First results from NA60 on low mass muon pair production in In-In collisions at 158 GeV/nucleon

. The NA60 experiment at the CERN SPS studies dimuon production in proton-nucleus and nucleus-nucleus collisions. The combined information from a novel vertex telescope made of radiation-tolerant silicon pixel detectors and from the muon spectrometer previously used in NA50 allows for a precise measurement of the muon vertex and a much improved dimuon mass resolution. We report on first results from the data taken for Indium-Indium collisions at 158 AGeV/nucleon in 2003, concentrating on a subsample of about 370000 muon pairs in the mass range < 1 . 2 GeV/ c 2 . The light vector mesons ω and φ are completely resolved, with a mass resolution of about 23 MeV/ c 2 at the φ . The transverse momentum spectra of the φ are measured over the continuous range 0 < p T < 2 . 5 GeV/c; the inverse slope parameter of the spectra is found to increase with centrality, with an average value of T = 252 ± 3 MeV.