Self-Energy of Dirac Particles

Weisskopf's analysis of the electron self-energy in 1939 showed that the part of the self-energy that is due to the spin of the particle is negative. An extension of this analysis with the addition of a Pauli term to the Hamiltonian shows that the additional self-energy that arises is also negative and only adds to the original spin self-energy. The physical significance of this negative spin energy is discussed in detail.With the choice of an appropriate cutoff, the total self-energy of a Dirac particle with a Pauli anomalous magnetic moment can be made negative. A possible explanation of the neutron-proton mass difference in terms of their difference in electromagnetic self-energy, as first pointed by Feynman and Speisman, is considered in the light of such an analysis of the self-energy.