Isospin violation and the proton's neutral weak magnetic form factor - art. no. 073002

The effects of isospin violation on the neutral weak magnetic form factor o f the proton are studied using two-flavor chiral perturbation theory. The f irst nonzero contributions appear at O(p(4)) in the small-momentum expansio n, and the O(p(5)) corrections are also calculated. The leading contributio ns 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 numbe r of unknown parameters to contribute. However, it is found that no unknown parameters can appear within loop diagrams, and a single tree-level counte rterm at O(p(4)) is sufficient to absorb all divergences. The momentum depe ndence of the neutral weak magnetic form factor is independent of the count erterm, and is consistent with previous quark model estimates. Removal of h igher-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].