Isolating prompt photons with narrow cones

S Catani,J Ph Guillet,M Fontannaz,E Pilon

doi:10.1007/jhep09(2013)007

Abstract

Abstract We discuss the isolation of prompt photons in hadronic collisions by means of narrow isolation cones and the QCD computation of the corresponding cross sections. We reconsider the occurence of large perturbative terms with logarithmic dependence on the cone size and their impact on the fragmentation scale dependence. We cure the apparent perturbative violation of unitarity for small cone sizes, which had been noticed earlier in next-to-leading-order (NLO) calculations, by resumming the leading logarithmic dependence on the cone size. We discuss possible implications regarding the implementation of some hollow cone variants of the cone criterion, which simulate the experimental difficulty to impose isolation inside the region filled by the electromagnetic shower that develops in the calorimeter.

Highlights

We discuss possible implications regarding the implementation of some hollow cone variants of the cone criterion, which simulate the experimental difficulty to impose isolation inside the region filled by the electromagnetic shower that develops in the calorimeter
We have considered the standard cone isolation criterion that is used to measure promptphoton cross sections at TeV colliders
The dependence on the cone size R is dominantly logarithmic at small R, and the NLO predictions become unreliable when R becomes too small

Summary

Origin of the logarithmic dependence on the cone size at the NLO

We consider the isolation criterion in eqs. (1.1) and (1.2) and the ensuing small-R behaviour of the isolated-photon cross section at the NLO. We start with the contribution where the photon is accompanied by collinear parton radiation inside a cone of radius R (see figure 1) This part of the HO correction to the Born level direct (D) cross section leads to the NLO fragmentation (F) contribution. The effects of collinear parton radiation inside and outside the cone around the parton d, and the ensuing mismatch of the ln R dependence in the NLO isolated cross section produce a contribution that is analogous to the term in eq (2.6). The all-order summation of the ln R terms should cure the pathological behaviour observed at the NLO in both the direct and fragmentation components of the isolated cross section

Multiparton collinear radiation and ln R dependence

Isolated cross section and resummation of the ln R dependence

Quantitative results

Implications for a hollow-cone criterion

Conclusions