Abstract
The tau polarization in Z decay is one of the most sensitive electroweak observables [4]. It is measured from the charged particle momentum distribution in the semi-leptonic decays τ → eνeντ , μνμντ or in hadronic decays τ → hντ where h can be π, K, ρ, K∗, a1 etc. Each channel having its own analysis power, a clean separation is essential. The analysis of the τ polarization dependence on the scattering angle θ gives access to both the tau and electron chiral coupling asymmetries Aτ and Ae independently and serves as a crucial ingredient of a full lepton-by-lepton extraction of the Neutral Current chiral (or vector and axial-vector) couplings. A closely related outcome is a precise determination of the vector and axial vector spectral functions, which provide important information for the extraction of αs(m2τ ) and αQED(q2). A limiting systematic error is associated with cross-channel contamination, implying tight detector requirements on reconstruction of photons, π0s and other neutral particles, as well as K/π separation.