ISOSPIN-BREAKING CORRECTIONS TO NUCLEON ELECTROWEAK FORM-FACTORS IN THE CONSTITUENT QUARK-MODEL

We examine isospin breaking in the nucleon wave functions due to the u -d quark mass difference and the Coulomb interaction among the quarks, and their consequences on the nucleon electroweak form factors in a n onrelativistic constituent quark model. The mechanically induced isosp in breaking in the nucleon wave functions and electroweak: form factor s are exactly evaluated in this model. We calculate the electromagneti cally induced isospin admixtures by using first-order perturbation the ory, including the lowest-lying resonance with nucleon quantum numbers but isospin 3/2. We find a small (less than or equal to 1%), but fini te correction to the anomalous magnetic moments of the nucleon stemmin g almost entirely from the quark mass difference, while the static nuc leon axial coupling remains uncorrected. Corrections of the same order of magnitude appear in charge, magnetic, and axial radii of the nucle on. The correction to the charge radius in this model is primarily iso scalar, and may be of some significance for the extraction of the stra ngeness radius from, e.g., elastic forward angle parity violating elec tron-proton asymmetries, or elastic He-4((e) over right arrow; e(')) e xperiments.