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arXiv · hep-ex/0409063

Measurement of the Michel Parameter $ρ$ in Muon Decay

TWIST Collaboration·J. R. Musser
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Abstract

The TWIST Collaboration has measured the Michel parameter $ρ$ in normal muon decay, $μ^+ \to e^+ ν_e \barν_μ$. 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 $\pm$ 0.00044(stat.) $\pm$ 0.00093(syst.) $\pm$ 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.

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TWIST Collaboration, J. R. Musser. 2004-09-29. Measurement of the Michel Parameter $ρ$ in Muon Decay. https://doi.org/10.1103/physrevlett.94.101805

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