P-odd observables at the $Υ$ peak

Abstract

We study the $γ$-Z interference in the process $e^+ e^-\to Υ\to τ^+ τ^-$ as a means to measure the neutral current coupling of the b-quark. The helicity amplitudes are calculated from resonant and background diagrams and the spin density matrix of the final state is discussed. The spin analyzer of the $τ$'s is illustrated with the decays $πν$ and $ρν\to (ππ) ν$. With $10^8 Υ$ a sensitivity to $g_V^b$ of a few per cent could be reachable.