Photon production in relativistic nuclear collisions at SPS and RHIC energies

Chiral Lagrangians are used to compute the production rate of photons from the hadronic phase of relativistic nuclear collisions. Special attention is paid to the role of the pseudovector a_1 meson. Calculations that include reactions with strange mesons, hadronic form factors and vector spectral densities consistent with dilepton production, as well as the emission from a quark-gluon plasma and primordial nucleon-nucleon collisions, reproduce the photon spectra measured at the Super Proton Synchrotron (SPS). Predictions for the Relativistic Heavy Ion Collider (RHIC) are made.Comment: Work presented at the 26th annual Montreal-Rochester-Syracuse-Toronto conference (MRST 2004) on high energy physics, Montreal, QC, Canada, 12-14 May 2004. 8 pages, 3 figure

Quark Recombination and Heavy Quark Diffusion in Hot Nuclear Matter

We discuss resonance recombination for quarks and show that it is compatible with quark and hadron distributions in local thermal equilibrium. We then calculate realistic heavy quark phase space distributions in heavy ion collisions using Langevin simulations with non-perturbative T-matrix interactions in hydrodynamic backgrounds. We hadronize the heavy quarks on the critical hypersurface given by hydrodynamics after constructing a criterion for the relative recombination and fragmentation contributions. We discuss the influence of recombination and flow on the resulting heavy meson and single electron R_AA and elliptic flow. We will also comment on the effect of diffusion of open heavy flavor mesons in the hadronic phase.Comment: Contribution to Quark Matter 2011, submitted to J.Phys.G; 4 pages, 5 figure

Medium Modifications of the Rho Meson at CERN/SPS Energies

Rho meson propagation in hot hadronic matter is studied in a model with coupling to $\pi\pi$ states. Medium modifications are induced by a change of the pion dispersion relation through collisions with nucleons and $\Delta's$ in the fireball. Maintaining gauge invariance dilepton production is calculated and compared to the recent data of the CERES collaboration in central S+Au collisions at 200 GeV/u. The observed enhancement of the rate below the rho meson mass can be largely accounted for.Comment: 10 pages RevTeX and 2 figures (uuencoded .ps-files

The Vector Probe in Heavy-Ion Reactions

We review essential elements in using the $J^P=1^-$ channel as a probe for hot and dense matter as produced in (ultra-) relativistic collisions of heavy nuclei. The uniqueness of the vector channel resides in the fact that it directly couples to photons, both real and virtual (dileptons), enabling the study of thermal radiation and in-medium effects on both light ($\rho, \omega, \phi$) and heavy ($\Psi, \Upsilon$) vector mesons. We emphasize the importance of interrelations between photons and dileptons, and characterize relevant energy/mass regimes through connections to Quark-Gluon-Plasma emission and chiral symmetry restoration. Based on critical analysis of our current understanding of data from fixed-target energies, we identify open key questions to be addressed.Comment: Invited Talk at the Hot Quarks 2004 Workshop, July 18-24, 2004 (Taos Valley, NM, USA), 15 pages latex incl 14 figs and iop style files, to appear in the proceeding

Hadro-Chemistry and Evolution of (Anti-) Baryon Densities at RHIC

The consequences of hadro-chemical freezeout for the subsequent hadron gas evolution in central heavy-ion collisions at RHIC and LHC energies are discussed with special emphasis on effects due to antibaryons. Contrary to naive expectations, their individual conservation, as implied by experimental data, has significant impact on the chemical off-equilibrium composition of hadronic matter at collider energies. This may reflect on a variety of observables including source sizes and dilepton spectra.Comment: 4 pages ReVTeX incl. 3 ps-figs, submitted to PR

Interpretation of Recent SPS Dilepton Data

We summarize our current theoretical understanding of in-medium properties of the electromagnetic current correlator in view of recent dimuon data from the NA60 experiment in In(158 AGeV)-In collisions at the CERN-SPS. We discuss the sensitivity of the results to space-time evolution models for the hot and dense partonic and hadronic medium created in relativistic heavy-ion collisions and the contributions from different sources to the dilepton-excess spectra.Comment: To appear in the proceedings of the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2006) v2: references added, minor typos correcte

Dileptons in High-Energy Heavy-Ion Collisions

The current status of our understanding of dilepton production in ultrarelativistic heavy-ion collisions is discussed with special emphasis on signals from the (approach towards) chirally restored and deconfined phases. In particular, recent results of the CERN-SPS low-energy runs are compared to model predictions and interpreted. Prospects for RHIC experiments are given.Comment: Invited talk at ICPAQGP, Jaipur, India, Nov. 26-30, 2001; 1 Latex and 9 eps-/ps-files Reoprt No.: SUNY-NTG-02-0

Heavy-Quark Diffusion and Hadronization in Quark-Gluon Plasma

We calculate diffusion and hadronization of heavy quarks in high-energy heavy-ion collisions implementing the notion of a strongly coupled quark-gluon plasma in both micro- and macroscopic components. The diffusion process is simulated using relativistic Fokker-Planck dynamics for elastic scattering in a hydrodynamic background. The heavy-quark transport coefficients in the medium are obtained from non-perturbative $T$-matrix interactions which build up resonant correlations close to the transition temperature. The latter also form the basis for hadronization of heavy quarks into heavy-flavor mesons via recombination with light quarks from the medium. The pertinent resonance recombination satisfies energy conservation and provides an equilibrium mapping between quark and meson distributions. The recombination probability is derived from the resonant heavy-quark scattering rate. Consequently, recombination dominates at low transverse momentum ($p_T$) and yields to fragmentation at high $p_T$. Our approach thus emphasizes the role of resonance correlations in the diffusion and hadronization processes. We calculate the nuclear modification factor and elliptic flow of $D$- and $B$-mesons for Au-Au collisions at the Relativistic Heavy Ion Collider, and compare their decay-electron spectra to available data. We also find that a realistic description of the medium flow is essential for a quantitative interpretation of the data.Comment: 16 pages, 14 figure

Isospin Fluctuations in QCD and Relativistic Heavy-Ion Collisions

We address the role of fluctuations in strongly interacting matter during the dense stages of a heavy-ion collision through its electromagnetic emission. Fluctuations of isospin charge are considered in a thermal system at rest as well as in a moving hadronic fluid at fixed proper time within a finite bin of pseudo-rapidity. In the former case, we use general thermodynamic relations to establish a connection between fluctuations and the space-like screening limit of the retarded photon self-energy, which directly relates to the emissivities of dileptons and photons. Effects of hadronic interactions are highlighted through two illustrative calculations. In the latter case, we show that a finite time scale $\tau$ inherent in the evolution of a heavy-ion collision implies that equilibrium fluctuations involve both space-like and time-like components of the photon self-energy in the system. Our study of non-thermal effects, explored here through a stochastic treatment, shows that an early and large fluctuation in isospin survives only if it is accompanied by a large temperature fluctuation at freeze-out, an unlikely scenario in hadronic phases with large heat capacity. We point out prospects for the future which include: (1) A determination of the Debye mass of the system at the dilute freeze-out stage of a heavy-ion collision, and (2) A delineation of the role of charge fluctuations during the dense stages of the collision through a study of electromagnetic emissivities.Comment: 12 pages ReVTeX incl. 4 ps-fig