Measurement of the tau polarisation at the Z resonance

Abstract

Using 18.8 pb⁻¹ of data collected in 1990 and 1991, ALEPH has measured the tau polarisation in the decay modes τ→ev $\bar v$, τ→μv $\bar v$, τ→πν, τ→ρν and τ→a ₁ν, using both the individual tau decay kinematics and the event acollinearity. The measurement of the tau polarisation as a function of the production polar angle yields the two parametersA _τ andA _e , whereA _l =2g ^l _v g ^l _A /(g ^l _v )²+(g ^l _A )²] The resultsA _τ=0.143±0.023 andA _e =0.120±0.026 are consistent with the hypothesis of electron-tau universality. Assuming universality yields a measurement of the effective weak mixing angle sin²θ ^eff _w =0.2332±0.0022.