[dphidt]

Hi, neutrino physicist here. There are indeed a handful of results that point to a more-or-less consistent picture with sterile neutrinos, including MiniBooNE (now updated with 2x more data), LSND, antineutrinos from nuclear reactors, and calibrations of solar neutrino experiments (GALLEX/GNO and SAGE). All very different experiments with different uncertainties, which makes it hard to explain away. Meanwhile, there are a bunch of other experiments that should see this effect but don't (IceCube and MINOS, KARMEN, NOMAD, CDHS, CCFR, ...). With all this tension, most of the possible parameters for sterile neutrinos are ruled out, but there is still a little room. Next-generation experiments will go after the parameter space that remains, and definitively confirm/reject the sterile neutrino hypothesis at high confidence. See e.g. the Fermilab Short-Baseline Neutrino Program, which puts a set of three detectors in the same neutrino beam as MiniBooNE: https://sbn.fnal.gov/.

[DoctorOetker]

Hi, I have a couple of questions:

If I understand correctly, as far as we thought there seem to be 3 flavours of neutrino (electron, muon, tau neutrinos), and neutrinos carry (kinetic) energy (and possibly some rest mass).

Historically often "different" or "new" particles just turned out to be the same particle with different energy:

Cathode rays and electrons are the same thing, but nobody would describe the electron in hydrogen as a cathode ray orbitinng the proton.

Beta rays also turned out to be electrons, and similarily nobody describes the electron in hydrogen to be a beta ray orbiting the proton.

X-rays and gamma-rays are both photons, yet initially we did not know they were the same particle, just higher kinetic ennergy.

Now my question: how do we know the neutrino flavours aren't really the same particle but in some kind of different state, causing them to be differentially absorbed/detected?

consider red and blue light photons and pigments, the red light would only be absorbed by the blue pigment, and the blue light woud only be absorbed by the red pigment, but does that mean they are different particles?

How do we know a sterile neutrino isn't just one of the known neutrinos with little kinetic energy, or perhaps too much kinetic energy to interact?

[dphidt]

The other comments are spot-on. I'll just add that the neutrinos flavors are defined by the way they interact: electron neutrinos interact to produce electrons, never muons. So they're quite different in that sense, and in the current Standard Model of particle physics, they're treated as independent particles. (This is not to say that in the future, we won't require a more comprehensive model that could relate particles in a deeper way.)

We know from experiments like LEP (electron-positron collisions) that there are only three kinds of neutrinos that participate in weak interactions (electron, muon, and tau). Thus the fourth neutrino type suggested by these anomalous oscillation measurements cannot interact via the weak force, meaning it doesn't interact at all,* hence sterile. The only way to detect them is through their influence on the oscillations of other neutrino types.

* They'd still feel gravity, which isn't included in the Standard Model anyway.

[ianai]

Thank you so much for replying! I learned quite a bit.

What is the source of neutrinos in the experiments suggesting sterile nutrinos? Is it different from the non confirmations?

[sanxiyn]

By definition, neutrinos interact with weak force and sterile neutrinos don't.

[bdonlan]

What interactions are left then? Neutrinos don't interact with the electromagnetic or strong forces, and I don't think we can observe gravitational effects in an accelerator, so how are we detecting these sterile neutrinos?

[danharaj]

Neutrinos oscillate between different flavors. If a sterile neutrino exists then we will see deviations in the distribution of detected neutrino flavors because they will oscillate differently.

[sanxiyn]

On reactor antineutrino anomaly, what do you think of https://arxiv.org/abs/1806.00574 ? It seems to me both Daya Bay collaboration and RENO collaboration suggest anomaly is explained by nuclear fuel evolution.

[dphidt]

It's really interesting work! Just a year or so ago, the features in the energy spectrum appeared to be independent of burn-up, so it's exciting to see higher precision data coming in. No matter what, we'll get a much better model for reactor antineutrino spectra.

It's about a year old, but I like this talk from Patrick Huber (one of the developers of the new reactor models), in particular his "Score Card" for the various evidence on slide 25: https://absuploads.aps.org/presentation.cfm?pid=13003. I keep this in mind as I am updating my personal priors :).