  We investigate the breaking of Lorentz symmetry caused by the inclusion of an
external four-vector via a Chern-Simons-like term in the Duffin-Kemmer-Petiau
Lagrangian for massless and massive spin-one fields. The resulting equations of
motion lead to the appearance of birefringence, where the corresponding photons
are split into two propagation modes. We discuss the gauge invariance of the
extended Lagrangian. Throughout the paper, we utilize projection operators to
reduce the wave-functions to their physical components, and we provide many new
properties of these projection operators.

High Energy Physics - Theory