Muon-Pair and Tau-Pair Production in Two-Photon Collisions at LEP

Abstract

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 < sqrt{s} < 209 GeV. The tau-pairs are observed through the associated decay of one tau into eννand the other into ππν. The cross sections are measured as a function of sqrt{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 < W_{γγ} < 40 GeV. Good agreement is found between these measurements and the O(α^4) QED expectations. In addition, limits on the anomalous magnetic and electric dipole moments of the tau lepton are extracted.