Muon-pair and tau-pair production in two-photon collisions at LEP

The QED processes e+e−→e+e−μ+μ− and e+e−→e+e−τ+τ− are studied with the L3 detector at LEP using an untagged data sample collected at centre-of-mass energies 161 GeV⩽s⩽209 GeV. The τ-pairs are observed through the associated decay of one τ into eνν and the other into ππν. The cross sections are measured as a function of s. For muon pairs, the cross section of the γγ→μ+μ− process is also measured as a function of the two-photon centre-of-mass energy for 3 GeV⩽Wγγ⩽40 GeV. Good agreement is found between these measurements and the O(α4) QED expectations. In addition, limits on the anomalous magnetic and electric dipole moments of the τ lepton are extracted.