An improved method to access initial states in relativistic heavy-ion collisions

TL;DR

This paper improves a method for selecting specific initial states in relativistic heavy-ion collisions, enabling better analysis of initial conditions by reducing impact parameter fluctuation effects.

Contribution

It introduces an enhanced spectator neutron binning technique that more effectively isolates initial state properties from impact parameter variations.

Findings

Higher initial state density correlates with increased particle production.

Spectator binning reduces sensitivity to impact parameter fluctuations.

Method helps distinguish geometric effects from fluctuations in collision data.

Abstract

Observables in heavy-ion collisions are generally categorized into centralities, which reflect an average over events within a range of impact parameter including a wide variety of initial state configurations. A multiple binning method using spectator neutrons within each centrality has been previously shown to provide access to events with rare initial state conditions. This work suggests an improvement in quantifying the difference between standard centrality and spectator neutron binning towards accessing the initial state properties. A selection of events with higher initial state density at fixed participating nucleon number was observed to result in larger final state particle production and smaller elliptic flow. The relative difference between observables in centrality and spectator binning shows reduced sensitivity for the observables dominated by impact parameter fluctuations in initial state such as triangular flow. This property makes the spectator binning method a good candidate to separate geometric contributions from random fluctuations in initial state towards final state observables.