# Systematics of quark/gluon tagging

## TL;DR

This paper investigates the complexities of quark/gluon jet discrimination beyond leading-order approximations, analyzing perturbative and nonperturbative effects through simulations and calculations, with implications for collider physics.

## Contribution

It provides a detailed study of higher-order effects on quark/gluon tagging and proposes key measurements to enhance jet modeling at the LHC.

## Key findings

- Higher-order effects significantly influence quark/gluon discrimination.
- Nonperturbative hadronization impacts jet substructure observables.
- Proposed measurements can reduce uncertainties in jet tagging models.

## Abstract

By measuring the substructure of a jet, one can assign it a "quark" or "gluon" tag. In the eikonal (double-logarithmic) limit, quark/gluon discrimination is determined solely by the color factor of the initiating parton (C_F versus C_A). In this paper, we confront the challenges faced when going beyond this leading-order understanding, using both parton-shower generators and first-principles calculations to assess the impact of higher-order perturbative and nonperturbative physics. Working in the idealized context of electron-positron collisions, where one can define a proxy for quark and gluon jets based on the Lorentz structure of the production vertex, we find a fascinating interplay between perturbative shower effects and nonperturbative hadronization effects. Turning to proton-proton collisions, we highlight a core set of measurements that would constrain current uncertainties in quark/gluon tagging and improve the overall modeling of jets at the Large Hadron Collider.