Nuclear dependence of light neutral meson production in p-A collisions at 400 GeV with NA60

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 proton beam at the CERN SPS. The transverse momentum spectra of the $rho/omega$ and $phi$ mesons are measured in the full $p_mathrm{T}$ range accessible, from $p_mathrm{T} = 0$ up to 2 GeV/$c$. The nuclear dependence of the production cross sections of the $eta$, $omega$ and $phi$ mesons has been found to be consistent with the power law $sigma_mathrm{pA} propto mathrm{A}^alpha$, with the $alpha$ parameter increasing as a function of $p_mathrm{T}$ for all the particles, and an approximate hierarchy $alpha_eta approx alpha_phi > alpha_omega$. The cross section ratios $sigma_eta/sigma_omega$, $sigma_rho/sigma_omega$ and $sigma_phi/sigma_omega$ have been studied as a function of the size A of the production target, and an increase of the $eta$ and $phi$ yields relative to the $omega$ is observed from p-Be to p-U collisions.