2023 Impact factor 4.2
Particles and Fields
Eur. Phys. J. C 8, 3-21
DOI 10.1007/s100529901080

Measurement of the Michel parameters
in leptonic tau decays

The OPAL Collaboration

K. Ackerstaff8 - et al.

8 CERN, European Organisation for Particle Physics, CH-1211 Geneva 23, Switzerland

Received: 25 June 1998 / Published online: 1 March 1999

The Michel parameters of the leptonic \ensuremath{\mathrm{\tau}} decays are measured using the OPAL detector at LEP. The parameters \ensuremath{\rho_\ensuremath{\mathrm{\ell}}}, \ensuremath{\xi_\ensuremath{\mathrm{\ell}}}, \ensuremath{(\xi\delta)_\ensuremath{\mathrm{\ell}}} (with $\ensuremath{\mathrm{\ell}}= \ensuremath{\mathrm{e}},\ensuremath{\mathrm{\mu}}$) and \ensuremath{\eta_\ensuremath{\mathrm{\mu}}} 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 $\sqrt{s}\allowbreak =m_\ensuremath{\mathrm{{Z^0}}}$ corresponding to an integrated luminosity of 155 pb-1. If \ensuremath{\mathrm{e}}-\ensuremath{\mathrm{\mu}} universality is assumed and inferring the \ensuremath{\mathrm{\tau}} polarization from neutral current data, the measured Michel parameters are:

{r@{$\ \ =\ \ $}l@{\qquad}r@{$\ \ =\ \ $}l}
 ...nsuremath{\xi\delta}}& $\mbox{$\space 0.65 \pm 0.14 $} \pm 0.07$,
where the value of \ensuremath{\eta} has been constrained using the published OPAL measurements of the leptonic branching ratios and the \ensuremath{\mathrm{\tau}} lifetime. 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.

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