bmonk wrote: WIMP wrote:
Pfhorrest wrote:Question: Is the equal production of matter and antimatter strictly guaranteed by the laws responsible for it, or is it merely a statistical thing? E.g. is it possible to condense a bunch of energy into just an electron or positron, but on average a volume of high energy density will condense into equal numbers of each; or does the production of an electron demand the production of a positron to maintain conservation of something?
Always, 100% of the time, equal amounts of matter and antimatter are produced by any process, if you're purely counting antiparticles and particles. CP violation allows for some asymmetries, though. For example, a kaon decays very slightly more often into a positron and neutrino than electron and antineutrino. At the end of the day you've created as many particles as antiparticles (positron + antineutrino = neutrino + electron), but if all you care about is positrons and electrons, you've created more antimatter than matter. It's this kind of asymmetry, presumably amplified at the high energies of the early Universe, that led to today's apparent matter-antimatter imbalance. That is, lots of electrons (and baryons like the proton), not so many positrons (and antibaryons like the antiproton). But, many trillions of antineutrinos from the sun pass through your body every second, from above during the day and from below after passing through the Earth at night. Don't say antimatter doesn't exist in normal circumstances!
That's not quite true. If 100% equal matter and antimatter is produced, then how does the observed universe have such an excess of matter? There are theories that some processes must create, or destroy, matter and antimatter unequally--even if only by a billionth of one percent or less--or no matter would remain at all.
Because the observed particles are the equivalent of the electron, and the unobserved particles are the equivalent of the antineutrino. That is, the Universe is dominated by protons and electrons, not antiprotons and positrons, but something else somewhere is picking up the slack for antimatter.
The dominant theory for the imbalance is enhanced CP violation in the early Universe (ie a "dynamical" source). CP violation *does not* produce one electron and nothing else. It produces an electron and antineutrino more often than a positron and neutrino (or vice versa, and the equivalents for other particles). For example, the long lived kaon decays to a positron and neutrino (plus other stuff) more often than to an electron and antineutrino (plus other different stuff), to the tune of a part in 10,000 if memory serves. That is not the same thing as decaying to an electron and neutrino alone, thus cutting antimatter out of the equation entirely. The decay treats matter and antimatter differently, meeting the definition of CP violation, and at first glance it appears to produce antimatter more often than matter, but only if you're biased towards easily detectable particles (electrons and positrons) instead of hard-to-detect particles (the neutrinos).
It is impossible, in the fundamental structure of quantum field theories, to produce unequal amounts of matter and antimatter. CP violation treats matter and antimatter differently, allowing for an imbalance in, say, electrons and positrons (this is what's observed in the Universe--in favor of electrons), but not an overall matter-antimatter imbalance (so there'd be, for example, a reverse imbalance in favor of antineutrinos over neutrinos). The Standard Model CP violation is far too small to account for the observed effect, so there must be enhanced CP violation at high energies, but the mechanism is widely agreed to be similar to the small effect seen at low energies, just on a different order of magnitude.
And the comment about Majorana neutrinos is correct but in the opposite direction. What I say is *false* in a sense if neutrinos turn out to be Majorana particles, and true iff they are Dirac particles. You can make a case that Majorana neutrinos make sense, but there is an ever so slightly more compelling case that they're Dirac particles, and that's the Standard Model interpretation. I'll come back and edit this if anyone finds neutrinoless double beta decay == they're Majorana.