How does CPT violation differ from violations of C, P, T, CP?
Violations of all the symmetries C, P, T, CP are predicted by the Standard Model of particle physics and are observed in experiments. Only the combination CPT is required by the Standard Model to be a symmetry of nature. For example, processes are known in nature that violate C but not CPT. The Standard-Model Extension allows for violations of Lorentz and CPT symmetry that cannot occur in the usual Standard Model or Einstein’s General Relativity. We have shown, for instance, that it allows for CPT violation unaccompanied by C or P or CP violation, causing effects such as a difference between the spectra of hydrogen and antihydrogen. Similarly, it allows for CPT violation unaccompanied by P or T or PT violation, producing effects such as modifications of the behavior of kaons and antikaons. All these effects are forbidden in conventional theories obeying the CPT theorem.
