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 2 GeV/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^α , with the α parameter increasing as a function of p_T for all the particles, and an approximate hierarchy α _η≈α _ϕ > α _ω . The cross section ratios σ _η /σ _ω , σ _ρ /σ _ω and σ _ϕ /σ _ω 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.