Model dependence of the number of participant nucleons and observable consequences in heavy-ion collisions

TL;DR

This paper investigates how the estimated number of participant nucleons in heavy-ion collisions depends heavily on the model used, affecting centrality determination and suggesting alternative observables like baryon number balance.

Contribution

It demonstrates the strong model dependence of $N_{part}$ estimates and proposes using total baryon number balance as a more reliable centrality indicator.

Findings

$N_{part}$ varies significantly between models

Impact parameter and charged particle count are more model-independent

Baryon number balance may serve as an alternative centrality measure

Abstract

The centrality determination and the estimated fluctuations of number of participant nucleons $N_{part}$ in Au-Au collisions at 1.23 $A$GeV beam kinetic energy suffers from severe model dependencies. Comparing the Glauber Monte Carlo (MC) and UrQMD transport models, it is shown that $N_{part}$ is a strongly model dependant quantity. In addition, for any given centrality class, Glauber MC and UrQMD predicts drastically different $N_{part}$ distributions. The impact parameter $b$ and the number of charged particles $N_{ch}$ on the other hand are much more correlated and give an almost model independent centrality estimator. It is suggested that the total baryon number balance, from integrated rapidity distributions, can be used instead of $N_{part}$ in experiments. Preliminary HADES data show significant differences to both, UrQMD simulations and STAR data in this respect.