Influence of Impact Parameter Fluctuations on Transverse Momentum Fluctuations

The preliminary NA49 results on the energy dependence of transverse momentum fluctuations over the whole SPS energy range exhibit an unexpected effect. The $\Phi_{p_{T}}$ fluctuation measure - used by the NA49 experiment - manifests a different behavior for different charge combinations. Whereas the $\Phi_{p_{T}}$ is consistent with zero and independent of energy for negatively charged particles, it significantly increases for both all charged and positively charged particles at lower SPS energies. The string-hadronic model UrQMD is applied here to explain this effect. The UrQMD simulations show that the number of protons is strongly correlated with impact parameter and that the event-by-event impact parameter fluctuations are responsible for the event-by-event transverse momentum fluctuations of positively charged and all charged particles where protons are included. The observations presented in this article are important for all experiments measuring event-by-event fluctuations, especially for those using all charged particles in the analysis. The results can be also crucial for detectors with acceptances extending to the beam/target spectator domains.