Measurement of the Michel parameters in leptonic tau decays

The OPAL Collaboration: K. Ackerstaff - et al.

doi:doi:10.1007/s100529901080

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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

Abstract
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:

$\begin{tabular} {r@{$\ \ =\ \ $}l@{\qquad}r@{$\ \ =\ \ $}l} {\ensuremath{\rho}}... ...nsuremath{\xi\delta}}& $\mbox{$\space 0.65 \pm 0.14 $} \pm 0.07$, \end{tabular}$

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.