Origin of Electrons: Big Bang & Annihilation

[mathman]

My understanding of the origin of matter is that it arose from pair production after the big bang. The process leading to electrons also produces an equal number of positrons. The universe now has lots of electrons and essentially no positrons. Since positrons are inherently stable, I presume they disappeared through electron annihilation. Where did the surplus electrons come from?

 

[Drakkith]

THAT is one of the big mysteries! Antimatter in general, not just positrons, should have been produced in equal amounts as matter after the big bang. The reason why we live in a matter dominated universe is an unanswered question that many many people seek!

 

[BillSaltLake]

Although this is (sort of) substituting one mystery for another, it's reasonably possible that the excess of matter over antimatter occurred only once, in the form of an excess of neutrons over antineutrons. Then the excess electrons came from the neutrons that decayed into an electron and proton (and an antielectron neutrino).

 

[Tanelorn]

I hope this doesn't sound completely silly, but has the presence of undetected anti-matter in our universe been completely ruled out? Would we know if distant isolated galaxies or the earliest super massive black holes were composed of anti-matter?


Having problems accessing this site today, hope all is well now, it is great site.

 

[Chronos]

Pockets of anti matter cannot be ruled out, but, appear improbable. Collisions with matter pockets would be betrayed by highly energetic gamma ray emissions. Such emissions, with appropriate properties, are not observed

 

[apeiron]

Although this is (sort of) substituting one mystery for another, it's reasonably possible that the excess of matter over antimatter occurred only once, in the form of an excess of neutrons over antineutrons. Then the excess electrons came from the neutrons that decayed into an electron and proton (and an antielectron neutrino).

Hi Bill - do you have a source for that idea? That electrons came from neutron beta decays rather than being spawned from a bath of hot photons via pair production?

 

[twofish-quant]

I hope this doesn't sound completely silly, but has the presence of undetected anti-matter in our universe been completely ruled out?

No, but there are constraints. Here is an old review paper on the topic

http://adsabs.harvard.edu/abs/1976ARA&A..14..339S

Would we know if distant isolated galaxies or the earliest super massive black holes were composed of anti-matter?

Yes. There is this paper that gives a good argument that if there are anti-matter pockets, they have to be at least the size of galaxy clusters.

http://arxiv.org/abs/0808.1122

The idea is that galaxies organize themselves into clusters so if the clusters that we see had a mix of matter/anti-matter galaxies, they would produce much more radiation than we see. So based on the radiation that we see, we can limit the amount of matter/anti-matter difference.

It's still possible that the galaxy clusters that we see are all *anti*matter, but we should be able to beat those limits down.

 

[twofish-quant]

Hi Bill - do you have a source for that idea?

see http://en.wikipedia.org/wiki/Baryogenesis#Sakharov_conditions

That electrons came from neutron beta decays rather than being spawned from a bath of hot photons via pair production?

That's a pretty standard view of how things happened.

We are pretty certain that the electron/positron asymmetry didn't happen via pair-production. The reason for this is that pair-production reactions happen at relatively low energies at which we have very good models. Both our current models (the electro-weak model) and our observations (pair-production happens in particle accelerators all of the time) say that you can't get an asymmetry via standard pair-production.

Now we think the asymmetry happened with baryon production, and that the universe produced slightly more baryons than anti-baryons. The reason for this is that 1) it's harder to rule out things by observation 2) our models for how neutrons work are a *lot* less firm than for how electrons work and 3) we do see some nuclear processes that are "asymmetric" is sort of the same way that we expect matter/anti-matter to be.

Also at extremely high temperatures, it's more useful to think of neutrons and (proton+electrons) as one type of particle. What happens is that at "ordinary temperatures" the reaction neutron-> proton+electron+anti-neutrino happens a lot faster than the reverse reaction proton+electron+anti-neutrino -> neutron. Once you start increasing the temperatures, the reverse reaction happens almost as quickly as the forward reaction so at extremely high temperatures, it's easier to think of this as a soup of baryons that are changing their identities very quickly.

 

[twofish-quant]

My understanding of the origin of matter is that it arose from pair production after the big bang.

Pair production is only one of many particle reactions. The problem with pair production is that it produces equal numbers of particles and anti-particles, so standard pair production *isn't* what caused the imbalance.

As far as what did, the place that we think it happened in was when neutrons formed. We think it happened then, because we don't understand how that worked, so putting a mystery process in something we don't quite understand, makes sense.

 

[apeiron]

Also at extremely high temperatures, it's more useful to think of neutrons and (proton+electrons) as one type of particle. What happens is that at "ordinary temperatures" the reaction neutron-> proton+electron+anti-neutrino happens a lot faster than the reverse reaction proton+electron+anti-neutrino -> neutron. Once you start increasing the temperatures, the reverse reaction happens almost as quickly as the forward reaction so at extremely high temperatures, it's easier to think of this as a soup of baryons that are changing their identities very quickly.

Thanks, the baryon soup explanation makes more sense.

So this means that the uncertain part of the baryogenesis story remains the strong CP problem? Getting the right degree of matter/antimatter asymmetry out strong force couplings? It is not that neutrons have been observed to have the necessary CP violation as yet (ie: no recent discovery of the neutron electric dipole moment).

The baryon soup also raises a supplementary question.

It seems to be saying that up and down quarks ought to be generated in equal numbers out of the Big Bang (due to pair production). And this is why in turn you even get leptons being produced.

You have to have equal numbers of neutrons and protons (a neutron/proton pair giving you three up quarks to match every three down quarks). And this in turn leaves over enough energetic slop to make a further lepton pair.

So the origin of the electron would seem to sheet back to this further grand coincidence of nature - one that is not a CP violation issue, a matter/antimatter asymmetry, but just a matter asymmetry (due to the differing masses of up and down quarks)?

 

[twofish-quant]

So this means that the uncertain part of the baryogenesis story remains the strong CP problem?

According to Sakharov, you need to violate CP to get matter/anti-matter asymmetry, and you need to violate CP by just the right amount.

t is not that neutrons have been observed to have the necessary CP violation as yet (ie: no recent discovery of the neutron electric dipole moment).

Right. Neutrons and protons have never been observed to violate CP. There is only one known process that violates CP and that is the decay of the neutron Kaon. The belief is that if you increase the temperature high enough so that neutrons and protons start changing into other types of particles that something will happen then that will give you an matter imbalance.

If this sounds vague and unclear, that's because it is, we haven't worked out the details, and that's partly because we can't practically get particle accelerators up to those energies.

It seems to be saying that up and down quarks ought to be generated in equal numbers out of the Big Bang (due to pair production).

Nope. Pair production will produce equal amounts of up and anti-up or down and anti-down. Up and down quarks are different types of quarks and you won't get equal amounts.

You have to have equal numbers of neutrons and protons (a neutron/proton pair giving you three up quarks to match every three down quarks).

Actually you don't. Down quarks and neutrons are slightly heavier than up quarks and protons. Also all of the reactions preserve charge so that the number of protons and electrons are the same.

So what happens is that, you produce slightly fewer neutrons than protons. You can take the mass difference, and calculate how many fewer neutrons get made.

The really cool part is that the neutrons that exist get paired up with protons to become helium. The protons that are left end up making hydrogen. You then being able to show that given that difference in proton/neutron masses, that about 25% of the universe should be helium, and voila, it is...

Here is a sketch of the argument

http://adsabs.harvard.edu/abs/1991ApJ...376...51W

Also the first section is interesting to read, because you see the process of "figuring this out"

So the origin of the electron would seem to sheet back to this further grand coincidence of nature - one that is not a CP violation issue, a matter/antimatter asymmetry, but just a matter asymmetry (due to the differing masses of up and down quarks)?

If the charge is conserved and you have protons, then you have to have electrons. Now why protons are heavy and electrons are light is a mystery of the universe that we don't currently understand.

 

[apeiron]

If the charge is conserved and you have protons, then you have to have electrons. Now why protons are heavy and electrons are light is a mystery of the universe that we don't currently understand.

I'm finding this rather confused. Now remember I was querying BillSaltLake's original statement...

the excess of matter over antimatter occurred only once, in the form of an excess of neutrons over antineutrons

So the implication was that neutrons were the prime decay path because of some CP mechanism. And as you agree, we have no evidence yet for strong force CP violation, just EW violation (by K mesons AND B mesons). So my question is why should the focus be on neutrons rather than protons or quarks, or even supersymmetry and other exotica?

Neutrons may perhaps be the best candidate because of the electric dipole moment hypothesis. Yet then measurement also seems to have pretty much eliminated that now? Therefore I was wondering what was motivating BillSaltLake's comment. Because as far as I was aware, there are still a ton of other suggested baryogenesis mechanisms. See this post for instance...https://www.physicsforums.com/showthread.php?t=219026. What is the argument that favours neutrons so strongly?

On charge conservation, it seems to me that charge would be adequately conserved by proton/antiproton pair production and annihilation. So the proton/electron deal only kicks in if there is already an excess of neutrons over antineutrons. Again, it is that first imbalance that has to be explained (if neutron CP violation is indeed the primary pathway).

But thanks for correcting me on the up and down quarks. Yes, more lighter up quarks ought to be produced. So the baryon soup would in fact be biased towards protons in the first place?

 

[BillSaltLake]

If in a certain high-temperature process, you ended up with more protons than antiprotons (starting with equal numbers), and no excess electrons were formed, you would violate charge cons. To avoid this, a "double coincidence" of excess protons and excess electrons must have occurred simultaneously, even though proton formation temperature is 2000x higher than electron.
In contrast, a formation of excess neutrons does not violate any "strong" conservation rule (baryon # is not absolutely cons.) and does not require a double coincidence.

 

[Multi]

What about X-boson?

 

[BillSaltLake]

There are many other possibilities, but the direct creation of excess protons is unlikely compared to direct creation of excess neutrons.

 

[mathman]

There are many other possibilities, but the direct creation of excess protons is unlikely compared to direct creation of excess neutrons.

What is the physics behind this assertion?

 

[apeiron]

In contrast, a formation of excess neutrons does not violate any "strong" conservation rule (baryon # is not absolutely cons.) and does not require a double coincidence.

Thanks, so are you saying that neutrons are fingered just for these indirect, circumstantial reasons, or because there are also plausible CP violation mechanisms (like the electron dipole moment)?

If you could point to any recent papers discussing the issue in basic fashion, that would be helpful. I've searched, but there was surprisingly little.

 

[BillSaltLake]

These are just guesses on my part. In studying particle processes, many reactions are disallowed, and those which are allowed but require a double coincidence are generally rarer than other reactions. With present knowledge, the excess of matter is "disallowed", so this is all still just conjecture.

 

[Chronos]

Another suspicious issue to consider is free neutrons spontaneously decay in a matter of minutes except under extreme gravity conditions - e.g., neutron stars. I think neutrons conceal more secrets than we currently suspect.