A study of the Lorentz structure in tau decays

Abstract

Abstract.

This paper describes a measurement of the Michel parameters, \(\eta\), \(\rho\), \(\xi\), \(\xi \delta\), and the average \(\nu_{\tau}\) helicity, \(h_{\nu_{\tau}}\), in \(\tau\) lepton decays together with the first measurement of the tensor coupling in the weak charged current. The \(\tau^{+} \tau^{-}\) pairs were produced at the LEP \(\mathrm{e^{+} e^{-}}\) collider at CERN from 1992 through 1995 in the DELPHI detector. Assuming lepton universality in the decays of the \(\tau\) the measured values of the parameters were: \(\eta = -0.005 \pm 0.036 \pm 0.037\), \(\rho = 0.775 \pm 0.023 \pm 0.020\), \(\xi = 0.929 \pm 0.070 \pm 0.030\), \(\xi\delta = 0.779 \pm 0.070 \pm 0.028\), \(h_{\nu_{\tau}} = -0.997 \pm 0.027 \pm 0.011\). The strength of the tensor coupling was measured to be \(\kappa^W_\tau = -0.029 \pm 0.036 \pm 0.018\). The first error is statistical and the second error is systematic in all cases. The results are consistent with the \(V\!-\!A\) structure of the weak charged current in decays of the \(\tau\) lepton.