Observing Proton Decay & Antineutrinos

[swampwiz]

antineutrino?

I was looking at this article, which says that a proton that interacts with an antineutrion transforms (or whatever the proper verb is here) into a neutron & positron. But this begs the question that if we're trying to observe a proton decaying, how would we know that it had not interacted with an antineutrino?

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

 

[Dr.AbeNikIanEdL]

First, proton decays are not to neutrons, so if you have perfect knowledge of your final state you know what happened. Practically that won't be the case however, so indeed you won't know what happened in an individual event.
In general, most experiments have some background that you need to be able to handle. If you are looking for proton decay by counting positrons you are essentially asking "do we see more positrons then we would expect from neutrino+proton interactions (+ other backgrounds)". If you can detect neutrons you can use this to keep the background lower.

 

[malawi_glenn]

Proton decays are also very model-dependent, what the final states are

 

[mathman]

current theory doesn't have proton decay.

 

[malawi_glenn]

current theory

What you mean is that there are no proton decays in the Standard Model of particle physics (Glashow-Salam-Weinberg model).

 

[mathman]

What you mean is that there are no proton decays in the Standard Model of particle physics (Glashow-Salam-Weinberg model).

Also there is no experimental evidence for it.

 

[Vanadium 50]

As others have written, few experiments claim discovery after one event. Lots of things can happen at that level. Also, as others have mentioned, these decays are model dependent.

As such, the question, "Gosh, couldn't you just get confused?" is so broad as to be unanswerable. If the decay you are looking for is [itex]p \rightarrow \pi^0 e^+[/itex] which is a common one in many models, it gives you a monoenergetic and high-energy positron, a neutral pion, and no neutron. That is distinguishable from IBD where you get no pion, a neutron, and a lower energy positron with variable energy.

You can repeat this exercise for your favorite supposed decay modes.

 

[snorkack]

First, proton decays are not to neutrons, so if you have perfect knowledge of your final state you know what happened. Practically that won't be the case however, so indeed you won't know what happened in an individual event.
In general, most experiments have some background that you need to be able to handle. If you are looking for proton decay by counting positrons you are essentially asking "do we see more positrons then we would expect from neutrino+proton interactions (+ other backgrounds)". If you can detect neutrons you can use this to keep the background lower.

Or in other words, it is a matter of completeness and certainty of your knowledge of the final state. Searching for a proton decay might be a matter of distinguishing between "we can detect neutrons, but sometimes we miss them, so this was just another time when we missed a neutron" vs. "we are massively unlikely to miss a neutron, so if we did not see it, this must have been the time when baryon really decayed for good". Since baryon decay is certainly a rare process, can you really rule out just missing a neutron to that probability?

Many searches for proton decay are looking for high energy products, like fast positron or pion, on the guess that a large fraction of proton rest mass turns into energy. The same products might be produced by energetic neutrinos - but then especially the neutron would also tend to get a large recoil energy, and be easy to detect.

 

[Dr.AbeNikIanEdL]

"we can detect neutrons, but sometimes we miss them, so this was just another time when we missed a neutron" vs. "we are massively unlikely to miss a neutron, so if we did not see it, this must have been the time when baryon really decayed for good"

But what's actually done is intermediate between these two. Let's for concreteness look at the Super-Kamiokande analysis (arXiv:1610.03597). They basically say "we know how often we see electrons from neutrino interactions, so if we see significantly more than that some of those have to come from proton decay". Of course, electrons here means "electrons looking like they come from a proton decay" which gets rid of backgrounds. But only in their last run period do they actually have an explicit veto on neutrons (you can see from table 1 this reduces their background by a factor of ~2). At the end of the day they even see two events (in the muon channel). This is compatible with their expected background expectation, but they don't make a definite claim what those events are.