Centrality determination with a forward detector in the RHIC Beam Energy Scan

In heavy ion collisions, centrality is the amount of overlap between nuclei in a collision event and is not a direct observable. Various methods are employed by experiments which use different observables as a proxy for centrality, and these centrality proxies form an important eventwise characteristic for many analyses. In the RHIC Beam Energy Scan (BES), centrality is usually determined at mid-rapidity where pseudorapidity η ≤ 1. Determining centrality within this η acceptance, however, could lead to the autocorrelation effect (ACE), which can suppress values such as kurtosis for net proton distributions (κσ²). Forward η centrality determination could be possible with the STAR Event Plane Detector (EPD), which would avoid ACE; however, the presence at lower collision energy (√s_NN) of spectator protons from the initial collision complicates an EPD based centrality to the point that, when using a simple ``sum of particles" method, EPD centrality is not of sufficient resolution for use in BES analyses. This talk will discuss a technique of linearly weighting η rings of the EPD, which rectifies the forward centrality resolution issue. This technique will be demonstrated using UrQMD simulations of Au+Au collisions in the range of √s_NN = 7.7-200 GeV. Implications of an EPD based centrality on measured κσ² will also be discussed, as well as more sophisticated techniques that could be employed to form an EPD centrality measure.