New Wired article on "where's the antimatter"

[jnorman]

I am a simple layman, but I hope one of you wizards can help me understand something here.
I just read the new article in Wired magazine on the problem of the missing antimatter, a topic I have read about before in several popular books on physics and cosmology, and I have a basic understanding of the issue.
In the article, they discuss a purported answer to why there is an abundance of matter and apparently no significant amount of antimatter- a radioactive process called neutrinoless double-beta decay where two neutrons turn into two protons and produces two electrons.
Now please forgive my ignorance, but it seems to me that to have a radioactive atom to begin with would require that you already have stars in which a radioactive atom could be created. For a star to form, the universe would have to have already progressed far beyond the time when the matter-antimatter balance was problematic. I do not see how the proposed decay process in the article could possibly account for the missing antimatter problem.
Can someone please enlighten me a bit on this? Thank you.

 

[PeterDonis]

I just read the new article in Wired magazine

Please give a link.

In the article, they discuss a purported answer to why there is an abundance of matter and apparently no significant amount of antimatter- a radioactive process called neutrinoless double-beta decay where two neutrons turn into two protons and produces two electrons.

Is there a link in the Wired article to the actual peer-reviewed paper that gives this hypothesis? If not, it's going to be hard to discuss since Wired itself is not a valid source.

 

[Bandersnatch]

This is the wired article:
https://www.wired.com/2017/04/hunt-universes-missing-antimatter/
It's referencing this Nature paper:
http://www.nature.com/nature/journal/v544/n7648/full/nature21717.html

Now please forgive my ignorance, but it seems to me that to have a radioactive atom to begin with would require that you already have stars in which a radioactive atom could be created. For a star to form, the universe would have to have already progressed far beyond the time when the matter-antimatter balance was problematic. I do not see how the proposed decay process in the article could possibly account for the missing antimatter problem.

While the experiments performed in order to observe the process mentioned above use large nuclei of xenon and germanium, it is a general process that can happen in other circumstances. The important bit is that it is one of the possible processes that violate the CP symmetry (here, the lepton number is not conserved). The CP violation processes allow for more matter to be produced than antimatter, rather than equal parts of both, so identifying and quantifying these let's you build a model of baryogenesis in the early universe that naturally produces enough matter overabundance to account for what we observe today.
This article on the wiki talks about it in detail:
https://en.wikipedia.org/wiki/CP_violation#CP_violation_and_the_matter.E2.80.93antimatter_imbalance

 

[jnorman]

Thanks bander- okay so it is a process which doesn't necessarily require heavy atoms. However, if this process can occur with matter particles decaying to yield a new pair of electrons, it seems that it would similarly occur with antimatter particles decaying to yield a new pair of anti-electrons. How does this solve the problem at hand?

 

[bahamagreen]

Isn't it considered true that among the identical properties of particles and antiparticles are their lifetimes? Aren't lifetimes, half lives, etc. probabilistic periods... in that the figures may be precise and accurate, but they are aggregates; that individual particles will show variation? At a very early local scale couldn't a fluctuation in lifetime come to bias and subsequently dominate what later becomes the observable universe?

I'm thinking it seems that a present balance of matter and antimatter might be exceedingly unlikely because it would assume that there were no early particle lifetime fluctuations of consequence. If I do a series of 100 coin tosses the individual series that come out 50:50 would probably be in the minority.

If early local variation in particle lifetimes do not have enough influence in the initial production and annihilation of particles to cascade a bias for what becomes the present observable universe, what principal suppresses it? Or is it that the initial imbalance is thought to occur earlier than the greatest short time variations of particle lifetimes, and so back to square one?

 

[Chalnoth]

Thanks bander- okay so it is a process which doesn't necessarily require heavy atoms. However, if this process can occur with matter particles decaying to yield a new pair of electrons, it seems that it would similarly occur with antimatter particles decaying to yield a new pair of anti-electrons. How does this solve the problem at hand?

The relationship isn't at all simple. The basic idea can be thought of like this:
1. Some beyond-standard-model theories which are developed to explain the matter/anti-matter imbalance have a peculiar property: neutrinos are their own anti-particles.
2. If these theories are true, then one observable consequences is a specific type of nuclear decay that does not occur under the standard model.
3. If we see these decays occurring, then that provides evidence that this kind of extension of the standard model may be correct (we'd need more than just this to gain anything approaching real confidence).

So there's no direct link between the specific decays being looked for. It's just that the theory that allows for the matter/anti-matter imbalance being studied has other ways to measure it.

 

[rootone]

I think neutrinos (and their unspeakable relatives) might be the guilty party,
since they are their own antimatter and have a history of misbehaving when being questioned.

 

[PeterDonis]

I think neutrinos (and their unspeakable relatives) might be the guilty party,
since they are their own antimatter

No, they're not. The fact that a particle is electrically neutral does not mean it's its own antiparticle. There are other properties besides electric charge involved (in the case of neutrinos, weak isospin and hypercharge).

 

[Bandersnatch]

No, they're not. The fact that a particle is electrically neutral does not mean it's its own antiparticle. There are other properties besides electric charge involved (in the case of neutrinos, weak isospin and hypercharge).

I thought the whole point of the neutrinoless double beta decay mentioned in the article was to provide evidence for neutrinos annihilating with themselves as their own antiparticles. Isn't that the case?

 

[PeterDonis]

I thought the whole point of the neutrinoless double beta decay mentioned in the article was to provide evidence for neutrinos annihilating with themselves as their own antiparticles. Isn't that the case?

Hm, from looking at the Wikipedia page on double beta decay, it does reference papers talking about the process, if it were observed, being evidence for a Majorana neutrino:

https://en.wikipedia.org/wiki/Double_beta_decay

Unfortunately the papers are behind paywalls so I can't access them. But any such theory would go beyond the current Standard Model, so discussion of it probably belongs in the Beyond the Standard Model forum. My comments were based on the Standard Model as it currently exists; in that model, as I understand it, neutrinos are not Majorana fermions.