A Study of the Lorentz Structure in Tau Decays

TL;DR

This study measures the Lorentz structure in tau decays by analyzing Michel parameters and helicity, providing the first measurement of tensor coupling in weak charged currents, confirming the V-A theory.

Contribution

It presents the first measurement of tensor coupling in tau decays and precise values of Michel parameters, testing the Lorentz structure of weak interactions.

Findings

Results are consistent with the V-A structure of the weak charged current.

Measured tensor coupling strength is close to zero, indicating no significant tensor contribution.

Michel parameters align with Standard Model predictions.

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 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 +/- 0.036 +/- 0.037, \rho = 0.775 +/- 0.023 +/- 0.020, \xi = 0.929 +/- 0.070 +/- 0.030, \xi\delta = 0.779 +/- 0.070 +/- 0.028, h_{\nu_{\tau}} = -0.997 +/- 0.027 +/- 0.011. The strength of the tensor coupling was measured to be \kappa^W_\tau = -0.029 +/- 0.036 +/- 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.