Low Mass Dimuon Production in p-A Collisions at 400 GeV/c with the NA60 Detector

The NA60 experiment has studied low-mass muon pair production in proton-nucleus collisions with a system of Be, Cu, In, W, Pb and U targets using a 400 GeV/c proton beam at the CERN SPS. The mass spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons eta, rho, omega, eta' and phi. A new measurement of the electromagnetic transition form factors of the eta and omega was performed. The values found agree with the previous available measurements, improving their uncertainty thanks to the higher statistics, and confirm the discrepancy with the prediction of the Vector Meson Dominance (VMD) model in the case of the electromagnetic form factor of the omega meson; for this latter, the comparison of the data to an improved calculation including also a direct point-like term is discussed. The pT spectra for the omega and phi mesons are studied in the full pT range accessible, up to pT = 2 GeV/c. The pT spectrum of the eta meson is also considered, starting from pT = 0.6 GeV/c. The cross section ratios rho/omega, phi/omega and eta/omega have been studied as a function of the size A of the production target, and an increase of the eta and phi production with respect to the omega is observed from p-Be to p-U collisions. The nuclear dependence of the production cross sections of the eta, omega and phi mesons has been parameterized with the power law A^alpha, where the alpha parameter has been found to increase as a function of pT.