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 P T 1 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 P T 2 , 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 P T 2 the two decay parameters α ′ / A and β ′ / A are deduced. Part of the data ( 40 × 10 8 annihilation events) have been evaluated. Both P T 1 and P T 2 , averaged over the positrons' energy, are consistent with zero in full agreement with the standard model. The precision obtained is Δ 〈 P T 1 〉 = Δ 〈 P T 2 〉 = 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].