We present a new high precision measurement of parity violation in the weak interaction, using polarized muon decay. The TWIST collaboration has measured $P_mu^pi xi$, where $P_mu^pi$ is the polarization of the muon in pion decay and $xi$ describes t
he intrinsic asymmetry in muon decay. We find $P_mu^pi xi = 1.00084 pm 0.00029,(textrm{stat.})_{-0.00063}^{+0.00165},(textrm{syst.})$, in good agreement with the standard model prediction of $P_mu^pi=xi=1$. Our result is a factor of 7 more precise than the pre-TWIST value, setting new limits in left-right symmetric electroweak extensions to the standard model.
The time dependence of muon spin relaxation has been measured in high purity aluminum and silver samples in a longitudinal 2 T magnetic field at room temperature, using time-differential musr. For times greater than 10 ns, the shape fits well to a si
ngle exponential with relaxation rates of $lambda_{textrm{Al}} = 1.3 pm 0.2,(textrm{stat.}) pm 0.3,(textrm{syst.}),pms$ and $lambda_{textrm{Ag}} = 1.0 pm 0.2,(textrm{stat.}) pm 0.2,(textrm{syst.}),pms$.
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.
