MEASURING DIMUONS IN HEAVY ION COLLISIONS - Highlights from the NA60 experiment at CERN

par Dr Michele Floris (INFN/Università - Cagliari)

jeudi 5 mars 2009, 11:00 → 12:00 Europe/Paris

Salle Master (IPHC/DRS)

NA60 is a fixed target heavy ion experiment at the CERN SPS. Thanks to state-of-the-art silicon detectors in the vertex region, it measured dimuon production in indium-indium collisions at 158 AGeV with unprecedented precision. After a brief review of the motivations which lead to the NA60 experiment, the detector concept will be introduced, with particular emphasis on the silicon vertex tracker. Some of the highlights of the experiment will then be discussed. A clear excess of dimuon pairs above expected sources was measured in the low and intermediate mass regions. In the low mass region, this is consistent with a dominant contribution from $\pi\pi \to \rho \to \mu\mu$ annihilation. In the intermediate mass region, the excess is found to be prompt, not due to enhanced charm production. The slope parameter $T_{eff}$ extracted from the spectra rises with dimuon mass up to the $\rho$, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline indicates a transition to an emission source with much smaller flow, possibly of partonic origin. $\phi\to\mu\mu$ production was measured with high statistics and good $p_T$ coverage. This is discussed in the framework of the $\phi$ puzzle, i.e. the difference in the inverse $T_{eff}$ slopes and absolute yields measured by NA49 and NA50 in the kaon and lepton channel, respectively. The slope parameter $T_{eff}$ shows a rapid increase with centrality, followed by a saturation. Variations of $T_{eff}$ with the fit range of the order of 10~MeV were observed, possibly as a consequence of radial flow. The $\phi$ meson yield normalized to the number of participants increases with centrality and is consistently higher than the yield measured by the NA49 experiment at any centrality. Finally, the motivation and possible strategies for a continuation of the experiment at low energy will be discussed.