Measurement of τ polarisation at LEP

Abstract

Using the data collected with the L3 detector at LEP between 1990 and 1995, corresponding to an integrated luminosity of 149 pb⁻¹, the Τ longitudinal polarisation has been measured as a function of the production polar angle using the τ decays τ ⁻→ h⁻ν_τ (h=π,ρ,a₁) and Τ⁻→ℓ^-ν̅_ℓν _τ (ℓ=e,μ) From this measurement the quantities A_c and A_τ , which depend on the couplings of the electron and the τ to the Z, are determined to be A_c = 0.1678±0.0127;±0.0030 and A_τ = 0.1476 ± 0.0088 ± 0.0062, consistent with the hypothesis of e-τ universality Under this assumption a value of Al = 01540 ± 0.0074 ± 0.0044 is obtained, yielding the value of the effective weak mixing angle sin²θ̅_W=0.2306±0.0011.