Isovector electromagnetic exchange currents in the chiral approach

Summary

The problem studied in the paper concerns the structure of the isovector e.m. MEC operators of the A₁₋, ϱ- and π ranges in the interval of intermediate energies. The two main dynamic principles which we invoke in our considerations are the current conservation and the gauge chiral invariance. Respecting them consistently allows us to describe correctly the interaction of NA₁ ϱπ system with the external e.m. field. Our main results are as follows:

• (i) We verified that our Lagrangian approach is consistent with the prediction of the low energy theorem at threshold.

• (ii) We showed explicitly the continuity equation which the longitudinal parts of our currents obey.

• (iii) We proved the equivalence relation for the MEC operator of the pion range and demonstrated the existence of the seagull current in the MEC operator built up using PS πN coupling. This new term influences strongly the exchange charge density.

In our approach, the vector dominance Dirac e.m. form factors appear naturally in formulae for MEC operators. However, any other choice of parametrization (e.g. dipole form factors) is possible for computational purposes and it does not result in any alteration of our formulae and conclusions. Also, the continuity equation does not change when putting form factors F_BNN(q₂) into the BNN vertices (B = = π. ϱ. A₁) of the MEC operators and OBE potentials.