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.