From dileptons in heavy-ion collisions to chiral restoration at high temperature

Hadronic many-body theory predicts a strong broadening of the rho-meson spectral function in hot and dense matter, leading to a melting of its resonance structure as the pseudo-critical temperature is approached from below. Pertinent calculations of thermal dilepton spectra in heavy-ion collisions, which additionally include radiation from the quark-gluon plasma phase, are largely consistent with experimental measurements which now cover a rather large range of collision energies, from SPS to RHIC. The main part of this talk is devoted to analyzing the implications of this scenario for the long-standing question of chiral symmetry restoration. Toward this end, a combination of QCD and chiral Weinberg sum rules is utilized with inputs from lattice-QCD for in-medium condensates and order parameters. Rather stringent constraints on the a1(1260) spectral function - the chiral partner of the rho - are deduced. Solutions are found which satisfy the temperature-dependent sum rules accurately, thus suggesting that the rho melting scenario is compatible with chiral restoration.