A precision measurement of the muon decay parameter delta

The muon decay parameter delta characterizes momentum dependence of the parity-violating muon decay asymmetry. A new measurement of delta has been performed using the first physics data recorded by the TWIST experiment at TRIUMF. The obtained value, delta = 0.74964 +/- 0.00066 (stat.) +/- 0.00112 (syst.), is consistent with the Standard Model expectation delta = 3/4. This is the first determination of delta performed using a blind analysis technique. Combined with other data, the measurement sets new model-independent limits on effective right-handed couplings of the muon. Improved limits on the product of another muon decay parameter, xi, and the muon polarization in pion decay, Pmu, are obtained in the form: 0.9960 < Pmuxi ≤ xi < 1.0040, at 90% confidence level. Implications for left-right symmetric models are discussed.