Deciphering initial states of high energy heavy-ion collisions using spectators

The standard technique of categorizing observables into centralities based on final state multiplicity in heavy-ion collisions only reflects an average over events, including a wide variety of initial-state configurations. It thus restricts the study to a limited range of initial conditions. There are also event-by-event geometric and quantum fluctuations in the initial condition of heavy-ion collisions. A multiple binning method using spectator neutrons within each centrality has been shown to provide access to events with rare initial conditions. In this work, we developed a new method to quantify the difference in slope of correlations between observables in standard centrality and spectator neutron binning to access essential properties of the initial states in heavy-ion collisions. Selection of events with a wide range of initial state densities at fixed participating nucleon numbers via spectator neutron binning would result in a different final-state particle production and elliptic flow. The relative difference between observables in centrality and spectator binning shows reduced sensitivity for the observables dominated by impact parameter fluctuations in the initial state, such as triangular flow. Therefore, the multiple binning method is a good candidate for separating geometric contributions from random fluctuations in the initial state toward final-state observables.