Identification and Rejection of Fake Reconstructed Jets From a Fluctuating Heavy Ion Background in ATLAS

TL;DR

This paper addresses the challenge of identifying and rejecting fake jets caused by background fluctuations in heavy ion collisions, proposing a new variable $ ext{Σ}j_{T}$ for effective fake jet suppression in ATLAS data analysis.

Contribution

Introduction of the $ ext{Σ}j_{T}$ variable to efficiently identify and reject fake reconstructed jets in heavy ion collision data, improving jet purity.

Findings

Fake jet rate exceeds binary-scaled p+p jet rate below 50 GeV

The $ ext{Σ}j_{T}$ variable reduces fake jets by a factor of 100

The method is effective regardless of jet energy profile modifications

Abstract

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 background 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 constituent's $E_{T}$ 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.