SEMINAR

First Results of the NA60 Experiment at~the~CERN~SPS

Dr. Hermine W\"ohri

(CERN,CFTP)

Sala P7, IST, Edifício Pós-Graduação
Tuesday, November 16th, 2004 at 04:00 PM

Abstract

The study of dilepton production in ultra-relativistic heavy-ion collisions is expected to provide crucial improvements in our understanding of the interactions at play in dense and hot QCD matter. The anomalous suppression of charmonium resonances (NA50), the dimuon enhancement in the intermediate mass region (NA50) and the excess of low-mass dielectrons (CERES) are among the key observations performed in nuclear collisions at the SPS. The presently available results are likely to the formation of a state where the quarks and gluons become momentarily, at least in high-energy central Pb-Pb collisions. However, several issues on the interpretation of such data remain unclear and are presently b eing addressed by NA60. This new experiment complements the dimuon spectrometer and the zero degree calorimeter previously used by NA50 with a vertex spectrometer made of Silicon pixel and/or microstrip placed inside a 2.5 T dipole magnetic field, just after the target. This allows us to track the muons before they suffer multiple scattering and energy loss in the thick hadron absorber, placed upstream of the muon spectrometer, thereby vastly improving the dimuon mass resolution and the signal to background ratio. We will also be able to separate prompt dimuons (Drell-Yan, thermal dimuons) from muon pairs coming from displaced vertices (decays of D mesons). In 2003, NA60 collected around 230 million dimuon triggers from Indium-Indium collisions at 158 GeV/nucleon, with an excellent dimuon mass resolution, broad phase space very good signal-to-background ratio and record statistics. The presentation will show several results from this run, which places NA60 on a solid path to significantly contribute to the exciting field of QCD and hadronic interactions, besides improving our knowledge of dilepton and charmonia production in heavy-ion collisions. For the interpretation of the In-In results it is crucial to perform comparisons to reference data, collected in proton-nucleus collisions. Besides, the results achieved in these collision systems are interesting by themselves. NA60 is currently collecting data with 400 and 158 GeV protons incident on Be, Al, Cu, In, Pb and U targets. In this presentation we will also show results from the data collected during the short p-A run in 2002, yielding the nuclear dependence of the $\eta$, $\rho/\omega$ and $\phi$ production cross-sections, apart from the elementary pp production cross-sections of these mesons at $\sqrt{s}=27$ GeV.