= Time-reversal parity of a fermion electric dipole operator
For real $a$, the Hermitian effective interaction $ia\bar\psi\sigma^{\mu\nu}\gamma^5\psi F_{\mu\nu}$ is odd under <time-reversal symmetry>. The spinor tensor and <gauge field strength> both acquire $-t_\mu t_\nu$, while the <chirality matrix> is even. Their signs cancel, leaving the minus sign from the explicit $i$ under <antiunitarity>. By the <CPT theorem> this is also <CP violation>. It is not a renormalizable tree-level <Standard Model> term, but a radiatively generated low-energy dipole is allowed because of the <CP-violating phase> in the <CKM matrix>.
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