QCD anatomy of photon isolation

TL;DR

This paper provides a detailed QCD-based theoretical framework for understanding and calculating photon isolation effects in high-energy physics, including resummation of logarithms and a conversion formula for different isolation criteria.

Contribution

It introduces a comprehensive factorization and resummation approach for photon isolation effects, including the development of a fragmentation function and a formula to translate NNLO results between isolation schemes.

Findings

Resummation of logarithms of the cone radius R.

Factorization of the cone fragmentation function into collinear and soft functions.

A simple formula for converting NNLO cross sections between isolation schemes.

Abstract

To separate the energetic photons produced in hard scattering processes from those from other sources, measurements impose isolation requirements which restrict the hadronic radiation inside a cone around the photon. In this paper, we perform a detailed factorization analysis of the QCD effects associated with photon isolation. We show that for small cone radius $R$, photon isolation effects can be captured by a fragmentation function describing the decay of a parton into a photon accompanied by hadronic radiation. We compute this fragmentation function for different isolation criteria and solve the associated renormalization group equations to resum logarithms of $R$. For small isolation energy, the cone fragmentation function factorizes further, into collinear functions describing energetic quarks and gluons near the cone boundary and functions encoding their soft radiation emitted into the cone. Based on this factorization we also resum the non-global logarithms of the ratio of the photon energy and the isolation energy, so that we control all logarithmically enhanced terms in the cross section. Finally, we provide a simple formula to convert NNLO cross section results from smooth-cone isolation to fixed-cone isolation.