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<Pmu*xi<=xi<1.0040, at 90% confidence level. Implications for left-right symmetric models are discussed.