A study of the Lorentz structure in tau decays

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.