Charge parity symmetry violations (left, right, plus, minus) is equivalent to time symmetry violations (forward and backward), in which quantum processes happen at different rates for particles and anti-particles.
This doesn't happen in strong force or electromagnetic force events. It doesn't happen in Z boson mediated weak force events (see, e.g., here looking at D meson events). It doesn't happen in gravity. It only happens in charged weak force events mediated by W bosons.
It is more specific than that, however. It also has only been observed in events that involve one of two kinds of neutral charge mesons made up entirely of bottom type quarks: kaons comprised of both strange quarks and down quarks (one of which must, of course, being an anti-quark), and B mesons comprised entirely of down type quarks of differing generations (down, strange or bottom). All B mesons, of course, have bottom quarks in them. But, it the latest rumor from LHCb are true only neutral B mesons (one type of which has a down quark and a bottom quark, and the other type of which has a strange quark and a bottom quark) which exhibit oscillation, have been observed.
The possibility of charmed B meson involvement in CP violations had been hinted in the dimuon channel discussed in the previous post at this blog, but it appears that this result from D0 may not hold. I've also never heard of CP violation in B mesons with an up quark and bottom quark. (It is believed that bottom and top quarks never form mesons because the top quark decays to quickly for a meson to form).
Similarly, while all kaons, have one strange quark, CP violation has been observed only in the neutral kaons, with a down quark and strange quark, not charged kaons with up quarks and strange quarks. (Kaons with top quarks don't form for the same reason that there are no B mesons which have top quarks; CP violation is also not observed in mesons with both a strange quark and a charm quark).
There are some notable pieces to this observation.
First, all CP violation has been observed in the context of neutral particles that have oscillations. Neutrinos also oscillate, but have not been observed to be CP symmetry violating at this time.
Neutrons are neutral and have internal flavour, but while there "are theoretical proposals that neutron–antineutron oscillations exist, a process which would occur only if there is an undiscovered physical process that violates baryon number conservation."
Notably, lepton family number violation, which is observed in oscillating neutrinos, has also not been observed in charged leptons (electrons, muons and taus) (lepton number violation as opposed to lepton family number violation has not been observed at all).
The fact that particles that are electromagnetically charged and that electromagnetism is not CP violating could be relevant to this point.
Of course, leptons have not been observed to form composite particles at all. They are also not observed in any charged composite particles.
Second, CP violation has not been observed in the decays of electrically neutral mesons that contain up quarks or charm quarks a.k.a. neutral D mesons (top quarks don't form mesons), despite rigorous searches for this as recently as 2008, even though 2009 research has shown that neutral D meson mixing does take place (without CP violation), just as it does for other neutral particles (see also here).
Third, they are not observed in baryons (i.e. three quark composite particles) that contain up type quarks (or for that matter any type of baryon) or in neutrinos.
Fourth, CP violations have not been observed in mesons with a quark and antiquark of the same type (e.g. the Phi meson made of a strange and anti-strange quark or the J/Psi meson made of a charm and anti-charm quark or an upsilon meson made of a bottom and anti-bottom quark aka bottomium).
Only neutral mesons with different generations of bottom type quarks in them exhibit CP violations.
Of course, this could be because there is no experimental way to observe CP violation in an oscillating same quark type meson. The only way that these mesons can form at all is because they differ in color charge (otherwise that would be antiparticles of each other and annihilate). But, due to quark confinement, it isn't possible to observe color asymmetries.
It is a fine technical point that I could be overlooking about the interaction between the quantum mechanical considerations involved in neutral particle oscillation and the the CKM matrix or the way that the need for two out of phase complex conjugates works to lead to CP violation, but I am not aware of anything in the Standard Model CKM matrix or weak force interaction that makes the presence or absence of CP violation in the weak force interactions of quarks to be influenced in any way by the components of a composite particle. Yet, both the fact that protons do not decay and the absence of CP violation in mesons of mixed up and down type quarks, is suggestive of the possibility that this could be an omission.
The extremely specific and narrow circumstances in which CP violation and lepton family number violation are observed may provide some hints about what is going on in these interactions at a deeper level.
If same type quark-antiquark mesons are truly impossible to observe CP violations in due to confinement at even a theoretical level, and there are only three generations of fermions, as the Standard Model proposes, then we have a complete set of CP violating particles in the neutral kaon and neutral B meson, both of which are extreme exotic and fleeting creatures never seen in anything outside laboratory conditions in the real world. What an oddly limited way for quantum mechanics to acknowledge an arrow of time.
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