Chiral Symmetry Restoration and Dileptons in Relativistic Heavy-Ion Collisions

The current theoretical status in the analysis and interpretation of low-mass dilepton measurements in (ultra-) relativistic heavy-ion experiments is reviewed. Special emphasis is put on potential signals of (partial) restoration of dynamically broken chiral symmetry in a hot and dense hadronic medium. It follows from chiral symmetry alone that parity partners of hadronic correlation functions must become identical when the symmetry is restored. The assessment of medium effects in the vector channel, which governs the dilepton production, thus necessitates a simultaneous treatment of the vector and axialvector degrees of freedom. While significant progress in this respect has been made some open questions remain in establishing a rigorous link in the mass region below 1 GeV. From the present calculations a suggestive 'quark-hadron duality' emerges near the phase boundary. It implies substantial medium effects in the dilepton signal from the hadronic phase which smoothly matches a perturbative description within the plasma phase.Comment: 164 pages LaTeX including 88 eps-/ps-figures, Review Article to appear in Adv. Nucl. Phy

Probing Chiral Symmetry Restoration with Heavy Ions

It is discussed how chiral symmetry restoration manifests itself through mixing of vector and axial-vector correlators. The vector correlator is directly accessible in relativistic heavy-ion collisions. Within models of the vector correlator its implications for low-mass dilepton spectra are reviewed.Comment: 11 pages LaTeX, incl. 6 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

Modifications of the Rho Meson from the Virtual Pion Cloud in Hot and Dense Matter

The modification of the rho-meson self-energy due to the coupling to in-medium pions is calculated consistently at finite baryon density and temperature, keeping the full 3-momentum dependence in a gauge invariant way. As a function of nucleon density, the rho-meson spectral function is strongly enhanced in the invariant mass region M < 650 MeV, while the maximum, i.e. the pole mass, is slightly shifted upwards. As a function of temperature, for fixed nucleon density, the imaginary part of the self-energy increases further due to Bose-enhancement. At the same time the mass shift from the real part becomes very large. As a consequence of these medium effects, the dilepton rate in the low-mass region M < 650 MeV increases strongly, while the peak at M = 770 MeV disappears.Comment: 18 pages, 9 figures; resonance contributions adde

Rho Meson Propagation and Dilepton Enhancement in Hot Hadronic Matter

A realistic model for the free rho meson with coupling to two-pion states is employed to calculate the rho propagator in a hot and dense hadron gas. The medium modifications are based on hadronic rescattering processes: intermediate two-pion states are renormalized through interactions with surrounding nucleons and deltas, and rho meson scattering is considered off nucleons, deltas, pions and kaons. Constraints from gauge invariance as well as the full off-shell dynamics of the interactions are accounted for. Within the vector dominance model we apply the resulting in-medium rho spectral function to compute $e^+e^-$ production rates from $\pi^+\pi^-$ annihilation. The calculation of corresponding $e^+e^-$ spectra as recently measured in central collisions of heavy-ions at CERN/SpS energies gives reasonable agreement with the experimental data.Comment: 27 pages RevTeX, 9 eps-figures, submitted to Nucl. Phys.

Ideal Hydrodynamics for Bulk and Multistrange Hadrons in sNN\sqrt{s_{NN}}=200\,AGeV Au-Au Collisions

We revisit the use of ideal hydrodynamics to describe bulk- and multistrange-hadron observables in nuclear collisions at the Relativistic Heavy Ion Collider. Toward this end we augment the 2+1-dimensional code "AZHYDRO" by employing (a) an equation of state based on recent lattice-QCD computations matched to a hadron-resonance gas with chemical decoupling at $T_{\rm ch}\simeq$160\,MeV, (b) a compact initial density profile, (c) an initial-flow field including azimuthal anisotropies, and (d) a sequential kinetic decoupling of bulk ($\pi$, $K$, $p$) and multistrange ($\phi$, $\Xi$, $\Omega$) hadrons at $T\simeq110$\,MeV and 160\,MeV, respectively. We find that this scheme allows for a consistent description of the observed chemistry, transverse-momentum spectra and elliptic flow of light and strange hadrons.Comment: 9 pages, 5 figure