Measurement of the Michel Parameters in Leptonic Tau Decays

TL;DR

This paper measures the Michel parameters in leptonic tau decays using a novel likelihood fit method on OPAL data, testing the Standard Model and setting limits on new physics.

Contribution

Introduces a new global likelihood fit method for extracting Michel parameters from tau decay data with full detector simulation and background treatment.

Findings

Results agree with Standard Model V-A predictions

Set limits on non-standard couplings

Constrain masses of hypothetical new gauge bosons

Abstract

The Michel parameters of the leptonic tau decays are measured using the OPAL detector at LEP. The Michel parameters are extracted from the energy spectra of the charged decay leptons and from their energy-energy correlations. A new method involving a global likelihood fit of Monte Carlo generated events with complete detector simulation and background treatment has been applied to the data recorded at center-of-mass energies close to sqrt(s) = M(Z) corresponding to an integrated luminosity of 155 pb-1 during the years 1990 to 1995. If e-mu universality is assumed and inferring the tau polarization from neutral current data, the measured Michel parameters are extracted. Limits on non-standard coupling constants and on the masses of new gauge bosons are obtained. The results are in agreement with the V-A prediction of the Standard Model.