Muon Decay and the Majorana Neutrino

Abstract

Recently, the Berkeley group reported the precise and excellent experimental result on the e+ spectrum end point in /1+ decay.') In order to get the lower limit for the possible heavier (righthanded) gauge boson mass M( Wz»380 GeV, they have used the following assumptions made by Beg, Budny, Mohapatra and Sirlin;Z) (i) the manifest left-right symmetry in the charged current sector, (ii) no mirror leptons, (iii) the CP invariance, (iv) the smallness of all neutrino masses (Ll j =(mjlmp)4{.l), m" being the f.I. mass. In this note, we shall reinterpret this Berkeley result from the viewpoint of the massive Majorana neutrino, and show the information which we can draw from this result for the case where the right-handed Majorana neutrino (Nn+k) has a quite large mass mn+kl m,,~ 1. Namely, the fourth assumption (Ll j 4{.l) is reexamined by taking into account the effect due to the neutrino mixing. In order to be self-contained, we shall start from the effective lepton-hadron weak interaction Hamiltonian to treat the 7[+ decay;