Hadrons in Hot and Dense Matter

The description of excitations in hot and dense (hadronic) matter is discussed with emphasis on the use of correlation functions as a common framework for comparing different model (and QCD lattice) calculations with each other. Typical regimes of applicability of hadronic approaches are assessed, together with possibilities to confront them with experiment. We also elaborate on recent developments to relate baryonic in-medium effects to chiral symmetry restoration.Comment: Convener Talk at the Int. Conf. on Quark Nuclear Physics, June 9-14, 2002 (Juelich, Germany); 3 pages LaTeX including 5 eps-figures and EPJ style file

Comprehensive Interpretation of Thermal Dileptons at the SPS

Employing thermal dilepton rates based on medium-modified electromagnetic correlation functions we show that recent dimuon spectra of the NA60 collaboration in central In-In collisions at the CERN-SPS can be understood in terms of radiation from a hot and dense hadronic medium. Earlier calculated \rho-meson spectral functions, as following from hadronic many-body theory, provide an accurate description of the data up to dimuon invariant masses of about M\simeq 0.9 GeV, with good sensitivity to details of the predicted \rho-meson line shape. This, in particular, identifies baryon-induced effects as the prevalent ones. We show that a reliable description of the \rho contribution opens the possibility to study further medium effects: at higher masses (M \simeq 0.9-1.5 GeV) 4-pion type annihilation is required to account for the experimentally observed excess indicating precursor effects of chiral symmetry restoration (``chiral mixing''), while remaining structures in the \omega and \phi region are suggestive for modifications in their line shapes as well.Comment: 4 pages, 4 figures, v2: slightly improved estimate of four-pion contributions; accepted for publication in Phys. Rev. Let

Measurement of low mass dielectron continuum in sqrt(s_NN)=200GeV Au+Au collisions with the PHENIX experiment at RHIC

Electromagnetic probes are ideally suited to investigate hot and dense matter produced in high energy heavy ion collisions. They do not undergo strong interactions and thus probe the time evolution of the collision. The dielectron continuum is rich in physics. Dalitz decays of light hadrons and direct decays of vector mesons, which might be modified in the medium, and correlated charm decays contribute to the spectrum. Furthermore, exotic states, e.g quasi-particles predicted to exist in the medium only beyond the phase transition, might leave their footprint in the continuum. Although correlated e+e- pairs are rare, the 0.24 nb^-1 collected by PHENIX for Au+Au collisions at sqrt(s_NN)= 200 GeV in 2004 provides a significant sample to investigate the dilepton continuum. The continuum and its resonances are separated from the combinatorial background via an event mixing technique. Mass spectra are presented and compared with the expectations from decays of hadronic sources.Comment: 4 pages, 5 figures, conference proceedin

Dileptons and Medium Effects in Heavy-Ion Collisions

We discuss the status of calculating in-medium modifications of vector-meson spectral functions in hot and dense matter, their application to dilepton spectra in ultrarelativistic heavy-ion collisions, and possible relations to chiral symmetry restoration. We emphasize the importance of constraining in-medium spectral functions by empirical information from scattering data, QCD sum rules, and lattice QCD. This is a mandatory prerequisite to arrive at reliable predictions for low-mass dileptons in heavy-ion collisions.Comment: 8 pages latex, 15 figure

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

Duality and Chiral Restoration from Dilepton Production in Relativistic Heavy-Ion Collisions

We discuss the recent status in the theoretical understanding of dilepton production in central heavy-ion reactions with the Pb-beam at the full CERN-SpS energy of 158 AGeV. In the low-mass region ($M\le$~1 GeV) a strong broadening of the vector meson resonances in hot and dense matter (especially for the $\rho$ meson) entails thermal dilepton rates very reminiscent to perturbative $q\bar q$ annihilation close to the expected phase boundary of the chiral symmetry restoring transition. A consistent description of the experimentally observed enhancement at both low and intermediate masses (1.5 GeV~$\le M \le$~3 GeV) in terms of thermal radiation from an expanding fireball can be obtained.Comment: Talk given at ISMD '99 on 'QCD & Multiparticle Production' (Brown University, Providence, Aug. 9-13), 7 pages LaTeX including 6 ps-/eps-figures and sprocl.st