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 --> d pi(0) at leading order in chiral perturbation theory. A new leading-order operator is included. From our analysis we extract delta m(N)(str), the strong contribution to the neutron-proton mass difference. The value obtained, delta m(N)(str) = (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. (C) 2009 Elsevier B.V. All rights reserved.