Precise measurement of the asymmetry parameterδin muon decay

Highly polarized μ⁺ 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}{→}e{σ}, where m({σ})80 MeV/c²].