The TWIST Collaboration has completed its measurement of the three muon decay parameters rho, delta, and P_muxi. This paper describes our determination of rho, which governs the shape of the overall momentum spectrum, and delta, which controls the mo
mentum dependence of the parity-violating decay asymmetry. The results are rho=0.74977pm 0.00012(stat.)pm 0.00023(syst.) and delta = 0.75049pm 0.00021(stat.)pm 0.00027(syst.). These are consistent with the value of 3/4 given for both parameters in the standard model, and each is over a factor of 10 more precise than the measurements published prior to TWIST. Our final results on rho, delta, and P_muxi have been incorporated into a new global analysis of all available muon decay data, resulting in improved model-independent constraints on the possible weak interactions of right-handed particles.
The TWIST Collaboration has completed a new measurement of the energy-angle spectrum of positrons from the decay of highly polarized muons. A simultaneous measurement of the muon decay parameters {rho}, {delta}, and (P_{mu}){xi} tests the Standard Mo
del (SM) in a purely leptonic process and provides improved limits for relevant extensions to the SM. Specifically, for the generalized left-right symmetric model |(g_R/g_L){zeta}|<0.020 and (g_L/g_R)m_2> 578 GeV/c^2, both 90% C.L.
The TRIUMF Weak Interaction Symmetry Test (TWIST) experiment was designed to test the standard model at high precision in the purely leptonic decay of polarized muons. A general four-fermion interaction model is used to describe the muon decay. TWIST
measures three of the four muon decay parameters of this model, $rho$, $delta$ and $P_{mu}^{pi} xi$, from the shape of the momentum-angle spectrum. The results of this model independent approach are compared to the standard model predictions and used to constrain new physics. Our collaboration has finalized the blind analysis of the final experimental data taken in 2006 and 2007. This analysis mostly reached our goal of a precision of an order of magnitude improvement over the pre-TWIST measurements.
