Photon isolation and jet substructure

TL;DR

This paper introduces soft drop isolation, a new photon isolation method inspired by jet substructure, which is collinear safe, democratic, and useful for probing QED splitting functions within jets.

Contribution

The paper presents soft drop isolation, a novel photon isolation criterion that is collinear safe, democratic, and enables the study of photon subjets and QED splitting functions.

Findings

Soft drop isolation is equivalent to Frixione isolation at leading order.

It allows treating photons and hadrons equivalently in jet clustering.

The method successfully exposes the QED splitting function in jet substructure.

Abstract

We introduce soft drop isolation, a new photon isolation criterion inspired by jet substructure techniques. Soft drop isolation is collinear safe and is equivalent to Frixione isolation at leading non-trivial order in the small R limit. However, soft drop isolation has the interesting feature of being democratic, meaning that photons can be treated equivalently to hadrons for initial jet clustering. Taking advantage of this democratic property, we define an isolated photon subjet: a photon that is not isolated from its parent jet but is isolated within its parent subjet after soft drop declustering. The kinematics of this isolated photon subjet can be used to expose the QED splitting function, in which a quark radiates a photon, and we verify this behavior using both a parton shower generator and a perturbative calculation in the collinear limit.