Influence of Impact Parameter Fluctuations on Transverse Momentum Fluctuations

TL;DR

This paper investigates how impact parameter fluctuations influence transverse momentum fluctuations in heavy-ion collisions, explaining experimental observations with UrQMD simulations and emphasizing their importance for fluctuation measurements.

Contribution

It demonstrates that impact parameter fluctuations significantly affect transverse momentum fluctuation measurements, especially for positively charged particles, using UrQMD model analysis.

Findings

Impact parameter fluctuations cause observed pT fluctuations for positively charged particles.

UrQMD simulations reproduce the energy-dependent behavior of fluctuations.

Protons' correlation with impact parameter explains the fluctuation patterns.

Abstract

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.