Lorentz and CPT violations from Chern-Simons modifications of QED

The possibility of a small modification of spinor Quantum Electro-Dynamics isreconsidered, in which Lorentz and CPT non-covariant kinetic terms for photonsand fermions are present. The corresponding free field theory is carefullydiscussed. The finite one-loop parity-odd induced effective action isunambiguously calculated using the physical cutoff method, which manifestlyencodes the maximal residual symmetry group allowed by the presence of theLorentz and CPT breaking axial-vector. This very same induced effective action,which is different from those ones so far quoted in the Literature, is alsore-derived by means of the dimensional regularization, provided the maximalresidual symmetry is maintained in the enlarged $D$-dimensional space-time. Asa consequence, it turns out that the requirement of keeping the maximalresidual symmetry at the quantum level just corresponds to the physicalrenormalization prescription which naturally fixes the one-loop parity-oddinduced effective action.Comment: 24 pages, Latex, JHEP style, some modifications in design and abstrac