Electromagnetic self-energy of quarks confined to a sphere

The authors calculate the electromagnetic energy shift, deltaω, of a massless fermion confined to a sphere. They find deltaω > 0, in agreement with their previous result for a slab, and in disagreement with the sign needed to obtain the right value of the neutron-proton mass difference in a model in which the hadron consists of confined massless quarks. They consider the effect of a bare mass for the confined quarks, and show that it is then possible for deltaω to be negative. (Auth.)