Isolated photons in perturbative QCD

TL;DR

Addresses defining isolated-photon cross sections in perturbative QCD and suppressing fragmentation contributions. Introduces a smooth cone isolation (Frixione isolation) with a delta-dependent energy bound X(delta), ensuring IR safety and limiting fragmentation to a zero-measure region. Provides a precise, IR-safe observable via a measurement function that integrates with jet algorithms, applicable to $e^+e^-$, hadron-hadron, and photon-hadron collisions, with fragmentation effectively neglected. The approach enables robust, perturbatively calculable predictions for direct photons and has practical implications for QCD studies and Higgs searches.

Abstract

I present a definition of the cross section for the production of an isolated photon plus $n$ jets which only depends upon direct photon production, and it is independent of the parton-to-photon fragmentation contribution. This prescription, based on a modified cone approach which implements the isolation condition in a smooth way, treats in the same way quarks and gluons and can be directly applied to experimental data in hadron-hadron, photon-hadron and $\epem$ collisions. The case of several, isolated photons in the final state can also be dealt with in the very same way.