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Cross section of isolated prompt photons in hadron-hadron collisions

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

TL;DR

This work provides a rigorous, full next-to-leading order QCD treatment of isolated prompt-photon production in hadron-hadron collisions, keeping the complete dependence on the isolation cone radius $R$ without resorting to small-$R$ approximations. It establishes factorization for isolated photons, derives how isolation modifies the direct and fragmentation components, and implements a DIPHOX-based Monte Carlo program to evaluate the cross section with exact $R$-dependence. The study reveals that isolated cross sections are broadly similar in magnitude to non-isolated ones, though both direct and fragmentation pieces are highly sensitive to isolation parameters, with notable logarithmic enhancements as $R$ decreases. The results illuminate the theoretical limits of fixed-order calculations in the small-$R$ and tight-isolation regimes and have practical implications for backgrounds in Higgs to gamma gamma searches and for constraining gluon densities in colliding hadrons.

Abstract

We consider the production of isolated prompt photons in hadronic collisions. We present a general discussion in QCD perturbation theory of the isolation criterion used by hadron collider experiments. The isolation criterion is implemented in a computer programme of the Monte Carlo type, which evaluates the production cross section at next-to-leading order accuracy in perturbative QCD. The calculation includes both the direct and the fragmentation components of the cross section, without any approximation of the dependence on the radius R of the isolation cone. We examine the scale dependence of the isolated cross section, the sensitivity of the cross section to the values of the isolation parameters, and we provide a quantitative comparison between the full R dependence and its small-R approximation.

Cross section of isolated prompt photons in hadron-hadron collisions

TL;DR

This work provides a rigorous, full next-to-leading order QCD treatment of isolated prompt-photon production in hadron-hadron collisions, keeping the complete dependence on the isolation cone radius without resorting to small- approximations. It establishes factorization for isolated photons, derives how isolation modifies the direct and fragmentation components, and implements a DIPHOX-based Monte Carlo program to evaluate the cross section with exact -dependence. The study reveals that isolated cross sections are broadly similar in magnitude to non-isolated ones, though both direct and fragmentation pieces are highly sensitive to isolation parameters, with notable logarithmic enhancements as decreases. The results illuminate the theoretical limits of fixed-order calculations in the small- and tight-isolation regimes and have practical implications for backgrounds in Higgs to gamma gamma searches and for constraining gluon densities in colliding hadrons.

Abstract

We consider the production of isolated prompt photons in hadronic collisions. We present a general discussion in QCD perturbation theory of the isolation criterion used by hadron collider experiments. The isolation criterion is implemented in a computer programme of the Monte Carlo type, which evaluates the production cross section at next-to-leading order accuracy in perturbative QCD. The calculation includes both the direct and the fragmentation components of the cross section, without any approximation of the dependence on the radius R of the isolation cone. We examine the scale dependence of the isolated cross section, the sensitivity of the cross section to the values of the isolation parameters, and we provide a quantitative comparison between the full R dependence and its small-R approximation.

Paper Structure

This paper contains 19 sections, 68 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Mechanisms of prompt-photon production
  3. Inclusive cross section
  4. Isolated-particle production and factorization
  5. Isolated cross section
  6. Direct contribution with isolation
  7. Fragmentation contribution with isolation
  8. Numerical calculation and ${\cal O}(R^{2n})$ terms
  9. Brief presentation of the programme
  10. Numerical results
  11. Sensitivity to the isolation parameters
  12. Scale dependence
  13. Comparison with the collinear approximation
  14. Effect of the isolation as a function of $p_{T \, \gamma}$
  15. Conclusions and outlook
  16. ...and 4 more sections

Figures (3)

  • Figure 1: Kinematics of HO corrections to the direct contribution.
  • Figure 2: Kinematics of HO corrections to the fragmentation contribution.
  • Figure 3: Ratios 'isolated'/'non-isolated' for the direct contribution, the fragmentation contribution and the total contribution to the cross section $d \sigma/dp_{T \, \gamma}$ at NLO.