Muon decay: Measurement of the positron polarization and implications for the spectrum shape parameter eta, V-A and T-invariance

A new measurement of the longitudinal polarization Psub(L) of the e+ from μ+ decay is presented here. Combined with our previously published result, we obtain Psub(L) = 0.998 +- 0.045 (statistical plus systematic error). We also report on the first measurement of Psub(L)(theta) where theta is the angle between the e+ line of flight and the μ+ polarization. The implications on the ''low energy spectrum shape parameter eta'', on the ratio of A coupling to V coupling and on time-reversal invariance are discussed. The results are in agreement with the standard V-A form of the charged leptonic weak interaction. (orig.)