TWIST: precision measurement of the muon decay parameters

The TRIUMF Weak Interaction Symmetry Test (TWIST) collaboration has nearly completed the world's most precise measurement of the energy-angle spectrum of positrons from the decay of highly polarized muons. A simultaneous measurement of the muon decay parameters ρ, δ, and 𝒫μπξ tests the Standard Model (SM) in a purely leptonic process and provides improved limits for relevant extensions to the SM. Since the completion of data taking in 2007, the analysis has focused on reducing systematic uncertainties, estimating residual biases, and evaluating consistency checks. The analysis was blind with respect to the central values of the parameters, so the results were unknown until the values of the hidden parameters were revealed. The total uncertainties for the parameters are dominated by systematic uncertainties. While there are still some subtle systematic effects that preclude the determination of final values of the decay parameters, the goal of an order-of-magnitude improvement on pre-TWIST precisions has been achieved.