Probing Pb+Pb collisions at $\sqrt{S_{NN}}=2760$ GeV with spectators

TL;DR

This paper introduces spectator neutron binning in heavy-ion collisions to explore a wider range of initial conditions, revealing new scaling behaviors and enabling detailed study of viscosity effects beyond traditional centrality analysis.

Contribution

It proposes a novel spectator neutron binning method to access diverse initial conditions and uncovers how this affects flow scaling relations in heavy-ion collisions.

Findings

Spectator binning allows independent variation of $oldsymbol{ ext{E}_2}$ and $oldsymbol{ ext{E}_3}$.

Standard flow scaling relations are broken by spectator binning.

Acoustic scaling relation remains valid across different binning methods.

Abstract

There is event by event geometric as well as quantum fluctuations in the initial condition of heavy-ion collisions. The standard technique of analysing heavy-ion collisions in bins of centrality obtained from final state multiplicity averages out the various initial configurations and thus restricts the study to only a limited range of initial conditions. In this paper, we propose an additional binning in terms of total spectator neutrons in an event. This offers us a key control parameter to probe events with broader range of initial conditions providing us an opportunity to peep into events with rarer initial conditions which otherwise get masked when analysed by centrality binning alone. We find that the inclusion of spectator binning allows one to vary $\varepsilon_2$ and $\varepsilon_3$ independently. We observe that the standard scaling relation between $\displaystyle{v_2/\varepsilon_2}$ and $\frac{1}{S}\frac{dN_{\text{ch}}}{d\eta}$ exhibited by centrality bins is broken by the spectator neutron bins. However, the acoustic scaling relation between $\displaystyle{\ln\left( v_n/\varepsilon_n\right)}$ and transverse system size holds for both centrality as well as spectator bins for central to mid-central collisions. The introduction of the spectator binning allows us to tune over a wide range viscosity driven effects for events with varying initial states but similar final state multiplicity.