Anomalous propagation of neutrino beams through dense media

The phenomenon of fermion oscillations in matter is examined using the Dirac equation. The resonant oscillation observed by Mikheyev, Smirnov, Wolfenstein, and Bethe for solar neutrinos is reinterpreted as birefringence for a family of fermion waves. Particular attention is given to the presence of negative-energy states in the medium, and the resultant redefinition of the ground state. Such a redefinition is not expected to affect significantly the solution to the solar-neutrino puzzle posed by a resonant oscillation frequency within the solar core, but can alter the propagation of fermions through ultradense media.