2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/183417The 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.9 pages, REVTeX, 1 eps figureNuclear Theorytext