Light-quark and gluon jet discrimination in pp collisions at $\sqrt{s}$ = 7 TeV with the ATLAS detector

TL;DR

This paper develops and validates a likelihood-based method to distinguish quark-initiated jets from gluon-initiated jets in proton-proton collisions at 7 TeV, demonstrating effective tagging performance with real data and Monte Carlo comparisons.

Contribution

It introduces a new likelihood-based discriminant for quark-gluon jet identification validated with ATLAS data, showing systematic uncertainties and differences from Monte Carlo simulations.

Findings

Light-quark jets tagged at 50% efficiency with 25% mis-tag rate.

Discriminant performance consistent within 10-20% systematic uncertainty.

Gluon-jet rejection in data is lower than Pythia 6 Monte Carlo predictions.

Abstract

A likelihood-based discriminant for the identification of quark- and gluon-initiated jets is built and validated using 4.7 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 7 TeV collected with the ATLAS detector at the LHC. Data samples with enriched quark or gluon content are used in the construction and validation of templates of jet properties that are the input to the likelihood-based discriminant. The discriminating power of the jet tagger is established in both data and Monte Carlo samples within a systematic uncertainty of 10-20%. In data, light-quark jets can be tagged with an efficiency of 50% while achieving a gluon-jet mis-tag rate of 25% in a $p_T$ range between 40 GeV and 360 GeV for jets in the acceptance of the tracker. The rejection of gluon-jets found in the data is significantly below what is attainable using a Pythia 6 Monte Carlo simulation, where gluon-jet mis-tag rates of 10% can be reached for a 50% selection efficiency of light-quark jets using the same jet properties.