Diphoton production in lead-lead and proton-proton UPC

TL;DR

This paper investigates diphoton production in ultraperipheral lead-lead and proton-proton collisions at high energies, calculating cross sections for various mechanisms and exploring the potential for measuring elastic photon scattering at the LHC.

Contribution

It provides the first detailed calculation of diphoton production cross sections including multiple mechanisms in ultraperipheral collisions at LHC energies.

Findings

High cross section (~306 nb) for diphoton production in PbPb collisions.

Elastic gamma-gamma scattering measurable up to 15-20 GeV invariant mass.

Identification of phase space regions where two-gluon exchange dominates.

Abstract

We discuss diphoton semi(exclusive) production in ultraperipheral $PbPb$ collisions at energy of $\sqrt{s_{NN}}=$ 5.5 TeV (LHC). The nuclear calculations are based on equivalent photon approximation in the impact parameter space. The cross sections for elementary $\gamma \gamma \to \gamma \gamma$ subprocess are calculated including three different mechanisms: box diagrams with leptons and quarks in the loops, a VDM-Regge contribution with virtual intermediate hadronic excitations of the photons and the two-gluon exchange contribution (formally three-loops) to elastic photon-photon scattering in the high-energy approximation. We got relatively high cross sections in $PbPb$ collisions ($306$ nb). This opens a possibility to study the $\gamma \gamma \to \gamma \gamma$ (quasi)elastic scattering at the LHC. We find that the cross section for elastic $\gamma\gamma$ scattering could be measured in the lead-lead collisions for the diphoton invariant mass up to $W_{\gamma\gamma} \approx 15-20$ GeV. We identify region(s) of phase space where the two-gluon exchange contribution becomes important ingredient compared to box and nonperturbative VDM-Regge mechanisms. We perform a similar analysis for the $pp \to pp\gamma\gamma$ reaction at energy of $\sqrt{s_{NN}}=7$ and $100$ TeV.