Second-class currents in neutrino reactions

The influence of second-class currents in neutrino reactions is investigated. We show that Weinberg's unified model of electromagnetic and weak interactions implies both neutral and second-class currents. Possible experimental effects due to second-class neutral weak currents are discussed. Further, implications of second-class charged currents for hadronic neutral weak currents are investigated. It is shown that if second-class charged currents exist, the isospin I=1 axial-vector neutral current must be a 'new' current, or it must contain a 'new' I=1 part. Second-class charged currents are excluded if the I=1 axial-vector neutral current is 'old', i.e., if it is proportional to the third component of the charged current. Then also the proportionality constant is real. CP invariance is assumed throughout the discussion. (author)