Latest results from the NA57 experiment.pdfVIP

Latest results from the NA57 experiment G. E. Bruno on behalf of the NA57 Collaboration 1 Dipartimento IA di Fisica dell’Università e del Politecnico di Bari and INFN, Bari, Italy Abstract. The NA57 experiment at the CERN SPS has measured the production of strange and multi-strange particles in Pb-Pb (and p-Be) collisions at mid-rapidity. The collective dynamics of the collision is studied in the transverse and longitudinal directions as a function of centrality and beam momentum and analysed based on hydrodynamical models. Central-to-peripheral nuclear modification factors at 158 A GeV/c are presented and compared with other measurements and theory. Keywords: QGP, Heavy Ions, Strange particles, collective dynamics PACS: 12.38.Mh, 25.75.Nq, 25.75.Dw INTRODUCTION The measurement of strange particle production provides a powerful tool to study the dynamics of the reaction in heavy-ion collisions. In particular, an enhanced production of strange particles in nucleus–nucleus collisions with respect to proton–induced reactions was suggested long ago as a possible signature of the phase transition from colour confined hadronic matter to a Quark Gluon Plasma (QGP) [1]. Results on hyperon enhancements at 160 and 40 A GeV/c obtained by NA57 can be found in reference [2] and they will not be discussed in this paper. Important insights into the reaction dynamics can been determined from the p dis- tributions of strange particles: at low transverse momenta (p 
2 GeV/c) they allow to study the transverse expansion of the collision [3]; at higher values (p  2 GeV/c) the p dependence of the nuclear modification factors can be used to probe the properties of the medium produced after the collisions [4, 5]. Rapidity distributions provide a tool to study the longitudinal dynamics. Hydrody- namical properties of the expanding matter created in heavy ion reactions have been discussed by Landau [6] and Bjorken [7] using different initial conditions. In both sce- narios, therma