The quark condensate at finite temperature

The temperature evolution of the quark condensate is studied using three different methods. In the spirit of a many-body approach we make an expansion in the scalar density up to second order. Our result is consistent chiral perturbation theory to two-loop order.Comment: Latex 14 pages + 1 figure (postscript

Transport Properties of Strong-Interaction Matter

The properties of strong-interaction matter are probed in ultra-relativistic heavy-ion collisions. In the context of measurements of the elliptic flow at RHIC and the LHC the shear viscosity is of particular interest. In this presentation we discuss recent results for eta/s in hadronic matter at vanishing baryo-chemical potential within kinetic theory. Using the Nambu-Jona-Lasinio model, special attention is paid to effects arising from the restoration of spontaneously broken chiral symmetry with increasing temperature.Comment: 8 pages, 4 figures, presented at the HIC for FAIR Workshop and XXVIII Max Born Symposium "Three days on Quarkyonic Island", Wroclaw, May 19-21, 2011; v2: conversion to Pa in fig. 2 correcte

Momentum Dependence of the Pion Cloud for Rho Mesons in Nuclear Matter

We extend hadronic models for rho-meson propagation in cold nuclear matter via coupling to in-medium pions to include finite three-momentum. Special care is taken to preserve gauge invariance. Consequences for photoabsorption on the proton and on nuclei as well as for the dilepton production in relativistic heavy-ion collisions are discussed.Comment: 32 pages, 18 figures. Corrected version, accepted for publication in Nucl. Phys.

A Microscopic Calculation of Photoabsorption Cross Sections on Protons and Nuclei

A recently developed model for $\rho$-meson propagation in dense hadronic matter is applied to total photoabsorption cross sections in $\gamma$-proton and $\gamma$-nucleus reactions. Within the vector dominance model the photon coupling to the virtual pion cloud of the nucleon, two-body meson-exchange currents, as well as $\gamma$-nucleon resonances are included. Whereas the $\gamma p$ reaction is determined by the low-density limit of the model, higher orders in the nuclear density are important to correctly account for the experimental spectra observed on both light and heavy nuclei over a wide range of photon energies, including the region below the pion threshold. In connection with soft dilepton spectra in high-energy heavy-ion collisions we emphasize the importance of photoabsorption to further constrain the parameters of the model.Comment: 10 pages RevTeX, 2 eps-figure

Pionic susceptibility for charged pions in asymmetric nuclei

At low energies the particle-hole (ph) part of the pionic susceptibility in isospin-symmetric nuclear matter is known to behave very differently from the susceptibility in finite nuclei due to the presence of an energy gap in the ph excitation spectrum. In this note we show that for charged pions in N > Z nuclei the changes due to the gap are very similar to those in the symmetric case, except at very low momenta, where a qualitatively different behavior is found

Medium Dependence of the Vector-Meson Mass: Dynamical and/or Brown-Rho Scaling?

We discuss the similarities and differences for the theories of Rapp, Wambach and collaborators (called R/W in short) and those based on Brown-Rho scaling (called B/R), as applied to reproduce the dileptons measured by the CERES collaboration in the CERN experiments. In both theories the large number of dileptons at invariant masses $\sim$~$m_\rho/2$ are shown to be chiefly produced by a density-dependent $\rho$-meson mass. In R/W the medium dependence is dynamically calculated using hadronic variables defined in the matter-free vacuum. In B/R scaling it follows from movement towards chiral symmetry restoration due to medium-induced vacuum change, and is described in terms of constituent (or quasiparticle) quarks. We argue that the R/W description should be reliable up to densities somewhat beyond nuclear density, where hadrons are the effective variables. At higher density there should be a crossover to constituent quarks as effective variables scaling according to B/R. In the crossover region, the two descriptions must be ``dual''.Comment: 13 pages LaTeX, incl. 5 eps-figures and appb.sty; Talk given at the Workshop on 'The Structure of Mesons, Baryons and Nuclei', Cracow, May 1998, in honor of J. Speth's 60th birthday, to be published in Acta Physica Polonica

Temperature dependence of rho- and a_1-meson masses and mixing of vector and axial-vector correlators

Within a chiral model which provides a good description of the properties of rho and a_1 mesons in vacuum, it is shown that, to order T^2, the rho- and a_1-meson masses remain constant in the chiral limit, even if at tree level they are proportional to the chiral condensate, sigma_0. Numerically, the temperature dependence of the masses turns out to be small also for realistic parameter sets and high temperatures. The weak temperature dependence of the masses is consistent with the Eletsky-Ioffe mixing theorem, and traces of mixing effects can be seen in the spectral function of the vector correlator at finite temperature