Isolating Prompt Photons with Narrow Cones

TL;DR

This work addresses the reliability of prompt-photon predictions with narrow isolation cones by identifying and resumming the dominant $\ln R$ terms that cause unitarity violations at NLO. The authors derive the LL structure of multiparton collinear radiation using Mellin moments, implement a LL resummation for the isolated cross section with a cone scale $M_F\sim Rp_T^{\gamma}$, and integrate it with exact NLO results in Jetphox. Quantitative results show the resummation cures unphysical growth at small $R$ with modest corrections for typical cone sizes, and hollow-cone scenarios are explored, highlighting the need for further resummation of $\ln r$ terms. Overall, the paper provides a more robust framework for predicting isolated prompt-photon cross sections and informs experimental choices of isolation cones.

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.

Figures (2)

• Figure 1: A pictorial representation of the NLO contribution from final-state collinear radiation inside the isolation cone
• Figure 2: A HO correction to the fragmentation contribution