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 ρ/ω and ϕ mesons are measured in the full p${}_T$ range accessible, from p${}_T$=0 up to 2GeV/c. The nuclear dependence of the production cross sections of the η, ω and ϕ mesons has been found to be consistent with the power law σ${}_{pA}$∝A${}^{\alpha }$, with the α parameter increasing as a function of p${}_T$ for all the particles, and an approximate hierarchy α${}_{\eta }$≈α${}_{\varphi }$>α${}_{\omega }$. The cross section ratios σ${}_{\eta }$/σ${}_{\omega }$, σ${}_{\rho }$/σ${}_{\omega }$ and σ${}_{\varphi }$/σ${}_{\omega }$ have been studied as a function of the size A of the production target, and an increase of the η and ϕ yields relative to the ω is observed from p–Be to p–U collisions.