Transverse flow in heavy ion interactions at the CERN SPS

In the search for the quark-gluon plasma at CERN SPS, several signals were observed suggesting that this new state of matter was actually produced in the interactions induced by the lead beam at 158A GeV. The transverse flow phenomena characterize the distribution of the particles in the final state of the collision, and are expected to provide some indications about the initial state. More precisly, the existence of an elliptic flow directed along the reaction plane in the case of the non central collisions of two spherical nuclei, has been proposed to probe the thermalisation of the initial state. Indeed, the NA50 collaboration has measured these kind of effects in Pb-Pb collisions at 158A GeV, and we can conclude that the thermalisation of the initial state is actually present in these interactions. Conversely, there is no evidence for such a thermalisation in the NA38 data which were obtained with the sulphur beam at 200A GeV, neither in S-S nor in S-Cu interactions.