Extraction of the strong neutron-proton mass difference from the charge symmetry breaking in pn→dπ0

We perform a complete calculation of charge symmetry breaking effects for the reaction pn→dπ0 at leading order in chiral perturbation theory. A new leading-order operator is included. From our analysis we extract δmNstr, the strong contribution to the neutron-proton mass difference. The value obtained, δmNstr=(1.5±0.8 (exp.)±0.5 (th.)) MeV, is consistent with the result based on the Cottingham sum rule. This agreement provides a non-trivial test of our current understanding of the chiral structure of QCD.