Multiplicity fluctuations in proton-proton and nucleus-nucleus collisions

We study the scaled variances of multiplicity fluctuations in nucleus-nucleus collisions at SPS and RHIC energies within the HSD transport model. The HSD results are compared with proton-proton data and with predictions of the hadron-resonance gas statistical model. We find that the HSD scaled variances ωi for negatively, positively, and all charged hadrons in central nucleus-nucleus collisions remain close to the ωi values in proton-proton collisions and increase with collision energy as the corresponding multiplicities per participating nucleon. The statistical model predicts very different behavior of ωi. However, a comparison with preliminary NA49 data for the most central Pb + Pb collisions at SPS energies does not permit to distinguish the HSD and statistical model results. New measurements of the multiplicity fluctuations in nucleus-nucleus collisions in a wide energy region with large acceptance are needed to allow for a proper determination of the underlying dynamics.