Isospin violation and the proton's neutral weak magnetic form factor

The effects of isospin violation on the neutral weak magnetic form factor of the proton are studied using two-flavor chiral perturbation theory. The first nonzero contributions appear at O(p(4)) in the small-momentum expansion, and the O(p(5)) corrections are also calculated. The leading contributions from an explicit Delta(1232) isomultiplet are included as well. At such a high order in the chiral expansion, one might have expected a large number of unknown parameters to contribute. However, it is found that no unknown parameters can appear within loop diagrams, and a single tree-level counterterm at O(p(4)) is sufficient to absorb all divergences. The momentum dependence of the neutral weak magnetic form factor is independent of the counterterm, and is consistent with previous quark model estimates. Removal of higher-energy physics from the counterterm leaves a pion-cloud contribution to the form factor which is approximately 0.02 nuclear magnetons at q(2) = 0. [S0556-2821(99)02407-8].