Measurement of the Michel Parameter ρ in Muon Decay
Abstract
The TWIST Collaboration has measured the Michel parameter
ρ
in normal muon decay,
μ
+
→
e
+
ν
e
ν
¯
μ
. In the Standard Model,
ρ
= 3/4. Deviations from this value require mixing of left- and right-handed muon and electron couplings in the muon-decay Lagrangian. We find
ρ
= 0.75080
±
0.00044(stat.)
±
0.00093(syst.)
±
0.00023, where the last uncertainty represents the dependence of
ρ
on the Michel parameter
η
. This result sets new limits on the
W
L
−
W
R
mixing angle in left-right symmetric models.