The universe thrums with quantum fields; and the particles of matter and force emerge as vibrational manifestations of the deep symmetries of these fields.

The layers and reflections of those symmetries give us the wonderful richness of what we call the standard model of particle physics.

Three generations each of quarks and leptons with each generation bisected via isospin.

For the leptons, that splitting gives us the massive electrons, muons and taus and their corresponding ultra-light neutrino counterparts.

That's the simplest representation of the standard model, but all of these are split again into matter and antimatter.

And there's one more lesser known reflection-chirality.

This is a sort of fundamental handedness or spin relative to their direction of motion.

Left- and right-handed quarks, left- and right-handed electrons, muons and taus.

But, weirdly, here cosmic symmetry seems to fail: there are only left-handed neutrinos.

No right-handed neutrino has ever been detected, which seems odd at first but may not be so surprising.

Regular, left-handed neutrinos are famously very difficult to detect because they only interact via gravity and the weak nuclear force-both of which are notably weak.

The weak force, being 10^24 times stronger than gravity, is the only way we can actually see individual neutrinos.

However, the weak force is also victim to the broken cosmic symmetries: it only affects particles with left-handed chirality.

Right-handed particles simply don't feel it.

So, lacking any other type of charge, the right-handed neutrino has no way to interact with any of the forces of nature besides through its negligible gravity.

This is why we call the hypothetical particle the sterile neutrino-it's barren of any capacity for interaction by the quantum forces.

If regular neutrinos are difficult to detect, sterile ones seem to be impossible.

So are these gaps in the standard model missing because we haven't detected the particle?

But there are some major problems in physics that could be solved very neatly if sterile neutrinos do exist.

It's for this reason that physicists have been hunting for the sterile neutrino for decades, and why were getting excited as subtle evidence of the sterile neutrino started to grow.

And it's for this reason that the brand new results from the Fermilab MicroBOONE experiment are so shocking.

Our most advanced experiment yet suggests that this hole in the standard model really is empty and that the sterile neutrino doesn't exist at all.

Let's start with why the right-handed neutrino was really left out of the standard model in the first place.

When Weinberg, Salam, and Glashow put the finishing touches on the standard model, they were as economical as possible.

They only included the particles needed to explain our observations.

The chiral reflections of the quarks and the heavy leptons were non-negotiable, largely because these are needed to give these families of particles mass by granting a connection to the Higgs field.

But in the 1960s we didn't know whether the neutrino had mass-in fact, Weinberg et al.

assumed that it did not.

Not only was the right-handed neutrino not needed, but including them generated problems with certain conservation laws-lepton flavor to be precise.

Cleaner to just leave them out.

It wasn't until the 1990s that we discovered that neutrinos have mass after all.

We discovered the neutrinos spontaneously oscillate between their three flavours-electron, muon and tau.

That can only be true if they have mass.

As it turns out, absurdly tiny masses.

We don't know how neutrinos get their masses, but we just eliminated one of the main reasons that Weinberg et al.

left sterile neutrinos out of the standard Model in the first place.

They could, in principle, do the same job as the chiral reflections of electrons, granting mass via the Higgs mechanism.

In fact, they can solve two mysteries at once.

The extraordinarily tiny mass of the neutrino warrants its own explanation.

If sterile neutrinos exist, and if they're actually extremely massive, then they could a) explain why neutrinos have mass and b) drive down the mass of regular neutrinos through something called the see-saw process.

Not something to get into today.

There's another use of a hypothetical particle that's both massive and impossible to detect by any means other than gravity.

That's the exact requirement for the elusive dark matter particle.

So sterile neutrinos seem tailor made to explain why 80% of the matter in the universe is invisible.

And so the search for the sterile neutrino began.

But how do we go about detecting the undetectable?

Regular neutrinos are hard enough.

They only interact via the weak force, and the weak force has a very short range.

That means a neutrino has to pass very close to an atomic nucleus for anything to happen.

For a mid-energy solar neutrino passing through the Earth, there's only a one in a billion chance it'll be stopped by a nucleus.

But when that does happen, its energy gets converted into a cascade of particles that, in principle, can be seen.

So we can see neutrinos as long as we play the numbers right-enough neutrinos through a big enough detector.

But what about sterile neutrinos?

If they don't even interact by the weak force, what hope do we have?

Well, a sterile neutrino can't exchange force-carrying bosons with other particles.

But it can interact with itself in a certain sense.

In particular, it can participate in the neutrino oscillations that I mentioned earlier.

If electron, muon, and tau neutrinos can transform into each other, then they can also transform into their chiral-reflected counterparts-the sterile neutrino.

And this is where we have a prospect for "detecting" them-by looking for anomalies in the oscillations of known neutrino flavours.

Let's look at the first effort to catch the sterile neutrino as an example.

This was at Los Alamos in the 90s with the Liquid Scintillator Neutrino Detector.

The experiment goes like this: a beam of muon neutrinos from a particle accelerator is directed into a vat of mineral oil.

When a tiny fraction of those neutrinos collide with a neutron, one possible reaction is for the neutron and muon-neutrino to be converted into a proton and a muon.

The muon zips off through the oil at a near the vacuum speed of light.

In fact, the muon is moving faster than the diminished speed of that actual light has in the oil.

This leads to something like an optical sonic boom-what we call Cherenkov radiation, and it's that radiation that LSND detected.

A similar reaction can happen if the incoming particle is an electron neutrino instead of a muon neutrino.

In that case it produces a proton and an electron rather than a muon.

That electron will still produce Cherenkov radiation, but because the electron is much lighter than the muon it decelerates quickly and in the process emits photons, which themselves can undergo pair production to become electron-positron pairs, which emit further Cherenkov photons, which induce pair production, etc.

with the process continuing until the energy dissipates.

This is called an electromagnetic cascade.

The Cherenkov ring produced by an electron and its messy EM cascade is fuzzier and fainter than the crisp, bright ring produced by a muon.

This is what allows scientists to distinguish whether the incoming neutrino was of the muon or electron flavour.

Even though the LSND experiment used a muon-neutrino beam, there are two ways for it to see events from electron neutrinos.

One is that there's a very small contamination of electron neutrinos in the beam and from cosmic rays and things like that.

The other is that muon neutrinos can oscillate into electron neutrinos before reaching the tank.

However, the LSND experiment was set up so that the detector chamber was way too close to the beam source for the oscillations to occur in significant numbers.

Basically, it expected very few electron neutrinos.

And that design is the key to it finding our hypothetical neutrino type-the sterile neutrino.

If the sterile neutrino exists and has a relatively low mass, it gives the muon neutrino an intermediate step to oscillate into an electron neutrino.

That massively increases the chance of that transition, enabling a good number of muon neutrinos to flip before reaching the LSND chamber.

The outcome should be that LSND saw many more electron-induced electromagnetic cascades.

And it saw exactly that-enough to point to sterile neutrinos with a mass of around 1 eV, which puts it at the low end of possible masses.

This was compelling enough to inspire follow-ups.

The MiniBooNE experiment at Fermilab, built in the 2000s, was designed to test the emerging anomaly with higher sensitivity.

More neutrinos, a bigger tank of mineral oil, etc.

In 2018, MiniBooNE released its final data - and once again, they saw an excess of fuzzy Cherenkov rings consistent with electron-muon events, and supporting the 1 eV sterile muon hypothesis.

Another very different type of experiment also supported this.

The Italian GALLEX and the Soviet SAGE experiments looked for conversion of gallium into germanium after impact with electron neutrinos.

Too few conversions were observed in both experiments.

One explanation was that these electron neutrinos were depleted by converting into sterile neutrinos.

The interesting thing here is that the sterile neutrino would also have to be around 1 eV.

This feels like it's coming together in support of the sterile neutrino.

But there are two reasons not to colour-in the gaps in our standard model table just yet.

The first is that there are contradictions to the sterile neutrino hypothesis.

Other experiments similar in design to LSND and miniboone found no excess of electron neutrinos.

And several experiments found another contradiction-if muon-neutrinos are converting to sterile neutrinos, then there should be fewer muon neutrinos.

But that's never been detected, not from accelerator experiments, not from nuclear reactors, and even the neutrinos from the Sun as measured by the Ice Cube observatory at the south pole are perfectly consistent with neutrinos oscillating only between the three known types.

So we have these different types of experiments giving weirdly contradictory results.

Finding sterile neutrinos would be incredibly important, but it's also important to figure out this discrepancy.

MiniBoone needed to be upgraded - to MicroBoone.

That's short for Micro Booster Neutrino Experiment.

It was designed to eliminate the main source of uncertainty in its predecessor.

So one way to explain the electron excess in both miniBooNE and LSND is that these fuzzy electron Cherenkov rings may be due to something completely different.

Neutrino collisions often produce neutral pion particles, which decay into pairs of gamma rays, and those in turn can create electron-positron pairs via pair production.

And those particles then produce cones of Cherenkov radiation and electromagnetic showers.

If those showers overlap, the resulting ring can look just like the single rings from a proper electron-neutrino event.

MicroBooNE's main job is to filter out these false signals-we'll call them photon events-from the actual electron-neutrino events.

MicroBooNE sits just downstream of MiniBooNE on the same neutrino beam at Fermilab.

But it's built around a completely different technology - a liquid argon time projection chamber.

MiniBooNE distinguished between muon-like or electron-like events based on the quality of the Cherenkov rings.

But this new chamber type can actually track the detailed trajectory of the particles produced in the neutrino collision.

This gives a foolproof way of sorting real electron-neutrino events from the false ones I described.

So when a neutrino hits an atomic nucleus, there's always a spray of various charged particles like protons and pions from the point of impact-the "vertex"--that allows us to localize the collision point.

If muon is produced by a muon-neutrino, its track will begin at the vertex and it'll travel a long distance in a straightish line.

If an electron is produced by an electron-neutrino collision, it'll travel a shorter distance and then create an electromagnetic cascade like I already described.

Remember that with miniBooNE these electron events were hard to separate from photon events.

In the latter case we have a neutrino impact and the resulting spray of particles, which includes two gamma ray photons.

These then travel through the tank leaving no track at all until they themselves convert into electron-positron pairs, which can then start a visible electromagnetic cascade.

So the way to distinguish a genuine electron-neutrino event from one of these false events is that there's a clear gap between the collision vertex and the beginning of the EM cascade.

So, what did MicroBooNE find?

Well, not the sterile neutrino.

It found no excess of electron-neutrino events that might indicate a sterile neutrino in the mix, once MicroBooNE eliminated the false signal from photon events.

The first MicroBooNE data release and results were published in 2021, and already it wasn't looking good for the sterile neutrino.

And at the end of last year-December 2025-the MicroBooNE collaboration published new results in which a source of confusion was removed using a second neutrino beam.

The absence of an electron-neutrino excess was confirmed.

This final analysis was also able to confirm that the apparent excess observed by MiniBooNE can be completely accounted for by photon events, and it ruled out sterile neutrinos as the cause of the anomaly in the early gallium experiments.

So why is this a big deal?

The sterile neutrino is probably the simplest possible extension of the standard model, and so the anomalies observed in earlier experiments made its existence very plausible.

Likely, in the minds of some.

MicroBooNE has basically eliminated any of that evidence for the sterile neutrino.

That sends the particle back to the realm of pure speculation.

We have no empirical reason whatsoever to believe it exists.

But it may still exist.

MicroBooNE is sensitive to a mass range of around .1 to 10 eV.

But if the sterile neutrino has a much higher mass it could still be out there.

And that means the particle remains a solid prospect for explaining the tinyness of the other neutrino masses and for dark matter.

For those, the sterile neutrino would need a many orders of magnitude larger than the sensitivity of MicroBooNE.

In fact, the non-existence of light sterile neutrinos could even be seen as a boost for this particle's role in those mysteries.

But to detect massive sterile neutrinos we need to try other things, and there are various works in progress to look for the subtle signatures of their existence, both in labs and out there in the universe.

And these are topics for another time.

But in the meantime, our congratulations to the good folk at Fermilab for sorting out this strange anomaly, and for their continuing effort in understanding the neutrino-the most mysterious particle in the Standard Model.

If you want to learn more about their neutrino program, maybe watch that episode when Don Lincoln and Fermilab actually hosted PBS Space Time.