Multiplicity Fluctuations in Nucleus-Nucleus Collisions: Dependence on Energy and Atomic Number

TL;DR

This study analyzes how event-by-event multiplicity fluctuations in nucleus-nucleus and proton-proton collisions depend on energy and atomic number, using transport models to inform the search for the QCD critical point.

Contribution

It provides a detailed comparison of multiplicity fluctuations across different collision systems and energies, highlighting the importance of centrality selection in fluctuation analysis.

Findings

Participant number fluctuations dominate at finite impact parameters.

Centrality selection reduces participant fluctuation effects.

Results guide optimal experimental conditions for QCD critical point search.

Abstract

Event-by-event multiplicity fluctuations in central C+C, S+S, In+In, and Pb+Pb as well as p+p collisions at bombarding energies from 10 to 160 AGeV are studied within the HSD and UrQMD microscopic transport approaches. Our investigation is directly related to the future experimental program of the NA61 Collaboration at the SPS for a search of the QCD critical point. The dependence on energy and atomic mass number of the scaled variances for negative, positive, and all charged hadrons is presented and compared to the results of the model of independent sources. Furthermore, the nucleus-nucleus results from the transport calculations are compared to inelastic proton-proton collisions for reference. We find a dominant role of the participant number fluctuations in nucleus-nucleus reactions at finite impact parameter $b$. In order to reduce the influence of the participant numbers fluctuations on the charged particle multiplicity fluctuations only the most central events have to be selected. Accordingly, the samples of the 1% most central nucleus-nucleus collisions with the largest numbers of the projectile participants are studied. The results are compared with those for collisions at zero impact parameter. A strong influence of the centrality selection criteria on the multiplicity fluctuations is pointed out. Our findings are essential for an optimal choice of colliding nuclei and bombarding energies for the experimental search of the QCD critical point.