Phi production in In-In collisions and the phi puzzle

International audience; The NA60 experiment measured dimuon production in In-In collisions at 158 A GeV. This paper presents a high statistics measurement of phi → μμ with the specific objective to provide an insight into the phi 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 phi 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