Dirac-Foldy term and the electromagnetic polarizability of the neutron

We reconsider the Dirac-Foldy contribution mu(2)/m to the neutron electric polarizability. Using a Dirac equation approach to neutron-nucleus scattering, we review the definitions of Compton continuum (<(alpha)over bar>), classical static (alpha(E)(n)), and Schrodinger (alpha(Sch)) polarizabilities and discuss in some detail their relationship. The latter alpha(Sch) is the value of the neutron electric polarizability as obtained from an analysis using the Schrodinger equation. We find in particular alpha(Sch)=<(alpha)over bar>-mu(2)/m, where mu is the magnitude of the magnetic moment of a neutron of mass m. However, we argue that the static polarizability alpha(E)(n) is correctly defined in the rest frame of the particle, leading to the conclusion that twice the Dirac-Foldy contribution should be added to alpha(Sch) to obtain the static polarizability alpha(E)(n).