Measurement of the transverse e+ polarization from the decay of polarized μ+ and its implications on GF and time reversal invariance

The transverse polarization of the e+ from the decay of polarized μ+ has been measured at the Paul Scherrer Institute. From the energy dependence of the transverse polarization component PT1 which lies in the plane spanned by muon spin and positron momentum, the decay parameters η and η″ are derived, the first of which yields an improved model-independent value of the Fermi coupling constant. A non-zero value of the transverse component PT2, which is perpendicular to the plane mentioned above, would be the first observation of time reversal violation in a purely leptonic decay. From the energy dependence of PT2 the two decay parameters α′/A and β′/A are deduced. Part of the data (40×108 annihilation events) have been evaluated. Both PT1 and PT2, averaged over the positrons' energy, are consistent with zero in full agreement with the standard model. The precision obtained is Δ〈PT1〉=Δ〈PT2〉=8×10−3. which is an improvement by a factor of three as compared to the results of the previous experiment [H. Burkard et al., Phys. Lett. 160 B (1985) 343].