The elliptic flow of neutral transverse energy in heavy ion interactions at SPS energies

The threshold effect in the J/ψ suppression seen in the NA50 data in lead on lead interactions at the CERN SPS [1,2] can't be taken into account by conventional models. Other candidate models [3] use a phase transition, and therefore let suppose that the thermodynamic variables are defined everywhere that is the so-called thermalization. The in-plane elliptic flow, which has been proposed [4] in order to probe the thermalization at these beam energies, is found to be actually present in lead on lead interactions at 158A GeV/ c , but can't be observed in sulphur on sulphur at 200A GeV/ c in the NA38 data [5,6].