Low mass dimuons produced in relativistic nuclear collisions

The NA60 experiment has measured low mass muon pair production in In-In collisions at 158AGeV with unprecedented precision. We show that these data are reproduced very well by a dynamical model with parameters scaled from fits to measurements of hadronic transverse mass spectra and Hanbury Brown-Twiss correlations in Pb-Pb and Pb-Au collisions at the same energy. The data are consistent with in-medium properties of ρ and ω mesons at finite temperature and density as deduced from empirical forward-scattering amplitudes. Inclusion of the vacuum decay of the ρ meson after freeze-out is necessary for an understanding of the mass and transverse momentum spectrum of dimuons with M 0.9GeV/c2.