Measurement of the cross sections of the reactions e+e− → γγ and e+e− → γγγ at LEP

The cross section of the pure QED process e+e−→γγ has been measured using data accumulated during the 1989 and 1990 scans of the Z0 resonance at LEP. Both the energy dependence and the angular distribution are in good agreement with the QED prediction. Upper limits on the branching ratios of Z0→γγ, Z0→π0γ and Z0→ηγ have been set at 1.4×10 −4, 1.4×10−4 and 2.0×10−4 respectively. Lower limits on the cutoff parameters of the modified electron propagator have been found to be Λ+ > 117 GeV and Λ− > 110 GeV. The reaction e+e− → γγγ has also been studied and was found to be consistent with the QED prediction. An upper limit on the branching ratio of Z0→γγγ has been set at 6.6 × 10−5. All the limits are given at 95% confidence level.