Soft transverse expansion in Pb(158 AGeV) on Pb collisions: preequilibrium motion or first order phase transition?

Transverse expansion of centrally produced matter in Pb on Pb collisions at beam energies around 158 AGeV appears to be rather ‘soft’. Two possible reasons — an extended preequibrium stage and a first order phase transition from a quark-gluon plasma into hadronic matter — are discussed. The softening of transverse expansion caused by preequilibrium dynamics is estimated with the aid of the transport model RQMD which does not contain a first order phase transition. It is found that the anisotropy of transverse flow in non-central reactions is very different in the preequilibrium and hydrodynamic scenarios even if the latter are based on a strong 1 st order transition.