Measurement of the Tau Neutrino Helicity and the Michel Parameters in Polarized e+e- Collisions

TL;DR

This study measures tau neutrino helicity and Michel parameters using polarized electron-positron collision data, providing insights into tau decay spectra and testing the Standard Model predictions.

Contribution

It introduces a novel analysis method leveraging polarized beams to directly extract tau decay parameters from experimental data.

Findings

Tau neutrino helicity hnu = -0.93 ± 0.10 ± 0.04

Michel parameter rho = 0.72 ± 0.09 ± 0.03

Michel parameter xi = 1.05 ± 0.35 ± 0.04

Abstract

We present a new measurement of the tau neutrino helicity, hnu and the tau Michel parameters rho, eta, xi and the product delta xi. The analysis exploits the highly polarized SLC electron beam to extract these quantities directly from a measurement of the tau decay spectra, using the 1993-1995 SLD data sample of 4328 e+e- --> Z0 --> tau+tau- events. From the decays tau --> pi nu(tau) and tau --> rho nu(tau) we obtain a combined value hnu=-0.93+/-0.10+/-0.04. The leptonic decay channels yield combined values of rho=0.72+/-0.09+/-0.03, xi=1.05+/-0.35+/-0.04 and delta xi=0.88+/-0.27+/-0.04.