Constraints on the antisymmetric part of , which we denote , and come from the birefringence constraints on photon propagation (see Section 6.3). Here the loop corrections to the photon propagator induce a non-zero , which can be directly constrained. This yields a constraint on all three coefficients of order . A bound can be derived from the cyclotron frequencies of hydrogen ions and anti-protons. Bounds on the CPT violating term come from both the spin polarized torsion balance experiments and the noble gas maser. The torsion experiments bound the and components (where is parallel to the earth’s rotation axis) at the level of and the transverse components at . The maser system gives a better, although less clean, bound on the transverse components of order .
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