Measurement of the Michel Parameter xi" in Polarized Muon Decay and Implications on Exotic Couplings of the Leptonic Weak Interaction

TL;DR

This paper measures the Michel parameter xi" in polarized muon decay, achieving higher precision and providing new constraints on exotic weak interaction couplings beyond the Standard Model.

Contribution

It presents the most precise measurement of xi" to date, improving the relative accuracy and constraining new physics beyond the V-A interaction model.

Findings

Measured xi" = 0.981 ± 0.045 (stat) ± 0.003 (syst)

Result consistent with Standard Model prediction

Sets new limits on exotic leptonic weak couplings

Abstract

The Michel parameter xi" has been determined from a measurement of the longitudinal polarization of positrons emitted in the decay of polarized and depolarized muons. The result, xi" = 0.981 +- 0.045stat +- 0.003syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.