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 $\omega_i$ for negatively, positively, and all charged hadrons in central nucleus-nucleus collisions remain close to the $\omega_i$ 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 $\omega_i$. 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.