Precise measurement of the asymmetry parameter δ in muon decay

Highly polarized ${\mu }^{+}$ from the surface muon beam at TRIUMF have been analyzed by means of the muon-spin-rotation technique to determine the parity-violating muon decay asymmetry as a function of the daughter positron’s momentum. The primary result is a determination of the muon decay parameter δ to high precision [δ=0.7486±0.0026(stat)±0.0028(sys)] consistent with the prediction δ=0.75 of the standard model of weak interactions. The implications of this measurement for generalized four-fermion contact interaction models of muon decay are discussed. The data are also used to constrain the parameters in certain left-right-symmetric and supersymmetric extensions of the standard model, and to limit the existence of lepton-number-violating scalar decays of the muon [μ→eσ, where m(σ)<80 MeV/$c^2$].