Diffractive gamma-gamma production at hadron colliders

TL;DR

This work computes exclusive double-diffractive γγ production at the Tevatron and LHC, focusing on two mechanisms: gg t-channel exchange and qq̄ t-channel exchange. The gg contribution is treated with a factorized luminosity 𝓛_g built from skewed unintegrated gluon densities, Sudakov suppression, and survival probabilities, while the qq̄ channel is analyzed via its own luminosity 𝓛_q and a special treatment of the qq̄ → γγ subprocess. The results show gg exchange dominates the exclusive γγ rate, with qq̄ being subdominant due to angular-momentum and kinematic suppressions; backgrounds from QED and Pomeron-Pomeron processes are discussed, alongside uncertainties from PDFs and NLO corrections. The authors position exclusive γγ production as a practical standard candle to validate the exclusive gg luminosity used in predicting Higgs production at the LHC, enabling more reliable assessments of new physics in diffractive channels.

Abstract

We compute the cross section for exclusive double-diffractive gamma-gamma production at the Tevatron, $p{\bar p} \to p + γγ+ {\bar p}$, and the LHC. We evaluate both the gluon-gluon and quark-antiquark t-channel exchange contributions to the process. The observation of exclusive gamma-gamma production at the Tevatron will provide a check on the model predictions, and offer an opportunity to confirm the expectations for exclusive double-diffractive Higgs production at the LHC.

Figures (7)

• Figure 1: Schematic diagram for exclusive Higgs production at the LHC, $pp \to p+H+p$. The exclusive production of the states shown in the brackets are possible 'standard candles', where the model predictions may, in principle, be checked by measurements at the Tevatron. Here we are concerned with the last process, $p\bar{p} \to p+\gamma\gamma+\bar{p}$. Predictions already exist for the exclusive production of Higgs bosons KMRKMRProspKKMRext, $\chi_c$,$\chi_b$KMRmmKMRS and dijets KMRdijetKMRKMRProsp.
• Figure 2: (a) Exclusive $\gamma\gamma$ production driven by two-gluon $t$-channel exchange, and the backgrounds arising from, (b) $\gamma\gamma$ produced in a Pomeron-Pomeron initiated subprocess, accompanied by soft undetected hadrons and, (c) from $\gamma\gamma$ production via $t$-channel quark-antiquark exchange. All permutations of the particle lines are implied.
• Figure 3: The behaviour of the differential cross section for the $J_z=0$ hard subprocess $gg \to \gamma\gamma$ in exclusive $\gamma\gamma$ production, shown by the continuous curve for fixed $M_{\gamma\gamma}=10$ GeV and by the dashed curve for fixed $E_{T\gamma}=5$ GeV. The values of ${\rm cos}\theta$ corresponding to the rapidity differences of the two emitted photons $\eta_1-\eta_2=1,2$ or 3 are indicated.
• Figure 4: (a) The LO ${q\bar{q}}$ exchange diagram; (b) the helicity structure of the upper part of diagram (a); (c) the inclusion of the hard subprocess.
• Figure 5: The extra subamplitude we have to calculate for the $q\bar{q}$ subprocess for $\gamma\gamma$ production.