Polarized muon decay: Measurement of the polarization vector of the decay positrons

In the standard model (SM) of electroweak interactions the positron from the decay of polarized positive muons is mainly longitudinally polarized. The measurement of the two transverse polarization components, therefore, is a sensitive tool for contributions from additional, exotic, interactions. The energy dependence of the transverse polarization component PT1, which lies in the plane spanned by muon-spin and positron momentum, yields the low energy parameter η and thus an improved model-independent value of the Fermi coupling constant. A non-zero value of the transverse component PT2, which is perpendicular to the above mentioned plane, would be the first observation of time reversal violation in a purely leptonic decay. The μPT experiment at the Paul Scherrer Institute determines the three polarization components simultaneously with the same apparatus by making use of three different reactions (spatial and temporal dependence of annihilation-in-flight with polarized electrons as well as muon decay asy...