Results for muon decay parameters from TWIST

Muon decay offers an opportunity to test the Standard Model of particle physics in a purely leptonic situation where more ambiguous strong interaction processes are essentially absent. The TRIUMF Weak Interaction Symmetry Test (TWIST) was designed specifically to improve by an order of magnitude the precision of the decay parameters r ,d , and Pμx derived from measured energy and angle distributions of positrons from polarized positive muon decay. It tests the V-A structure of the decay by comparing the parameters to those predicted by the Standard Model in an analysis permitting more general Lorentz-invariant local terms.
    Since the completion of data taking in 2007, a careful analysis has been carried out with the aim of improving upon earlier intermediate results, by reducing systematic uncertainties, estimating residual biases, and evaluating consistency checks. The total uncertainties, representing improvements of 9, 12, and 7 in r ,d , and Pμx respectively, as compared to pre-TWIST experiments, are dominated by systematic uncertainties. The analysis was blind with respect to the central values of the parameters, and the hidden parameters were revealed in late January 2010.
   The talk will describe muon decay and the way in which we measure it. The experimental apparatus and analysis procedures will be presented, with particular attention to the reduction of leading systematic uncertainties. The results of the blind analysis and their uncertainties will be shown along with implications and limitations for physics beyond the Standard Model.