Soft transverse expansion in Pb(158 AGeV) on Pb collisions: preequilibrium motion or 1st 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 preequilibrium 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 1st order transition.