An operational definition of quark and gluon jets

TL;DR

This paper proposes a practical, data-driven operational definition of quark and gluon jets at the hadron level, enabling their identification and separation in collider experiments without relying on per-jet labels.

Contribution

It introduces a distribution-level, data-driven framework for defining and distinguishing quark and gluon jets, applicable directly to experimental data.

Findings

Successfully extracts pure quark and gluon distributions from collider data.

Demonstrates the feasibility of the operational definition using Z+jet and dijet samples.

Provides a practical method for jet flavor identification at the hadron level.

Abstract

While "quark" and "gluon" jets are often treated as separate, well-defined objects in both theoretical and experimental contexts, no precise, practical, and hadron-level definition of jet flavor presently exists. To remedy this issue, we develop and advocate for a data-driven, operational definition of quark and gluon jets that is readily applicable at colliders. Rather than specifying a per-jet flavor label, we aggregately define quark and gluon jets at the distribution level in terms of measured hadronic cross sections. Intuitively, quark and gluon jets emerge as the two maximally separable categories within two jet samples in data. Benefiting from recent work on data-driven classifiers and topic modeling for jets, we show that the practical tools needed to implement our definition already exist for experimental applications. As an informative example, we demonstrate the power of our operational definition using Z+jet and dijet samples, illustrating that pure quark and gluon distributions and fractions can be successfully extracted in a fully well-defined manner.