Nucleus–nucleus collisions at high baryon densities

We study central collision of Pb + Pb at 20, 40, 80 and 160 A GeV within the UrQMD transport approach and compare rapidity distributions of π−,K+,K− and Λ with the recent measurements from the NA49 Collaboration at 40, 80 and 160 A GeV. It is found that the UrQMD model reasonably describes the data, however, systematically overpredicts the π− yield by ∼20%, whereas the K+ yield is underestimated by ∼15%. The K− yields are in a good agreement with the experimental data, the Λ yields are also in a reasonable correspondence with the data for all energies. We find that hadronic flavour exchange reactions largely distort the information about the initial strangeness production mechanism at all energies considered.