A study of the reaction e+e−→γγ at LEP

The pure QED reaction e+e−→γγ has been studied at centre of mass energies around the mass of the Z0 boson using data recorded by the OPAL detector at LEP. The results are in good agreement with the QED prediction. Lower limits on the cutoff parameters of the modified electron propagator are found to be Λ+>89 GeV and Λ. The lower limit on the mass of an excited electron is 82 GeV assuming the coupling constant λ=1. Upper limits on the branching ratios of Z0→γγ, Z0→π0γ and Z0→ηγ are set at 3.7×10−4, 3.9×10−4 and 5.8×10−4 respectively. Two events from the reaction e+e−→γγγ have been observed, consistent with the QED prediction. An upper limit on the branching ratio of Z0→γγγ is set at 2.8×10−4. All the limits are given at 95% confidence level.