Enhanced strange particle yields - signal of a phase of massless particles?

The yields ol strange particles are calculated with the ultrarelativistic q uantum molecular dynamics model for p, Pb(158 A GeV)Pb collisions and compa red with experimental data. The yields are enhanced in central collisions c ompared with proton-induced or peripheral Pb + Pb collisions. The enhanceme nt is due to secondary interactions. Nevertheless, only a reduction of the quark masses or equivalently an increase of the string tension provides an adequate description of the large observed enhancement factors (WA97 and NA 49). Furthermore, the yields of unstable strange resonances such as the Lam bda* or the phi are considerably affected by hadronic rescattering of the d ecay products.