Multi-photon production in e+e- collisions at sqrt(s)= 189 geV

TL;DR

This study measures multi-photon production in electron-positron collisions at 189 GeV, confirming QED predictions and setting new limits on excited electron masses, graviton exchange scales, and resonance production.

Contribution

It provides precise measurements of multi-photon events at 189 GeV and establishes improved constraints on new physics beyond the Standard Model.

Findings

Measured cross-section agrees with QED predictions

Set lower limits on excited electron mass (>306 GeV)

Excluded certain graviton exchange scales (<660 GeV)

Abstract

The process e+e- to 2 (or 3) gammas is studied using data recorded with the OPAL detector at LEP. The data sample taken at a centre-of-mass energy of 189 GeV corresponds to a total integrated luminosity of 178 pb-1. The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Lambda+ > 304 GeV and Lambda- > 295 GeV as well as a mass limit for an excited electron, Me* > 306 GeV assuming equal e*egamma and eegamma couplings. Graviton exchange in the context of theories with higher dimensions is excluded for scales G+ < 660 GeV and G- < 634 GeV. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons and produced in association with another photon.