CASIMIR ENERGY FOR A MASSIVE FERMIONIC QUANTUM-FIELD WITH A SPHERICALBOUNDARY

The vacuum energies corresponding to massive Dirac fields with the bou ndary conditions of the MIT bag model are obtained. The calculations a re carried out with the fields occupying the regions inside and outsid e the bag, separately. The renormalization procedure for each of the s ituations is studied in detail, in particular the differences occurrin g with respect to the case when the field extends over the whole space . The final result contains several constants that undergo renormaliza tion and can be determined experimentally only. The non-trivial finite parts which appear in the massive case are found exactly, providing a precise determination of the complete, renormalized zero-point energy in the fermionic case. The vacuum energy behaves as an inverse power of the mass, for large mass of the field.