Finite Mass Splitting of the Nucleons

We calculate the neutron-proton mass difference in second order in a simple semirealistic gauge model of the weak and electromagnetic interactions. The mass splitting is finite. It is found that in the limit of infinitely heavy gauge mesons, the mass difference becomes the divergent one of conventional quantum electrodynamics. In the course of the calculation, certain general features of such models, which might be relevant for analogous calculations with more complicated groups, will be exhibited.