New Results for Ultraperipheral Heavy Ion Collisions

TL;DR

This paper investigates diphoton production in ultraperipheral lead-lead collisions at the LHC, calculating cross sections for various mechanisms and exploring the potential to measure elastic gamma-gamma scattering up to 20 GeV invariant mass.

Contribution

It introduces detailed calculations of diphoton production mechanisms in ultraperipheral heavy ion collisions, including the two-gluon exchange, and assesses their significance for experimental measurements.

Findings

High cross sections for diphoton production in PbPb collisions.

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

Two-gluon exchange becomes significant in certain phase space regions.

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). We got relatively high cross sections in $PbPb$ collisions. 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 discuss also first results concerning production of two $e^+ e^-$ pairs in UPCs of heavy ions. We considered only double scattering mechanism.