Matter-antimatter symmetry and particle charge

[Hypatio]

If matter composed of quarks of positive charge, and anti-quarks have negative charge, why do we call electrons particles of matter rather than anti-matter?

Is it possible that the matter-antimatter symmetry problem exists because we are actually calling something that is a particle of anti-matter, a particle of matter?

Also, am I correct to assume that annihilation between particles of matter and anti-matter may not occur if they are different types of particles of each. For instance, an electron and positron will annihilate but an electron will not annihilate with another types of anti-matter.

I am probably confused, but I hope someone can critique my thought process.

 

[kurros]

If matter composed of quarks of positive charge, and anti-quarks have negative charge, why do we call electrons particles of matter rather than anti-matter?

Well, there is more to being antimatter than simply having a certain charge. But you do raise an interesting question to which I don't really know the answer: why in the early universe were electrons produced in approximately the same ratio as protons, such that the overall charge of the universe is approximately zero, as it more or less appears to be?

I suppose it must be the case that if we assume net zero charge to begin with, then we must end up with either an excess of protons and electrons, or an excess of anti-protons and positrons, but I don't know that we have to make this assumption, except that experimentally it seems to fit the bill and that it is a minimal sort of assumption. Plus I suppose there are nice ideas about the universe being some sort of inflated vacuum fluctuation, and the vacuum has net zero charge.

Is it possible that the matter-antimatter symmetry problem exists because we are actually calling something that is a particle of anti-matter, a particle of matter?

Well, no, because you would still have a set of particles which behaved (almost) exactly the same as the ones we see around us, yet not know why the universe should have chosen one set over the other.

Also, am I correct to assume that annihilation between particles of matter and anti-matter may not occur if they are different types of particles of each. For instance, an electron and positron will annihilate but an electron will not annihilate with another types of anti-matter.

Yes that is the case. Particles only annihilate with their own antiparticles. They come in pairs, unless they are their own antiparticles (like photons).

 

[Hypatio]

Well, if particles only annihilate with their own antiparticles, then if you call a particle like a positron the particle and an electron an anti-particle, then this changes the matter-antimatter symmetry.

The question might be why we have unequal amounts of matter and anti-matter of certain types of particles, while the net energy of anti-particles is the same as particles.

Also, it seems the picture is complicated by the fact that you can get anti-particles from particles, such as in proton decay into a neutron, positron, and a neutrino.

How do we really know then that this is a problem for early synthesis of particles, and not of later mechanisms rearranging things?

 

[kurros]

Well, if particles only annihilate with their own antiparticles, then if you call a particle like a positron the particle and an electron an anti-particle, then this changes the matter-antimatter symmetry.

The question might be why we have unequal amounts of matter and anti-matter of certain types of particles, while the net energy of anti-particles is the same as particles.

I am not quite sure what you are asking here, sorry. Semantics are not important. Perhaps we should try and speak in terms of baryon and lepton numbers. Those are the quantities that we are really talking about, and we want to know why they are not both zero (in the whole universe). In the Standard Model, both baryon number and lepton number are conserved (except for some weird thing about sphalerons that I don't understand, and which have never been observed, although they may have been important in the early universe). So if you move forward past the early universe, the number of baryons minus antibaryons and leptons minus antileptons in the universe is fixed. The numbers that they are fixed to are such that we have loads of protons/neutrons/electrons flying around instead of their antiparticles, and understanding why both these numbers are not zero, and what happened in the early universe to cause this, is the matter/antimatter asymmetry problem.

Also, it seems the picture is complicated by the fact that you can get anti-particles from particles, such as in proton decay into a neutron, positron, and a neutrino.

In this picture both baryon number and lepton number are conserved as the positron and neutrino have opposite lepton number. So nothing weird there. You made some matter and antimatter at the same time, so no asymmetry.

How do we really know then that this is a problem for early synthesis of particles, and not of later mechanisms rearranging things?

Well, because the Standard Model conserves baryon and lepton number (except for this non-perturbative sphaleron business and perhaps some other exotic things I don't know about), so there are no later mechanisms for rearranging things.