Sterile neutrino question

The physicsworld.com site has an article about the Chinese neutrino experiment. It says, “If they exist, sterile neutrinos would interact extremely weakly, if at all, with ordinary matter, and so would be even harder to detect than conventional neutrinos.” I’ve also read something similar elsewhere. I would have thought that if sterile neutrinos interact “not at all” with normal matter, including other neutrinos, it would be impossible for them to oscillate into those others.

Mu parsing of that quote is that “normal matter” does not include neutrinos. I checked Wikipedia, and I confirmed this. It seems that sterile neutrinos interact with leptons, and neutrinos are merely neutral leptons. They also interact with Higgs bosons.

I will leave it to others to describe these interactions, apparently called Yukawa interactions. And apparently the Higgs mechanismi s involved. I only have a casual understanding of any of this, at best. I mostly was commenting on the parsing of the quote.

Basically, oscillation into other neutrino flavors would be their only interaction of any note with anything else. Well, that and gravity, but you’d need a heck of a lot of them for that to be relevant.

My understanding of the matter is:

[ul]
[li]Particles have a properly called “chirality” which comes in two states, either “left-handed” or “right-handed” which is closely, but not exactly related to whether the spin axis is parallel or anti-parallel to the momentum direction (except for massless particles where it is exactly related).[/li][li]The weak interaction only works on left-handed particles or right-handed antiparticles.[/li][li]Since all the methods we have for creating or detecting neutrinos involve the weak interaction, we can only create or detect left-handed neutrinos and right-handed anti-neutrinos. Which is not to say that other-chirality neutrinos don’t exist, but they’re much harder to detect or create (and it’s not like detecting regular neutrinos is easy).[/li][li]For massless particles, chirality is a conserved quantity, but not for particles with mass. Since neutrinos have some mass they can convert from one chirality to the other, but because their mass is so puny it’s not like they’re doing it willy-nilly.[/li][li]The Chinese reactor experiment detects neutrinos and finds that there are fewer of them than expected, and we are speculating that this is because they are shifting from the insanely hard to detect regular neutrinos to the crazy-stupid-insanely hard to detect other-chirality neutrinos, which would verify the existence of other-chirality neutrinos and the rate of conversion would put bounds on neutrino mass probably.[/li][li]Other-chirality neutrinos are called “sterile” because they can’t have children.[/li][/ul]

Hm, most of the discussions I’ve seen of sterile neutrinos assumed a fourth flavor. The wrong chirality of a familiar flavor is a lot less exotic.

It’s also worth noting that there are two competing models describing neutrinos, the Majorana model and the Dirac model. The Dirac model behaves as leahcim describes, but under the Majorana model, the neutrino is its own antiparticle, and the only thing which distinguishes between what we call “neutrinos” and “antineutrinos” is the chirality. If the Majorana model is correct, then sterile neutrinos of that type would be impossible.

Interacting and oscillating are different things. To very good approximation, sterile neutrinos do not directly interact with anything, including regular neutrinos, and this is why they are called “sterile”. (As is common in particle physics, there is often a way to generate unfathomably puny interactions where there “ought to” be none at all. This is one of those cases.)

Unrelated to their interactions, neutrinos “mix” with one another. Just considering the three regular neutrinos for a moment: Mixing here means that the things you might want to call the three neutrinos – neutral leptons with specific flavors (electron, muon, or tau type) – do not match up one-to-one with the three neutrinos with specific masses. That is, if you had in your hand a neutrino with a specific mass (perhaps the lightest of the three) and asked what flavor it was, the answer would be random. Thank you, quantum mechanics.

This didn’t have to be the case, but empirically it is, and neutrino oscillations are one consequence. But, neutrino interactions are largely unrelated to neutrino mixing.

One can postulate that additional non-interacting neutrinos are present – sterile neutrinos – and separately you can postulate that these should mix with the regular neutrinos. If they do mix, then sterile neutrinos get involved in neutrino oscillations. But they still don’t interact with anything (save the caveat at the top of the post).

The article’s quote of:

is pretty poor. It implies that the extremely weak interaction might have something to do with “…would be even harder to detect”. For all intents and purposes, sterile neutrinos don’t interact, and no one is trying to detect them via interactions.

The experiment in question is measuring electron antineutrinos coming from nuclear reactors to measure (primarily) regular oscillations of regular neutrinos, and has answered some crucial questions in the field recently. A feature of this sort of experiment is that we have rather poor ability to calculate ahead of time the rates and energies (together: flux) of the (anti)neutrinos coming out of the isotopic soup that is a nuclear reactor core. By careful experimental design, the principle measurements with the apparatus are largely insensitive to these calculations. That is, you can have pretty large uncertainties in the flux and still have good precision on the quantities of interest.

However, you can still check how correct the flux calculations seem to be according to the data collected. And experiments like this one have, over the past couple of decades, seen a deficit of neutrino interactions relative to the calculated rates. Most people suspect the flux calculations are just harder than they seem (and they seem pretty hard). But another possibility is that the flux calculations are correct and the deficit is caused by sterile neutrinos mixing with regular neutrinos. This hypothesis is buttressed by a collection of other anomalies in neutrino data that could also be explained away by invoking sterile neutrinos. But it’s all quite murky at the moment.

The new result in the article actually goes against sterile neutrinos. There is still an overall deficit in detected rate, and they’ve measured this even better than others before. But they’ve also measured spectral differences that suggest that the flux prediction could be off in ways other than just the rate. And this points much more readily to “predicting the flux is hard” than to “sterile neutrinos fixes all this”.

On the chirality thing: The easiest way to make a sterile neutrino is to make a right-handed one, and you do need to do this in many models of neutrino mass generation. But in practice, the sterile neutrinos that affect neutrino oscillations are usually just considered a generic additional neutrino that just happens not to have any fundamental interactions.

Come to think of it, the first place I saw sterile neutrinos proposed, wrong-chirality neutrinos couldn’t account for it. Neutrino oscillation experiments are sensitive to the differences between the squares of the masses of two different neutrino species. But there were three different experiments of different types which claimed to observe three different mass splittings, and the largest of the three splittings was not equal to the sum of the two smaller ones. The only way this could be true would be if there were at least four different neutrino species, not three, but four neutrinos would play havoc with what’s known of particle physics unless one of them were sterile. And wrong-chirality neutrinos would have the same mass as normal ones.