Question about proton decay

[OripahsTrebor]

How does proton decay explain the CP-violation? I do not understand why proton decay would favor matter over antimatter.

[Jesse]

I don't think that proton decay is taken to explain CP-symmetry violation at all, the violation of CP-symmetry is just a fact about the way the weak force works according to the Standard Model of particle physics. But if you're asking about the explanation for the observed excess of matter over antimatter in the universe, from what I've read CP-symmetry-violating reactions that favor matter over antimatter are not on their own enough to explain the lack of antimatter observed in our universe--this page (http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/baryogenesis.html) mentions that Sakharov showed in 1967 that you need two other assumptions to explain it, one being violations of the conservation of something called "baryon number", and the other being the assumption that the early universe was in a far-from-equilibrium state. I'm not sure exactly what baryon number is (I assume each type of particle has a specific baryon number) or why the matter/antimatter asymmetry also implies baryon asymmetry, but the Standard Model predicts baryon number is always conserved in all reactions, and proton decay is part of an extension of the standard model which wouldn't conserve baryon number, which would make it relevant to this issue. Apparently the Standard Model can itself accomodate changes in baryon number without the need for any reactions which violate it though, based on some sort of of quantum tunneling between vacua with different values of baryon number, which is only thought to be possible at the high energies and temperatures that were present shortly after the Big Bang.

[Schneibster]

Proton decay, if it happens, could result in non-conservation of quark number; baryons are made from quarks, and there are two types: mesons, which are a combination of a quark and an antiquark, and hadrons, which are combinations of three quarks (and, of course, anti-hadrons, which are combinations of three anti-quarks). There may also be penta-quarks, but none have been detected; they would most likely be extremely short-lived, even by the scales of very short-lived high-energy hadrons, decaying extremely quickly into hadrons and mesons, and requiring enormous energy for their creation. It is possible that we will see pentaquarks on the main beamline of the LHC, if its energy is high enough, and if they are not merely mathematical chimarae.

However, extensive experiments have failed to detect proton decay.

On the other hand, there is theoretical reason to believe that the Cabibbo-Kobayashi-Maskawa matrix indicates that the quarks and antiquarks may not be entirely symmetric, and that this lack of symmetry explains both the CP-reversal violation, or T-reversal violation, inherent in certain weak interactions such as the decays of the K- and B-mesons, and the matter-antimatter asymmetry. The matter-antimatter asymmetry is manifest; the asymmetry of the K-meson is unquestioned. However, the precise mixing, or Cabibbo, angles of the different quark flavors in the CKM matrix is not yet fully known; top quarks, upon whose precise properties the last little bit of information about the mixing angles of the CKM matrix depend, can only reliably be produced at Fermilabs' Tevatron, and measurements of its mass are not yet precise as a result of the energy limitations of the Tevatron. The LHC should permit more precise measurements, and final determination of the precise missing mixing angle.

The hope is that the LHC will be capable of high enough energies to permit the final mixing angles of the CKM matrix to be measured conclusively, and the prediction is that those mixing angles will support both the matter-antimatter asymmetry and explain the K- and B-meson asymmetric decays and consequent CP- or T-reversal violations. It is extremely unlikely that the LHC will be incapable of finalizing this measurement.

If this turns out to be the case, then proton decay will not be seen, but the matter-antimatter asymmetry will be explained along with the CP- or T-reversal violations of the weak force. How this will affect supersymmetry or supergravity, or string physics, remains to be seen. Certain possible observations could conclusively support string theory, though it is not believed by most physicists that an absence of those observations would disprove it, and it is not considered enormously likely that the supporting observations will be made; the LHC is most likely not powerful enough to show conclusive supporting evidence for string physics.