phi meson production in In–In collisions and the phi puzzle

The NA60 experiment measured dimuon production in In-In collisions at 158 A GeV. This paper presents a high statistics measurement of o → μμ with the specific objective to provide an insight into the o puzzle, i.e. the difference in the inverse T slopes and absolute yields measured by NA49 and NA50 in the kaon and lepton channel, respectively. Transverse momentum distributions were studied as a function of centrality. The slope parameter T shows a rapid increase with centrality, followed by a saturation. Variations of T with the fit range of the order of 15 MeV were observerd, possibly as a consequence of radial flow. The o meson yield normalized to the number of participants increases with centrality and is consistently higher than the yield measured by the NA49 experiment at any centrality.