A study of muon pair production and evidence for tau pair production in photon-photon collisions at LEP

We have studied the reactionsγγ→μ+μ− andγγ→τ+τ− by measuring the properties of events of the typese+e−→e+e−μ+μ− ande+e−→e+e−τ+τ− ate+e− centre-of-mass energies between 88 and 94 GeV. The data sample corresponds to an integratede+e− luminosity of 40.4 pb−1 collected by the OPAL detector at LEP. The QED structure functionF2 is extracted from a sample of 1462γγ→μ+μ− events in which one photon is off the mass shell (single-tagged). TheQ2 range for these massive photons is 4–400 GeV2, with an averageQ2 of 8 GeV2. We have observed 48e+e−→e+e−μ+μ− events in which both final state electrons are detected. In the sample of single-tag events we have identified 34.9±6.7 events as due to the reactionγ;→τ+τ−, on an estimated background of 5.1±2.7 events. In all cases the measured event distributions agree with QED calculations.