Matter-Antimatter Asymmetry in Experiments?

Question

As I hope is obvious to everyone reading this, the universe contains more matter than antimatter, presumably because of some slight asymmetry in the amounts of the two generated during the Big Bang. This raises the question of whether there are any processes short of the Big Bang that produce more matter than antimatter. That is, is there any known process where a particle collider (or whatever) would convert some energy into matter not through the production of particle-antiparticle pairs but through some process that produced more matter than antimatter? This doesn't need to be restricted to current accelerators-- if there's some mechanism for this that requires impractically high energies, but is based on solid theories (i.e., the Standard Model or straightforward extensions thereof), that would be interesting, too.

I'm fairly certain that the answer is "no," because I know that the matter-antimatter asymmetry is related to CP violation, and I also know that existing measurements of CP violation are not enough to explain the asymmetry. If there were a known way to slam protons together and make more quarks than antiquarks, I wouldn't expect this to still be a mystery. My particle physics knowledge is far from comprehensive, though, so it can't hurt to ask.

(I was briefly confused into thinking that there was such an experiment a while back, but it turned out to just be sloppiness about marking the antiquarks on the part of the people writing about it...)

(This is another question prompted by the book-in-progress, on relativity, this time a single word: I wrote that matter created from energy in particle physics experiments is "generally" in the form of particle-antiparticle pairs. Then I started wondering whether that qualifier was really needed, and thus this question.)

Answer 1

Dear Chad, you misinterpret the statement that "the known sources of CP-violation are not enough to explain the matter-antimatter asymmetry in the Universe."

You seem to think that the statement means that the known CP-violating parameter (namely the CP-violating phase in the CKM matrix) and the processes based on it are qualitatively insufficient to produce matter-antimatter asymmetry. But they are just quantitatively insufficient. One simply doesn't get enough of the asymmetry - but qualitatively, the CKM phase would be enough.

However, there are additional conditions beyond the CP-violation that have (or had) to be satisfied for the Universe to produce matter-antimatter asymmetry. They're known as the Sakharov conditions:

1. CP-violation as well as C-violation
2. Violation of the conservation of the baryon number B (and/or lepton number L)
3. Evolution away from the thermal equilibrium.

All of these "violations" have be present simultaneously to produce quarks and antiquarks asymmetrically. If one of them is absent, the processes remain matter-antimatter symmetric.

As you can see, lab experiments may deviate from thermal equilibrium but all lab experiments we can perform conserve the baryon number $B$ (as well as the lepton number $L$). That's why we can't imitate the matter-antimatter asymmetry in the lab.

The attempted "lab experiments" violating $B$ are the proton decay experiments - those big reservoirs of pure water with sensitive detectors able to see every single proton decay. So far, none of them has been seen (even though the simplest grand unified theories predicted that the proton decay should have been observed rather quickly). For theoretical reasons, it still seems extremely likely that the proton is unstable (although its lifetime is longer than expected in the SU(5) GUT) and $B$ is not conserved. Consequently, $L$ is not conserved, either.

In particular, black holes radiate the Hawking radiation away and the composition of the Hawking radiation carries $B=0$ in average because the event horizon looks the same regardless of the value of $B$ of the initial star that has collapsed into the black hole. This paragraph was meant to be a proof that locality implies that $B$ has to be violated in quantum gravity (or earlier, e.g. in the GUT theory) as long as there are no gauge fields associated with $B$.

However, the combination $B-L$ may be in principle conserved - it may be a generator of a grand unified group. However, this symmetry is probably broken because there are no long-range forces acting on this combined charge. So all these charges unrelated to gauge symmetries have to be violated (non-conserved) at some level; this reflects the wisdom that quantum gravity doesn't allow any global symmetries. Any symmetry is either explicitly broken by some effects or it is a gauge symmetry.

Answer 2

CP violation is necessary but not sufficient condition for matter-anti matter asymmetry. You also need Baryon number violation (sought after but not yet found e.g. in proton decay), and strong deviations from thermal equilibrium. Those three conditions are known as the Sakharov criteria.

I don't think it is a realistic goal to achieve all 3 of them in experiment. For example, the reason the standard model does not provide an explanation of the observed asymmetry is partially insufficient CP violation, and partially because the electroweak phase transition is not sufficiently non-equilibrium (i.e. it is second order, or weakly first order transition). By any reasonable criteria, this phase transition is an extremely violent process...

Answer 3

I'd like to point out that there is a small probability that the assumption on which the question is based:

"As I hope is obvious to everyone reading this, the universe contains more matter than antimatter,"

may not be true, depending on the result of the Aegis experiment at CERN.

That's because, as Professor Orzel stated in his answer to this question:

the primary evidence for a lack of vast amounts of antimatter out there in the universe is a lack of radiation from the annihiliation

but if matter and antimatter repel each other gravitationally, there could be whole galaxy clusters of matter, whole galaxy clusters of antimatter, and no particle or antiparticle getting near the boundary region between the matter and antimatter areas.

Of course, this is just a low-probability case to keep in mind until the jury is out on Aegis. I see it as a lifeboat: people keep it just in case, but while all is OK with the main ship, they don't live in it.

Answer 4

In reply to the second partenthetical question, I wrote that matter created from energy in particle physics experiments is "generally" in the form of particle-antiparticle pairs .

This is too restrictive. Quantum numbers have to be conserved, and they are conserved in pair production, but there can also be associated production of mesons etc:

For example a K+and a Lamda (+other particles) may come out of a proton proton collision, conserving strangeness, and similar with other conserved numbers. All sorts of interesting combinations can arise.

Answer 5

Feynman proposed that antimatter is the same as matter, except travelling backwards through time. Let's assume that this is true. If the Big Bang created antimatter, then it would have travelled off in the opposite temporal direction, into Negative Time. So we won't be seeing it again. Any antimatter we might see in our Universe was probably created locally, on this side.

This explains why there's an apparent asymmetry in the antimatter/matter.

I think it explains the Ultimate Free Lunch question (entropy).

It indicates the existance of Negative Time; a concept long overdue.

It raises the question about different temporal speeds, +1, -1, ...and more?