New Experimental Constraints for the Standard Model from Muon Decay

TL;DR

This paper reports a precise measurement of muon decay parameters that tests the Standard Model and sets new limits on possible extensions, improving our understanding of leptonic weak interactions.

Contribution

The study provides the most accurate measurements of muon decay parameters and constrains new physics models beyond the Standard Model.

Findings

Improved limits on left-right symmetric model parameters

Enhanced constraints on new physics from muon decay data

Precise determination of muon decay spectrum parameters

Abstract

The TWIST Collaboration has completed a new measurement of the energy-angle spectrum of positrons from the decay of highly polarized muons. A simultaneous measurement of the muon decay parameters {\rho}, {\delta}, and (P_{\mu}){\xi} tests the Standard Model (SM) in a purely leptonic process and provides improved limits for relevant extensions to the SM. Specifically, for the generalized left-right symmetric model |(g_R/g_L){\zeta}|<0.020 and (g_L/g_R)m_2> 578 GeV/c^2, both 90% C.L.