Radiatively Induced Lorentz andCPTViolation in Electrodynamics
In a nonperturbative formulation, radiative corrections arising from Lorentz and $\mathrm{CPT}$ violation in the fermion sector induce a definite and nonzero Chern-Simons addition to the electromagnetic action. If instead a perturbative formulation is used, an infinite class of theories characterized by the value of the Chern-Simons coefficient emerges at the quantum level.