J/ψsuppression and low mass dileptons in the NA60 experiment

Abstract

The NA60 experiment, at CERN SPS, has studied muon pairs production in p-A and A-A collisions at 158 GeV. After an introduction to explain why the dimuon measurements are a useful tool to investigate the formation of a deconfined medium and how these measurements can be performed, from an experimental point of view, a review of the status of the field is presented, with particular emphasis on the NA60 results. The NA60 experimental apparatus, in fact, allows to perform high precision measurements, and therefore to obtain high quality results. Concerning the so called “low mass region” (0.2 annihilation through the ρ . Furthermore, the associated space-time averaged ρ spectral function shows a strong broadening, but no shift in mass. The extraction of the inverse slope parameter Teff , from the transverse momentum spectra, allows an even deeper understanding. Teff rises with mass up to the ρ, followed by a sudden decrease. While the initial rise is consistent with the expectations for the radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow, which may be of partonic origin. The “high mass region” ( Mμμ > 2.9 GeV) is dominated by the J/ψ whose suppression is usually considered one of the main signatures for the formation of a deconfined medium. However also cold nuclear matter effects, not related to the hot matter, may reduce the J/ψ yield. Therefore, the study of p-A collisions allows the determination of the J/ψ behaviour in cold nuclear matter, representing the reference with respect to which the J/ψ yield in A-A collisions must be compared. NA60 results show that indeed an “anomalous suppression” is present in central In-In collisions. However, it is clear that to correctly quantify the amount of suppression exceeding the one due to cold nuclear matter, a precise determination of this reference must be performed. To advance in this direction, NA60 has also collected high quality p-A data at the same energy as the one of the A-A interactions.