IIRC, there is another quantum number, the chirality. All the neutrinos measured are left chiral. The sterile neutrino, which does not pair with other lepton, is right chiral.

Is that the only possible sterile neutrino? Because if I recall correctly, a right chiral neutrino cannot change into one of the other types, because of the chirality. (That's why it's "sterile".)

So either they aren't produced by beta decay, or they are but they can't be captured by gallium, or we're seeing a sterile neutrino in a different sense. ("Sterile" because it can't interact with gallium, but not sterile due to chirality.)

If I understand, the conversion of gallium into germanium is exactly a beta interaction. (But I may not understand, because it seems to me that gallium->germanium should emit a neutrino, not absorb one.) So I have a hard time seeing how a neutrino could be emitted by the source, but not absorbed by gallium. I could more readily see it as saying that there's a fourth type of neutrino that they cycle through, and while in that fourth state it can't convert gallium. But that wouldn't be "sterile" in the chirality sense.

> If I understand, the conversion of gallium into germanium is exactly a beta interaction

In a "standard" beta decay the neutron turns into a proton and emits an electron and an anti-electron neutrino. In this reaction its absorbing an electron neutrino and emitting an electron (while still turning the neutron into a proton). Absorbing an electron neutrino is pretty much equivalent to emitting an anti-electron neutrino.

>gallium->germanium should emit a neutrino, not absorb one.

Neutron-Neutrino interaction?

https://www.vivaxsolutions.com/physics/feynman-diagrams.aspx

Emitting an antiparticle is (in Feynman diagrams at least) the same as absorbing a particle. IIRC, for neutrinos the anti aspect isn't that important because they don't quickly annihilate with regular matter and they have no electrical charge.

I even seem to remember that the most commonly produced neutrinos are antineutrinos (so the anti aspect is more fundamental than a convention for each particle type).

> What's the sterile neutrino the partner to?

It would be a very unusual partner that has no concerved quantum numbers, and only mass.

A sterile neutrino would also imply that the neutrino is a Majorana particle (because it would have not quantum numbers that could be "anti").

> It might mean 20% more neutrinos exist than we think.

Would it though? It's not like we know neutrino mass from measurement, rather we estimate it from nuclear reactions that have occurred. The estimate would not change if a neutrino oscillates into a sterile one.