sPHENIX Characterization of the Underlying Event in 200 GeV Au+Au Collisions

sPHENIX is the newest experiment at the Relativistic Heavy Ion Collider (RHIC), designed for precision measurements of hard probes of the QGP at RHIC collision energies. Jets in heavy ion collisions sit on top of a large uncorrelated fluctuating background due to the large energy density which must be understood to be corrected. We report the first characterizations of the underlying event and jet background fluctuations at RHIC, as well as direct comparisons of the fluctuations resulting from methods typically used by different heavy ion experiments, using $\sqrt{{s}_{NN}}$ = $200$ GeV Au+Au collision data collected during the 2023 commissioning run of the sPHENIX detector. We characterized the underlying event using a multi-faceted approach, including unbiased sampling of calorimeter window areas and random cones ($R=0.4$), as well as methods sensitive to jet reconstruction effects such as embedding high $E_T$ probes ($E_{T}^{\text{Probe}} = 30$ GeV) and PYTHIA8 + GEANT embedded in minimum bias Au+Au data taken in 2023. The non-Poissionian background fluctuations for multiple jet background subtraction methods~\cite{PhysRevC.86.024908,Mengel:2023mnw,Cacciari:2008gn} are characterized with the standard deviation of residual energy $\sigma(\delta E_T)$ observed both in random cones and high $E_T$ probes. We study the underlying event fluctuations measured in $\sqrt{s_{NN}} = 200$ GeV Au+Au collisions to simulated HIJING events and compare the impact of jet background fluctuations on jet reconstruction reconstruction between the various methods.