Fluctuations and Correlations from Microscopic Transport Theory

The multiplicity fluctuations in A+A collisions at SPS and RH IC energies are studied within the HSD transport approach. We find a dominant role of the fluctuat ions in the nucleon participant number for the final fluctuations. In order to extract physica l fluctuations one should decrease the fluctuations in the participants number. This can be done considering very central collisions. The system size dependence of the multiplicity fluctuations in central A+A collisions at the SPS energy range ‐ obtained in the HSD and UrQMD transport models ‐ is presented. The results can be used as a ‘background’ for experimental measurements of fluctuations as a signal of the critical point. Event-by-event fluctuations of the K/π, K/p and p/π ratios in A+A collisions are also studied. Event-by-event fluctuations of the kaon to pion number ratio in nucleus-nucleus collisions are studied for SPS and RHIC energies. We find that the HSD model can qualitatively reproduce the measured excitation function for the K/π ratio fluctuations in central Au+Au (or Pb+Pb) collisions from low SPS up to top RHIC energies. The forward-backward correlation coefficient measured by the STAR Collaboration in Au+Au coll isions at RHIC is also studied. We discuss the effects of initial collision geometry and centrality bin definition on correlations in nucleus-nucleus collisions. We argue that a study of the dependence of correlations on the centrality bin definition as well as the bin size may distingu ish between these ‘trivial’ correlations and correlations arising from ‘new physics’.