Low mass dimuons produced in relativistic nuclear collisions

The NA60 experiment has measured low mass muon pair production in In-In collisions at $158A\text{ }\text{ }\mathrm{GeV}$ 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\char21{}Twiss correlations in Pb-Pb and Pb-Au collisions at the same energy. The data are consistent with in-medium properties of $\ensuremath{\rho}$ and $\ensuremath{\omega}$ mesons at finite temperature and density as deduced from empirical forward-scattering amplitudes. Inclusion of the vacuum decay of the $\ensuremath{\rho}$ meson after freeze-out is necessary for an understanding of the mass and transverse momentum spectrum of dimuons with $M\ensuremath{\lesssim}0.9\text{ }\text{ }\mathrm{GeV}/{c}^{2}$.