  The two-body currents for s-wave pion photoproduction on the NN system are
derived to O(Q^4) in chiral perturbation theory. For the interesting case of
3S1 <-> 1S0 transitions, we show that an axial isovector two-nucleon contact
term connects the short-distance physics of pion photoproduction to pion
production and several important electroweak reactions. We also find that the
standard chiral Lagrangian gives a gamma pi pi N N vertex that have not been
explicitly mentioned in previous literature. The corresponding Feynman rule is
presented here and some processes where it should be important are briefly
discussed.
