Dimuon and charm production in nucleus-nucleus collisions at the CERN-SPS

Abstract

Muon pair production in p-A, S-U and Pb-Pb collisions has been studied by the NA38 and NA50 collaborations at the CERN SPS. In this paper we present an analysis of the dimuon invariant mass region between the φ and the J/ψ. After subtraction of the combinatorial background due to π and K decays, we find that the superposition of dimuons from semi-leptonic decays of D mesons and from the Drell-Yan process reproduces well the mass and transverse momentum spectra measured in p-A collisions. From the yield of open charm dimuons required to fit our data we extract a charm production cross section in good agreement with previous measurements. A linear extrapolation of the p-A sources with the product of the mass numbers of the projectile and target nuclei, A×B, underestimates the yield measured in S-U and Pb-Pb collisions. The excess increases with the number of participant nucleons, and the ratio between the observed dimuon yield and the expected sources reaches a factor 2 for central Pb-Pb interactions. The kinematical distributions of the measured dimuon excess are compatible with those expected from the open charm contribution.

Υcross section inp+pcollisions ats=200 GeV

“…We use the like-sign combinations of e þ e þ and e À e À to estimate the combinatorial background (see e.g. [30]) via…”

Highlights from the NA60 experiment

“…The dimuon intermediate mass region, located between the φ and the J /ψ peaks, is expected to be well suited for the search of thermal dimuons, due to its relative production yield with respect to the other expected contributions in this region. In p-A collisions, the IMR region is described as a superposition of the Drell-Yan and open charm sources [15]. The NA38, NA50 and HELIOS-3 experiments observed an excess in the IMR with respect to the extrapolation of these sources in S-U and Pb-Pb collisions [15,16].…”

“…In p-A collisions, the IMR region is described as a superposition of the Drell-Yan and open charm sources [15]. The NA38, NA50 and HELIOS-3 experiments observed an excess in the IMR with respect to the extrapolation of these sources in S-U and Pb-Pb collisions [15,16]. The mass spectrum of the excess has the same shape as the open charm.…”

Charmonium and open-charm production in proton–nucleus collisions at the CERN SPS

“…In figure 2(left), we show an example of a fit to the invariant mass spectrum. Thanks to the 'matching' procedure that efficiently rejects muons from π and K decay, the combinatorial background is considerably lower with respect to a previous measurement by NA50 at similar energy [10]. However, due to the low statistics for masses above the J/ψ, the Drell-Yan contribution cannot be constrained directly by the fit.…”