1+v,<>l"+ t→>μ+Y+Vr,Va+e→>+ve (a) (b) 图12-25(a)轻子过程;(b)半轻子过程;(c)非轻子过程l l + ν l l ↔ ' ' +ν τμ μ νμ e −− ννμτ , ν e −− +→+++→