The electron decay mode of the pion

The electron decay mode of the pion has been the subject of nmeh theoreticM a.nd experimental investig~tion. Some ye~rs ,~go ]~UDER~IAN and F1NKELSTEIn" (~) pointed out that, although it was difficult to prediet the ~bsolute decay rate of the pion, exact calculations could be made of the br,mehing-ratio ( : : ~ e + v ) / ( ~ : ~ i , + v ) . qheir model for the decay of the pion was that already used by SAXATA (2) of the pion decaying into a virtual nueleonantinueleon pair and the subsequent annihilation of the pair through a weak four-fermion interaction. They showed that for the usual five rel~tivistie interaetions~ the electron deea.y mode was forbidden if the wea.k intert~etion is soMa.r, vector, or tensor. In the ease of the pseudo-seMar interaction they showed 2 /ft2 /fie tha t the branehint~-ratio was given by the factor [ (m~-¢)/( ~--m~)]2; tha t is, the electron decay mode is 5.48 times more probable th,~n the muon decay