Muon decay: Measurement of the transverse e+ polarization and its implications on GF and time reversal invariance

The transverse polarization components of the positrons from the decay of polarized muons have been measured at the Paul Scherrer Institute by annihilating in flight the decay positrons with polarized electrons. The use of a stroboscopic method greatly reduces systematic errors. The energy dependence of the transverse polarization component PT1, which lies in the plane defined by muon spin and positron momentum, yields the low energy parameter η = (−2 ± 8) × 10 −3 and thus an improved model‐independent value of the Fermi coupling constant. For the transverse polarization PT2, averaged over the positron energy, we obtain 〈PT2〉 = (−4 ± 8) × 10−3. This improves the previous limit on the violation of time invariance in muon decay by a factor of three.