Low Mass Dimuon Production in p-A Collisions at $\sqrt{s} = 27.5$ GeV with NA60

Abstract

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 $η$, $ρ$, $ω$, $η'$ and $ϕ$. A new high-precision measurement of the electromagnetic transition form factors of the $η$ and $ω$ mesons is presented, complemented with a measurement of the temperature parameter of the $ρ$ meson in cold nuclear matter. The $\pt$ spectra for the $ω$ and $ϕ$ mesons are extracted in the full $\pt$ range accessible, up to $\pt = 2$ GeV/$c$. The nuclear dependence of the production cross sections for the $η$, $ω$ and $ϕ$ mesons has been investigated in terms of the power law $σ_\mathrm{pA} \propto \mathrm{A}^α$, and the $α$ parameter was studied as a function of $\pt$.