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Comment by T-A | Hacker News Reader

> if we have a kind of nucleus that undergoes beta decay, and we built the exact same nucleus except out of antimatter, it would not undergo beta decay. Is that correct?

No. Until 1964 you would have been told that applying CP, i.e. the combination of C transformation (charge conjugation, i.e. change the signs of all charges) and P transformation (parity, i.e. swap left and right) would result in exactly the same decay rate. You would get anti-neutrinos (which are right-handed) instead of neutrinos, but that would be the only difference.

Then it turned out that CP symmetry is also violated by weak interactions, though far from as neatly as just P:

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

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Then I don't understand something. (Or several somethings...)

I thought you said that going to antimatter meant that you reversed chirality, and that right-handed chirality meant that a particle did not take part in the weak interaction. I interpreted that to mean that it didn't take part at all, not with any probability. But the CP violation article seems to be saying that CP violation is a matter of differences in probabilities.

By the way, that article talks about anti-neutrinos, which from your initial post, I thought you were saying were impossible to detect if they existed?

I'm confused... can you make any of this clearer? I'm mis-understanding something - can you tell what?

Given a fermion species, let's say an electron, you can have:

- left-handed electron

- right-handed electron

- left-handed anti-electron

- right-handed anti-electron

Going from particle to anti-particle, you swap charge and chirality (that's the CP transformation). So for instance a left-handed electron with negative electric charge becomes a right-handed anti-electron with positive electric charge.

For neutrinos, given that left-handed neutrinos have weak interactions, then so do right-handed anti-neutrinos.

It's no harder to detect anti-neutrinos than neutrinos. It's right-handed neutrinos and left-handed anti-neutrinos you'd have a problem with.

The CP violation article talks about probabilities because CP violation is not nearly as neat as P violation alone. Instead of the clear-cut "left-handed neutrinos (and therefore right-handed anti-neutrinos) only" of P, you get small differences.

Thanks! I think that was what I needed.

One last question: I was under the impression that the difference between a neutrino and an antineutrino was only chirality. What other property is there to distinguish a neutrino from an anti-neutrino?

Weak isospin. Look at the table in

https://en.wikipedia.org/wiki/Weak_isospin#Relation_with_chi...

(including the text in the last two rows).

Thank you for your knowledge, patience, and persistence in these answers.

That was an amazing discussion. Thank you both.

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