Low mass dimuon production in proton-nucleus collisions at 400 GeV/c

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 are Lambda(-2)(eta) = 1.950 +/- 0.059 (stat.) +/- 0.042 (syst.) (GeV/c(2))(-2) and Lambda(-2)(omega) = 2.241 +/- 0.025 (stat.) +/- 0.028 (syst.) (GeV/c(2))(-2). They 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 model in the case of the electromagnetic form factor of the omega meson. The comparison of the data to an improved calculation including also a direct point-like term is discussed. The nuclear dependence of the production cross sections of the omega, rho and phi mesons was also studied. An increase of the phi production with respect to the omega is observed from p-Be to p-U collisions. Parameterizing the nuclear dependence of the production cross sections with the power law sigma(pA) infinity A(alpha) one finds alpha(omega) = 0.841 +/- 0.014 (stat.) +/- 0.030 (syst.) and alpha(phi) = 0.906 +/- 0.011 (stat.) +/- 0.025 (syst.). A possible bias induced by the use of the light Be target is found. If it is excluded, the results are alpha(omega) = 0.801 +/- 0.037 (stat.) +/- 0.041 (syst.) and alpha(phi) = 0.963 +/- 0.026 (stat.) +/- 0.017 (syst.), showing an increase of alpha(phi) and a decrease of alpha(omega)