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

We have studied the reactions γγ→μ+μ- and γγ→τ+τ- by measuring the properties of events of the types e+e-→e+e-μ+μ- and e+e-→e+e-τ+τ- at e+e-centre-of-mass energies between 88 and 94 GeV. The data sample corresponds to an integrated e+e- luminosity of 40.4 pb-1 collected by the OPAL detector at LEP. The QED structure function F2 is extracted from a sample of 1462 γγ→μ+μ- events in which one photon is off the mass shell (single-tagged). The Q2 range for these massive photons is 4-400 GeV2, with an average Q2 of 8 GeV2. We have observed 48 e+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 reaction γγ→τ+τ-, on an estimated background of 5.1±2.7 events. In all cases the measured event distributions agree with QED calculations. (orig.)