In-Medium Vector Mesons, Dileptons and Chiral Restoration

Medium modifications of the electromagnetic spectral function in hadronic and quark-gluon matter are reviewed. A strong broadening of the $\rho$ meson, which dominates the spectral function in the low-mass regime, is quantitatively consistent with dilepton excess spectra measured in photoproduction off cold nuclei (CLAS/JLab) and in fixed-target ultrarelativistic heavy-ion collisions (NA45,NA60/CERN-SPS). The large excess observed by PHENIX at RHIC remains unexplained to date, but is most likely not due to emission from the Quark-Gluon Plasma. Connections to thermal lattice QCD promise progress in the search for chiral symmetry restoration.Comment: 9 pages, 12 figures, Proceedings of Chiral 2010 (Valencia, June 21-24, 2010

Intermediate-Mass Dileptons at the CERN-SpS and RHIC

The significance of thermal dilepton radiation at intermediate invariant masses (1GeV<M<3GeV) in ultrarelativistic heavy-ion collisions is investigated. At CERN-SpS energies, a consistent explanation of the excess observed by NA50 can be given. At RHIC energies the thermal signal is dominated by early emission indicative for QGP formation. Chemical under-saturation effects and the competition with open-charm contributions are addressed.Comment: Invited talk at the International Workshop XXVIII on Gross Properties of Nuclei and Nuclear Excitations, 'Hadrons in Dense Matter', Hirschegg (Austria), Jan. 16-22, 2000; 8 pages LaTeX including 11 eps/ps-figure

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

Production of Light Nuclei at Thermal Freezeout in Heavy-Ion Collisions

We revisit the problem of the production of light atomic nuclei in ultrarelativistic heavy-ion collisions. While their production systematics is well produced by hadro-chemical freezeout at temperatures near the QCD pseudo-critical temperature, their small binding energies of a few MeV per nucleon suggest that they cannot survive as bound states under these conditions. Here, we adopt the concept of effective chemical potentials in the hadronic evolution from chemical to thermal freezeout (at typically $T_{\rm fo}$$\simeq$100\,MeV), which, despite frequent elastic rescatterings in hadronic matter, conserves the effective numbers of particles which are stable under strong interactions, most notably pions, kaons and nucleons. It turns out that the large chemical potentials that build up for antibaryons result in thermal abundances of light nuclei and antinuclei, formed at thermal freezeout, which essentially agree with the ones evaluated at chemical freezeout. Together with their transverse-momentum spectra, which also indicate a kinetic freezeout near $T_{\rm fo}$, this provides a natural explanation for their production systematics without postulating their survival at high temperatures.Comment: 5 pages, 7 figures, v2: "Note added" correcte