A CPT anomaly

We consider chiral gauge theories defined over a four-dimensional spacetime manifold with a Cartesian product structure for at least one compact spatial dimension. For a simple setup, we calculate the effective gauge field action by integrating out the chiral fermions, while maintaining gauge invariance. Due to a combination of infrared and ultraviolet effects, there appears a CPT-odd term in the effective gauge field action. This CPT anomaly could occur in chiral gauge theories relevant to elementary particle physics, provided the spacetime manifold has the appropriate topology. Two possible applications for cosmology are discussed.