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 $2 \, {\hbox {GeV/c}}$ . 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.