A Study of the Lorentz Structure in tau Decays

This paper describes a measurement of the Michel parameters, η, ρ, ξ, ξδ, and the average ντ helicity, hντ, in τ lepton decays together with the first measurement of the tensor coupling in the weak charged current. The τ+τ− 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 τ the measured values of the parameters were: η=−0.005±0.036±0.037, ρ=0.775±0.023±0.020, ξ=0.929±0.070±0.030, ξδ=0.779±0.070±0.028, hντ=−0.997±0.027±0.011. The strength of the tensor coupling was measured to be κWτ=−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 τ lepton.