Quark and Gluon Tagging at the LHC

Abstract

Being able to distinguish light-quark jets from gluon jets on an event-by-event basis could significantly enhance the reach for many new physics searches at the Large Hadron Collider. Through an exhaustive search of existing and novel jet substructure observables, we find that a multivariate approach can filter out over 95% of the gluon jets while keeping more than half of the light-quark jets. Moreover, a combination of two simple variables, the charge track multiplicity and the pT -weighted linear radial moment (girth), can achieve similar results. While this pair appears very promising, our study is only Monte Carlo based, and other discriminants may work better with real data in a realistic experimental environment. To that end, we explore many other observables constructed using different jet sizes and parameters, and highlight those that deserve further theoretical and experimental scrutiny. Additional information, including distributions of around 10,000 variables, can be found on the website http://jets.physics.harvard.edu/qvg.

Associated jet and subjet rates in light-quark and gluon jet discrimination

“…The number of subjets as a quark-gluon separation variable was considered earlier in ref. [1]. In this study, we compute the subjet rates to NDLA accuracy, and show a detailed comparison with different MC's.…”

“…A large number of variables have been surveyed in the context of quark-gluon discrimination, constructed out of either track based observables or calorimeter based ones [1][2][3][4][5]. While the former category has the practical advantage of being more accurate due to better track momentum resolution as well as being less prone to pile-up contamination, the latter category can be used for jets with larger rapidities outside the tracker coverage.…”

“…(n ch ), the jet width [1] and energy-energy-correlation (EEC) angularity [4]. The jet width is defined as…”

“…This includes both discrete variables like the number of charged tracks inside the jet cone, as well as continuous ones like the width of a jet and energy-energy-correlation (EEC) angularity [1][2][3][4][5]. ATLAS and CMS collaborations have also studied the discrimination of light quarks from gluons along these lines with the 7 and 8 TeV LHC data respectively [6,7].…”

“…Here, quark-or gluon-initiated jets (henceforth simply referred to as quark and gluon jets) refer to the parton in the hard process at leading order in perturbation theory that initiates the parton shower. Based on the difference in the radiation pattern of quarks and gluons, a likelihood based discriminant can be built to separate decay jets from QCD radiation jets with a certain efficiency [1].…”