NA60 low mass di-muon data

Abstract

Data taken by NA60 in 2002 with a 400 GeV proton beam on various nuclear targets confirms that di-muon production with a mass below the ϕ can be explained by meson decays. The production cross sections of these mesons agree with what earlier experiments have reported. In 2003, NA60 has recorded a large sample of interactions between a 158 GeV/n In beam and In targets. Pending a study of the background, we do not comment on the existence of an excess in this data sample, but our measurements shed new light on the apparent conflict between NA49 and NA50 measurements of the inverse transverse mass slope of ϕ production. The NA60 measurement of the ϕ/(ϕ+ω) ratio as function of number of participants agrees well with earlier measurements, both in proton and in ion data.