Polarized muon decay: Measurement of the polarization vector of the decay positrons as a test of time reversal invariance Original Research Article

The μPT experiment at the Paul Scherrer Institut has determined the three polarization components of positrons from polarized muon decay simultaneously with the same apparatus by making use of three different effects (spatial and temporal dependence of annihilation-in-flight with polarized electrons and muon decay asymmetry). The use of a stroboscopic method greatly reduces systematic errors. 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 preliminary results are PT1 = (5 ± 16) × 10−3, PT2 = (1 ± 16) × 10−3and PL = 1.09 ± 0.15.