Measurement of the Chirality Parameter in the Charged-Current Coupling of the $\tau$ Lepton

According to the Standard Model of electroweak interactions, the weak charged-current has a universal V- A Lorentz structure. In order to test the charged-current coupling of the τ lepton, the chirality parameter ξh was measured in the decays τsp- /to/pi/sp-/nu/sb/tau and τsp-/to/rho/sp- /nu/sb/tau. This measurement is based on an integrated luminosity of 148 pb-1 for e+e/sp- interactions at center-of-mass energies near the Z boson mass. These interactions were recorded with the L3 detector at the LEP e+e/sp- collider located at CERN. The data sample consists of 41608 selected Z/toτ+/tau/sp- events in which at least one of the τ decays is identified as τto/pi/nu or τto/rho/nu. The chirality parameter and the average τ polarization in Z/toτ+/tau/sp- are measured simultaneously by using the decay spectra of individual τ decays and by exploiting the correlations between the τ+ and τsp- in an event. Assuming universality in τ decays, the results are:/eqalign[ξh &= [-]1.031 /pm 0.031 /pm 0.014/cr [/cal P]/sb/tau &= [-]0.149 /pm 0.011 /pm 0.015,/cr]where the first error is statistical and the second error is systematic. This value of ξh agrees with the Standard Model construction of a pure V-A weak charged-current, and this value of [/cal P]/sb/tau agrees with determinations from other experiments.