Second Class Currents

Abstract

It is often tacitly assumed that the decay amplitude for

$$p \to n$$

is completely equivalent to the decay amplitude for

$$n \to p,$$

i.e., that there is a certain “charge symmetry” in ordinary 3-decay. Although the fundamental V-A coupling possesses this symmetry, this is not necessarily so. In fact one can write down nu-cleon currents which have the requisite transformation properties to match the V, A lepton currents but which violate this symmetry. These currents are called, after, a concept introduced by Weinberg 20 years ago,^[1] second-class currents. Let us clearly state at the outstart that such currents are presently in strong disfavor with first-class theorists, since they do not fit into their favorite schemes (quark model, unified weak and e.m. interactions). Our task here however, shall be to review the experimental evidence without undue prejudice.