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 (CF versus CA). 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.
Highlights
Gluon radiates soft gluons proportional to CA = 3, and quark/gluon tagging performance is a function of CA/CF
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
There is the ambiguity of which parton shower to use, so we investigate quark/gluon radiation patterns in several event generators: Pythia 8.215 [39], Herwig 2.7.1 [40, 41], Sherpa 2.2.1 [43], Vincia 2.001 [44], Deductor 1.0.2 [45], Ariadne 5.0.β [46], and Dire 1.0.0 [48]
Summary
A quark operator appearing in a hard matrix element in the context of a factorization theorem. This is clearly an idealization, though one that makes some sense in the context of e+e− collisions, since truth-level “quark” and “gluon” labels can be defined by the Lorentz structure of the production vertex. This in turn would help λκβ become a more robust and powerful discriminant in searches for new physics beyond the standard model
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