Study of Low Mass Dimuon Production in 400 GeV Proton-Nucleus Collisions at the CERN-SPS with the NA60 Apparatus

1Physics Motivation The present work was performed within NA60, which is a fixed target experiment at the CERN SPS accelerator. NA60 studies proton-nucleus and nucleus-nucleus collisions in the dimuonic channel; data were taken between 2002 and 2004, and the analyses are currently still in progress. The current chapter gives a short overview of the physics topics explored within NA60, with emphasis on the topics related to this thesis. Several experiments at the CERN SPS have measured lepton pair production in nuclear collisions over more than 15 years. The absence of significant reinteractions with the hadronic environment makes dileptons very attractive to study the early stages of the collision, when thermal dileptons from a deconfined phase are possibly emitted. Furthermore, the pseudoscalar and vector mesons produced in the collisions (η, ρ, ω, η ′ ,φ,J/ψ,χ c , ...) can be detected in a clean way through their leptonic decay. While a number of relevant results were obtained by the experiments which pre- ceeded NA60, several points deserved a more detailed investigation. NA60 has been specifically designed with the idea to perform further high precision measurements on (i) the properties of the ρ meson in the strongly interacting dense medium cre- ated in a heavy ion collision, (ii) the unambiguous determination of the open charm yield and the quantitative assessment of the production of thermal radiation in the intermediate mass region (approximately between the φ and J/ψ resonances), and (iii) the study of the J/ψ suppression in a collision system other than Pb-Pb in an attempt to shed light on the mechanisms driving the suppression. These three issues are briefly summarised in the following paragraphs.