Nuclear dependence of light neutral meson production in p–A collisions at 400 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 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.