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.

Highlights

  • The study of the production of low-mass vector and pseudoscalar mesons in proton–nucleus (p–A) collisions represents a unique tool to understand the role of cold nuclear matter in particle production mechanisms

  • The nuclear dependence of both the transverse momentum spectra and the total production cross sections is of special interest in this context, with the observations usually interpreted in terms of phenomenological models, since a first-principle description based on non-perturbative QCD is not yet available

  • In this letter we reported on a comprehensive study of lowmass dimuon production in p–A interactions at 400 GeV, ranging from p–Be to p–U, measured with the NA60 apparatus at the CERN SPS

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Summary

Introduction

The study of the production of low-mass vector and pseudoscalar mesons in proton–nucleus (p–A) collisions represents a unique tool to understand the role of cold nuclear matter in particle production mechanisms. C (2019) 79:443 and mass-shift in cold nuclear matter In this context, a recent measurement in p–A collisions at 400 GeV by the NA60 experiment [4], exploiting the same data set considered in the present analysis, ruled out the existence of any significant cold-nuclear matter effect at the SPS energies on the line shapes of the light vector mesons ρ, ω and φ, giving in particular, for the first time, a precise characterisation of the ρ-meson line shape in p–A collisions. The NA60 experiment complemented its main In–In programme with a high-luminosity proton–nucleus run, exposing to a 400 GeV proton beam six target materials: Be, Cu, In, W, Pb and U This allowed for a comprehensive and detailed study of the nuclear dependence of the η, ρ, ω and φ meson production. Particle production is further discussed via the study of the cross section ratios in the full phase space, with strangeness enhancement being addressed through the measurement of the nuclear dependence of the cross section ratios σφ/σω and ση/σω

Apparatus and event selection
Target identification
Background treatment
Signal extraction
Particle cross-section ratios in full phase space
Findings
Conclusions
Full Text
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