Low mass dimuon production in indium-indium collisions at 158 A GeV

The NA60 experiment studies open charm and prompt dimuon production in proton-nucleus and nucleus-nucleus collisions at the CERN SPS. During 2003 the experiment collected data in Indium-Indium collisions at 158 GeV per nucleon. Almost 240 million dimuon events were recorded. New results on J/ $\psi$ suppression, open-charm production and low mass dimuons should help clarify some interesting questions left open by previous experiments. After a brief detector description, this paper focuses on the analysis of the low mass dimuons. Preliminary results are presented on the $\phi/\omega$ production cross section ratios and on the $\phi$ transverse momentum distributions, both as a function of collision centrality.