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

TL;DR

This paper measures muon and tau pair production in two-photon collisions at LEP, confirming QED predictions and setting limits on tau's anomalous dipole moments.

Contribution

It provides the first detailed measurements of these processes at LEP energies and tests QED with high precision, also constraining tau dipole moments.

Findings

Measured cross sections for muon and tau pair production.

Found good agreement with QED predictions.

Set new limits on tau's anomalous magnetic and electric dipole moments.

Abstract

The QED processes e^+ e^- -> e^+ e^- \mu^+ \mu^- and e^+ e^- -> e^+ e^- \tau^+ \tau^- 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 e\nu\nu and the other into \pi\pi\nu . The cross sections are measured as a function of sqrt{s}. For muon pairs, the cross section of the \gamma\gamma -> \mu^+\mu^- process is also measured as a function of the two-photon centre-of-mass energy for 3 GeV < W_{\gamma\gamma} < 40 GeV. Good agreement is found between these measurements and the O(\alpha^4) QED expectations. In addition, limits on the anomalous magnetic and electric dipole moments of the tau lepton are extracted.