Particle number fluctuations in high-energy nucleus-nucleus collisions from microscopic transport approaches

Event-by-event multiplicity fluctuations in nucleus-nucleus collisions are studied within the hadron-string dynamics (HSD) and ultra-relativistic quantum molecular dynamics (UrQMD) transport models. The scaled variances of negative, positive, and all charged hadrons in Pb+Pb at 158A GeV are analyzed in comparison to the data from the NA49 Collaboration. We find a dominant role of the fluctuations in the nucleon participant number for the final hadron multiplicity fluctuations. This fact can be used to check different scenarios of nucleus-nucleus collisions by measuring the final multiplicity fluctuations as a function of collision centrality. The analysis reveals surprising effects in the recent NA49 data which indicate a rather strong mixing of the projectile and target hadron production sources even in peripheral collisions.