Precise measurement of the asymmetry parameter<i>δ</i>in muon decay

Abstract

Highly polarized ${\ensuremath{\mu}}^{+}$ 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 \ensuremath{\delta} to high precision [\ensuremath{\delta}=0.7486\ifmmode\pm\else\textpm\fi{}0.0026(stat)\ifmmode\pm\else\textpm\fi{}0.0028(sys)] consistent with the prediction \ensuremath{\delta}=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 [\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\sigma}, where m(\ensuremath{\sigma})80 MeV/${c}^{2}$].