Low mass dimuon production in proton-nucleus and indium-indium collisions

The NA60 experiment studied low-mass dimuon production in proton-nucleus and indium-indium collisions at the CERN SPS. While the p-A data can be well described by a superposition of light neutral meson decays, an excess is observed in the heavy-ion data, increasing with collision centrality, with respect to these expected sources. Thanks to the high statistics and good quality of our measurement, we can subtract the muon pairs due to h, h0, w and f decays from the signal dimuon mass distribution. The resulting spectra are compatible with a r spectral function broadening from peripheral to central collisions, with no change in the pole mass.