Reconstructed jets in heavy ion collisions are a crucial tool for understanding the quark-gluon plasma. The separation of jets from the underlying event is necessary particularly in central heavy ion reactions in order to quantify medium modification
s of the parton shower and the response of the surrounding medium itself. There have been many methods proposed and implemented for studying the underlying event substructure in proton-proton and heavy ion collisions. In this paper, we detail a method for understanding underlying event contributions in Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV utilizing the HIJING event generator. This method, extended from previous work by the ATLAS collaboration, provides a well-defined association of truth jets from the fragmentation of hard partons with reconstructed jets using the anti-$k_T$ algorithm. Results presented here are based on an analysis of 750M minimum bias HIJING events. We find that there is a substantial range of jet energies and radius parameters where jets are well separated from the background fluctuations (often termed fake jets) that make jet measurements at RHIC a compelling physics program.
Full jet reconstruction in relativistic heavy ion collisions provides new and unique insights to the physics of parton energy loss. Because of the large underlying event multiplicity in $A+A$ collisions, random and correlated fluctuations in the back
ground can result in the reconstruction of fake jets. These fake jets must be identified and rejected to obtain the purest jet sample possible. A large but reducible fake rate of jets reconstructed using an iterative cone algorithm on HIJING events is observed. The absolute rate of fake jets exceeds the binary-scaled p+p jet rate below 50 GeV and is not negligible until 100 GeV. The variable $Sigma j_{T}$, the sum of the jet constituents $E_{T}$ perpendicular to the jet axis, is introduced to identify and reject fake jets at by a factor of 100 making it negligible. This variable is shown to not strongly depend on jet energy profiles modified by energy loss. By studying azimuthal correlations of reconstructed di-jets, the fake jet rate can be evaluated in data.
An analysis of inclusive pion production in proton-beryllium collisions at 6.4, 12.3, and 17.5 GeV/c proton beam momentum has been performed. The data were taken by Experiment 910 at the Alternating Gradient Synchrotron at the Brookhaven National Lab
oratory. The differential $pi^+$ and $pi^-$ production cross sections ($d^2sigma/dpdOmega$) are measured up to 400 mRad in $theta_{pi}$ and up to 6 GeV/c in $p_{pi}$. The measured cross section is fit with a Sanford-Wang parameterization.
