Determination of the e+e- -> gamma gamma (gamma) cross-section at centre-of-mass energies ranging from 189 GeV to 202 GeV

TL;DR

This study measured the e+e- -> gamma gamma cross-section at energies up to 202 GeV, confirming QED predictions and setting new limits on possible new physics such as excited electrons, graviton exchange, and QED cut-off parameters.

Contribution

First measurement of gamma gamma production cross-section at LEP energies, providing tests of QED and constraints on new physics models.

Findings

Measured cross-sections agree with QED predictions.

Set lower limits on QED cut-off parameters (>330 GeV).

Established bounds on excited electron mass (>311 GeV/c^2) and string scale (M_S > 713 GeV/c^2).

Abstract

A test of the QED process e+e- -> gamma gamma (gamma) is reported. The data analysed were collected with the DELPHI detector in 1998 and 1999 at the highest energies achieved at LEP, reaching 202 GeV in the centre-of-mass. The total integrated luminosity amounts to 375.7 pb^{-1}. The differential and total cross-sections for the process e+e- -> gamma gamma were measured, and found to be in agreement with the QED prediction. 95% Confidence Level (C.L.) lower limits on the QED cut-off parameters of Lambda+ > 330 GeV and Lambda- > 320 GeV were derived. A 95% C.L. lower bound on the mass of an excited electron of 311 GeV/c^2 (for lambda_gamma = 1) was obtained. s-channel virtual graviton exchange was searched for, resulting in 95% C.L. lower limits on the string mass scale, M_S: M_S > 713 GeV/c^2 (lambda = 1) and M_S > 691 GeV/c^2 (lambda = -1).