Measurement of tau polarization at LEP

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,a 1 ) and τ − →ℓ − ν ̄ ℓ ν τ (ℓ=e,μ). From this measurement the quantities A e and A τ , which depend on the couplings of the electron and the τ to the Z, are determined to be A e =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 A ℓ =0.1540±0.0074±0.0044 is obtained, yielding the value of the effective weak mixing angle sin 2 θ W =0.2306±0.0011 .

electron positron: colliding beams
electron positron: annihilation
Z0: electroproduction
tau: pair production
tau-: semileptonic decay
tau-: leptonic decay
tau: polarization
polarization: longitudinal
universality: (tau electron)
Weinberg angle

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