One-pion-exchange-current contribution to neutron--proton bremsstrahlung

The electromagnetic current $j_2^{OPE}$ due to the exchange of a charged pion between a neutron and a proton is rederived by comparing the bremsstrahlung matrix element computed from the Schr\"odinger equation with that obtained from the lowest-order Feynman diagrams. No account is taken of renormalization effects or nucleon resonances. The operator $\int d^{3}x{\overrightarrow{j}}_2^{OPE}\left(\overrightarrow{x}\right)e^{-i\overrightarrow{k}·\overrightarrow{x}}$ which is needed for neutron-proton bremsstrahlung calculations is evaluated, and an approximation to it obtained as an expansion in powers of (photon energy)/(pion mass), keeping the first two terms. This is expected to be considerably more accurate than expanding the bremsstrahlung matrix element itself in powers of k. The second term in the expansion contributes to ¹S₀↔³S₁ transitions and may become significant even for small values of k/μ. As a preparation for actual calculations the operator is rewritten in terms of irreducible tensors. The treatment of nonlocalities other than charged-pion exchange is discussed briefly.