Speaker: Alexei KOUREPINE
Title
Charmonia States Suppression and Transverse Momentum Distribution in Pb-Pb Collision at the CERN SPS
Abstract
The results of the NA50 experiment at CERN SPS on charmonia states production in Pb -Pb collisions at 158 GeV/c per nucleon will be presented. An anomalous charmonium suppression in central collisions was observed. The ratio of the J/psi production cross section to the Drell-Yan cross section measured for several lighter colliding systems extending from p-p to S-U exibits the exponential decrease due to ordinary absorption of the J/psi in nuclear matter. For Pb-Pb interactions the experimental data agree with this ordinary absorption curve only for the most peripheral collisions. By increasing the centrality of the interaction, a threshold effect is observed for the onset of the anomalous suppression. The experimental data agree well with the prediction of a quark-gluon phase transition formed during the collision and with the interpretation of charmonium suppression due to Debye color screening in a deconfined plasma. The analysis of the transverse momentum distributions dependence on the centrality of collision was made. It was shown that the maximum J/psi suppression is observed for low Pt in central collisions. The change of the Pt distributions with the centrality of the collision will be discussed in the frames of the models of initial scattering of projectile and target partons and quark-gluon plasma formation.





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