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(C)harge conjugation + (P)arity transformation + (T)ime reversal - CPT - is observed to be an exact symmetry of nature (as far as anyone can tell so far anyway). Applying the C, P and T operators to the entire Universe would exchange every particle for its antiparticle (and also mirror-image everything, and flip all momenta). Provided CPT is in fact an exact ...


Almost, but not quite. The answer to the questions you ask Are all the constants (Boltzmann's, gravitational, c) the same? Would elements react differently? Would it look the same? are all "yes", but various processes controlled by the weak nuclear interaction violate charge symmetry (sometimes known a C-symmetry) and would be different. Most of those ...


The difference, I think, is that $\overline B{}^0 \to B^0$ and $B^0 \to \overline B{}^0$ are inverse processes, and so a difference between them can be ascribed to broken $CP$ symmetry, as you say, or to broken time-reversal symmetry. Only with the additional (strongly motivated) assumption that $CPT$ is an exact symmetry of nature can you definitely say ...


Answer transposed from a comment: the $K$, $D$, $B$ have nonzero "flavor quantum number" (strangeness, charm, and beauty, to be specific). The analogy you should pursue is the $J/\psi$ or "charmonium," made of a charm quark and charm anti-quark.

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