the anisotropic flow coefficients v_2 and v_4 as a function of both p_T and pseudorapidity. We were able to determine v_4 with high numerical precision, finding that it does not strongly depend on the choice of initial condition or equation of state.Comment: 16 pages, 11 figures, version accepted for publication in PRC, references added, minor typos corrected, more detailed discussion of freeze-out routine adde

Charge Transfer Fluctuations as a QGP Signal

In this study, we analyze the recently proposed charge transfer fluctuations within a finite pseudo-rapidity space. As the charge transfer fluctuation is a measure of the local charge correlation length, it is capable of detecting inhomogeneity in the hot and dense matter created by heavy ion collisions. We predict that going from peripheral to central collisions, the charge transfer fluctuations at midrapidity should decrease substantially while the charge transfer fluctuations at the edges of the observation window should decrease by a small amount. These are consequences of having a strongly inhomogeneous matter where the QGP component is concentrated around midrapidity. We also show how to constrain the values of the charge correlations lengths in both the hadronic phase and the QGP phase using the charge transfer fluctuations. Current manuscript is based on the preprints hep-ph/0503085 (to appear in Physical Review C) and nucl-th/0506025.Comment: To appear in the proceedings of 18th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2005 (QM 2005), Budapest, Hungary, 4-9 Aug 200

Pion Decay Constant at Finite Temperature in the Nonlinear Sigma Model

We calculate the pion decay constant near the critical temperature of the $O(N)$ nonlinear sigma model in the large $N$ limit. Making use of the known low temperature behavior, we construct a Pad\'{e} approximant to obtain the behavior of $f_\pi(T)$ at all temperatures.Comment: 8 pages, one latex file and one postscript file (uses psfig). Uuencode

Leading Order Calculation of Shear Viscosity in Hot Quantum Electrodynamics from Diagrammatic Methods

We compute the shear viscosity at leading order in hot Quantum Electrodynamics. Starting from the Kubo relation for shear viscosity, we use diagrammatic methods to write down the appropriate integral equations for bosonic and fermionic effective vertices. We also show how Ward identities can be used to put constraints on these integral equations. One of our main results is an equation relating the kernels of the integral equations with functional derivatives of the full self-energy; it is similar to what is obtained with two-particle-irreducible effective action methods. However, since we use Ward identities as our starting point, gauge invariance is preserved. Using these constraints obtained from Ward identities and also power counting arguments, we select the necessary diagrams that must be resummed at leading order. This includes all non-collinear (corresponding to 2 to 2 scatterings) and collinear (corresponding to 1+N to 2+N collinear scatterings) rungs responsible for the Landau-Pomeranchuk-Migdal effect. We also show the equivalence between our integral equations obtained from quantum field theory and the linearized Boltzmann equations of Arnold, Moore and Yaffe obtained using effective kinetic theory.Comment: 45 pages, 22 figures (note that figures 7 and 14 are downgraded in resolution to keep this submission under 1000kb, zoom to see them correctly

A classical Odderon in QCD at high energies

We show that the weight functional for color sources in the classical theory of the Color Glass Condensate includes a term which generates Odderon excitations. Remarkably, the classical origin of these excitations can be traced to the random walk of partons in the two dimensional space spanned by the SU(3) Casimirs. This term is naturally suppressed for a large nucleus at high energies.Comment: 19 pages. No figur

Coherence Time in High Energy Proton-Nucleus Collisions

Precisely measured Drell-Yan cross sections for 800 GeV protons incident on a variety of nuclear targets exhibit a deviation from linear scaling in the atomic number A. We show that this deviation can be accounted for by energy degradation of the proton as it passes through the nucleus if account is taken of the time delay of particle production due to quantum coherence. We infer an average proper coherence time of 0.4 +- 0.1 fm/c, corresponding to a coherence path length of 8 +- 2 fm in the rest frame of the nucleus.Comment: 11 pages in LaTeX. Includes 6 eps figures. Uses epsf.st

Particle yield fluctuations and chemical non-equilibrium at RHIC

We study charge fluctuations within the statistical hadronization model. Considering both the particle yield ratios and the charge fluctuations we show that it is possible to differentiate between chemical equilibrium and non-equilibrium freeze-out conditions. As an example of the procedure we show quantitatively how the relative yield ratio $\Lambda/K^-$ together with the normalized net charge fluctuation v(Q)=\ave{\Delta Q^2}/\ave{\Nch} constrain the chemical conditions at freeze-out. We also discuss the influence of the limited detector acceptance on fluctuation measurements, and show how this can be accounted for within a quantitative analysis.Comment: Accepted for publication by Physical Review

Size of Fireballs Created in High Energy Lead-Lead Collisions as Inferred from Coulomb Distortions of Pion Spectra

We compute the Coulomb effects produced by an expanding, highly charged fireball on the momentum distribution of pions. We compare our results to data on Au+Au at 11.6 A GeV from E866 at the BNL AGS and to data on Pb+Pb at 158 A GeV from NA44 at the CERN SPS. We conclude that the distortion of the spectra at low transverse momentum and mid-rapidity can be explained in both experiments by the effect of the large amount of participating charge in the central rapidity region. By adjusting the fireball expansion velocity to match the average transverse momentum of protons, we find a best fit when the fireball radius is about 10 fm, as determined by the moment when the pions undergo their last scattering. This value is common to both the AGS and CERN experiments.Comment: Enlarged discussion, new references added, includes new analysis of pi-/pi+ at AGS energies. 12 pages 5 figures, uses LaTex and epsfi

Random walks of partons in SU(N_c) and classical representations of color charges in QCD at small x

The effective action for wee partons in large nuclei includes a sum over static color sources distributed in a wide range of representations of the SU(N_c) color group. The problem can be formulated as a random walk of partons in the N_c-1 dimensional space spanned by the Casimirs of SU(N_c). For a large number of sources, k >> 1, we show explicitly that the most likely representation is a classical representation of order O(\sqrt{k}). The quantum sum over representations is well approximated by a path integral over classical sources with an exponential weight whose argument is the quadratic Casimir operator of the group. The contributions of the higher N_c-2 Casimir operators are suppressed by powers of k. Other applications of the techniques developed here are discussed briefly.Comment: 51 pages, includes 3 eps file