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Thank you to Babel for supporting PBS. 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. We've got a couple of quick announcements before we start. First, our eternal battle
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against the algorithm continues. The best way to encourage YouTube to share our videos is to like and comment. Doing both really makes a difference. And if you're new here, subscribe, hit the bell, and introduce yourself in the comments. We're friendly. We've got our first UV glow merch of 2026. The Kerr rotating black hole tea and hoodie are UV charged, so after some time in the sun, it starts to glow. Perfect for showing love for the place where light
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can't escape. And if you want to make sure your ideas don't escape either, pick up our black hole rotation notebook. UV shirts are limited time only and links are in the description. Now onto the episode. Three generations of quarks and lepttons with each generation bisected via iso spin for the leptons. That splitting gives us the mass of electrons, muons, and tails and their corresponding ultralight nutrino counterparts. That's the simplest representation
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of the standard model. But all of these are split again into matter and antimatter. And there's one more lesserknown reflection, kirality. This is a sort of fundamental handedness or spin relative to the direction of motion. Left and right-handed quarks, left and right-handed electrons, muons and tails. But weirdly here, cosmic symmetry seems to fail. There are only left-handed nutrinos. No right-handed nutrino has ever been detected, which seems odd at first, but may not be so
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surprising. Regular left-handed nutrinos 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 the 24 times stronger than gravity, is the only way that we can actually see individual nutrinos. However, the weak force is also victim to the broken cosmic symmetries. It only affects particles with left-handed kirality. Right-handed particles simply don't feel it. So,
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lacking any other type of charge, the right-handed nutrino has no way to interact with any of the forces of nature besides through its negligible gravity. And this is why we call this hypothetical particle the sterile nutrino. It is barren of any capacity for interaction by the quantum forces. If regular nutrinos are difficult to detect, sterile ones seem to be impossible. So these gaps in the
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standard model are missing because perhaps we haven't detected the particle. But there are some major problems in physics that could be solved very neatly if sterile nutrinos do exist. It's for this reason that physicists have been hunting for the sterile nutrino for decades and why they were getting excited as subtle evidence of the sterile nutrino started to grow. And it's for this reason that the brand new results from the Firmeny Lab microbon experiment are so shocking.
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Our most advanced experiment yet suggests that this hole in the standard model really is empty and that the sterile nutrino doesn't exist at all. Let's start with why the right-handed nutrino was really left out of the standard model in the first place. When Weinberg, Salam, and Glacial put the finishing touches on the standard model, they were as economical as possible. They only included
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particles needed to explain our observations. The chyal 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 Higs field. But in the 1960s, we didn't know whether the nutrino had any mass. In fact, Weineberg Edel assumed that it did not. Not only was the right-handed nutrino not needed, but including them generated problems with certain conservation
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laws, lepto flavor to be precise. Cleaner to just leave them out. It wasn't until the 1990s that we discovered that nutrinos do have mass after all. We discovered that nutrinos spontaneously oscillate between their three flavors, electron, muon, and toao. That can only be true if they have mass. And as it turns out, it's an absurdly tiny mass. We don't know how nutrinos get their masses, but we just eliminated one of the main reasons that Weinberg left out sterile nutrinos from
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the standard model. They could in principle do the same job as the kyo reflections of electrons granting mass via the Higs mechanism. In fact, they can solve two mysteries at once. The extraordinarily tiny mass of the nutrino warrants its own explanation. If sterile nutrinos exist, and if they're actually extremely massive, then they could a explain why nutrinos have mass, and b drive down the mass of regular nutrinos through something called the seessaw process.
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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 nutrinos seem tailorade 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
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about detecting the undetectable? Regular nutrinos are hard enough. They only interact via the weak force. And the weak force has a very short range. That means a nutrino has to pass very close to an atomic nucleus for anything to happen. For a midenery solar nutrino passing through the Earth, there's only around a one in a billion chance that it'll be stopped by a nucleus. But when that does
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happen, its energy gets converted into a cascade of particles that in principle can be seen. So we can see nutrinos as long as we play the numbers right, enough neutrinos through a big enough detector. But what about sterile nutrinos? If they don't interact by the weak force, what hope do we have? Well, a sterile neutrino can't exchange force carrying bzons with other particles, but it can interact with itself in a certain sense. In particular, it can participate in the
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nutrino oscillations that I mentioned earlier. If electron muon and tow nutrinos can transform into each other, then they can also transform into their chyal reflected counterparts, the sterile nutrino. And this is where we have a prospect for detecting them by looking for anomalies in the oscillations of no nutrino flavors. Let's look at the first effort to catch the sterile nutrino
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as an example. This was at Los Alamos in the '90s with the liquid cintillator nutrino detector. This experiment goes like this. A beam of muon nutrinos from a particle accelerator is directed into a vat of mineral oil. When a tiny fraction of those nutrinos collide with a neutron, one possible reaction is for the neutron and muon nutrino to be converted into a proton and a muon. The muon zips
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off through the oil at near the vacuum speed of light. In fact, the muon is moving faster than the diminished speed of actual light in that oil. This leads to something like an optical sonic boom, what we call churanov 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 nutrino. In that case,
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it produces a proton and an electron rather than a muon. That electron will still produce tranco radiation. But because that electron is much lighter than the muon, it decelerates quickly and in the process it emits other photons which themselves can undergo pair production to become electron posetron pairs which emit further shrank of photons which induce further pair production etc. with the process continuing until the energy dissipates. This is called an electromagnetic
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cascade. The tranov ring produced by the electron and the 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 flavor. Even though the LSND experiment used a muon neutrino beam, there are two ways for it to see events from electronutrinos. One is that there's a very small contamination of electronutrinos in the beam and from cosmic rays
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and things like that. The other is that mutrinos can oscillate into electronutrinos 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 finding our hypothetical nutrino type,
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the sterile nutrino. If the serial nutrino exists and has a relatively low mass, it gives the muon nutrino an intermediate step to oscillate into an electron neutrino that massively increases the chance of that transition, enabling a good number of muon nutrinos to flip before reaching the LSND chamber. The outcome should be that LSND saw many more electron induced electromagnetic cascades and
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it saw exactly that enough to point to sterile nutrinos with a mass of around one electron volt which puts them at the low end of possible masses. This was compelling enough to inspire follow-ups. The Miniboon experiment at Fermy Lab 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, Miniboon released its final data and once again they saw an excess of fuzzy tranov
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rings consistent with electron muon events and supporting the one EV sterile muon hypothesis. Another very different type of experiment also supported this. The Italian galaxy and the Soviet sage experiments looked for conversion of gallium into ggermanium after impact with electron neutrinos. Too few conversions were observed in both experiments and one explanation was that these electron neutrinos were depleted by converting into sterile neutrinos. The interesting
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thing here is that the sterile neutrino would also have to have a mass of around 1 electron volt. This feels like it's coming together in support of the sterile neutrino. But there are two reasons not to color in the gaps of our standard model table just yet. The first is that there are contradictions to the sterile nutrino hypothesis. Other experiments similar in design to LSND and Miniboon found no excess of electronutrinos and several experiments found another contradiction.
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If muon nutrinos are converting into sterile nutrinos, there should be fewer muon nutrinos, but that's never been detected. Not from accelerator experiments, not from nuclear reactors. And even the neutrons from the sun as measured by the ice cube observatory at the south pole are perfectly consistent with nutrinos oscillating only between the three known types. So we have these different types of experiment giving weirdly contradictory results. Finding
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sterile nutrinos would be incredibly important. But it's also important to figure out this discrepancy. Mini boon needed to be upgraded to microbon. That's short for micro booster nutrino experiment. It was designed to eliminate the main source of uncertainty in its predecessors. So one way to explain the electron excess in both Miniboon and LSND was that these fuzzy electron shrankov rings may be due to something completely different. Nutrino collisions often produce
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neutral pion particles which then decay into pairs of gamma rays and those in turn can create electro posetron pairs via this pair production and those particles can then produce their own cones of tranquil radiation and electromagnetic showers. If those showers happen to overlap, the resulting ring can look just like the single rings from a proper electron neutrino event. Microbon's main job was to filter out these false signals. We'll call them
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photon events from the actual electron neutrino events. Microbon sits downstream of Minibbo on the same neutron beam at Firmeny Lab, but it's built around a completely different technology, a liquid argon time projection chamber. Miniboon distinguished between muonlike or electron-like events based on the quality of the Tranco rings. But this new chamber type can actually track the detailed trajectory of the particles produced in the nutrino collision. This gives a foolproof way
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of sorting real electronutrino events from the false ones that I described. So when a nutrino 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 a muon is produced by a muon neutrino, its track will begin at the vertex and it'll travel a long distance in
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a straightish line. If an electron is produced by an electronutrino event, it'll travel a shorter distance and then create an electromagnetic shower like I described. Remember that with Miniboon, these electron events were hard to separate from photon events. In the latter case, we have a nutrino impact and the resulting spray of particles which includes two gammaray photons. These then travel through the tank leaving no track at all until they themselves convert into
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electron posetron pairs which then start a visible electromagnetic cascade. So the way to distinguish a genuine electronutrino 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 Microbon find? Well, not the sterile nutrino. It found no excess of electronutrino events that might indicate a sterile nutrino in the mix once Microbon eliminated the false signal from the photon
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events. The first microboom data release and results were published in 2021 and already it was not looking good for the sterile nutrino. And at the end of last year, December 2025, the microboom collaboration published new results in which a source of confusion was removed using a second nutrino beam. The absence of an electron neutrino excess was confirmed. This final analysis was also able to confirm that the apparent excess observed by mini boon can be completely accounted
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for by photon events and it ruled out sterile nutrinos as the cause of the anomaly in the early gallium experiments. So why is this a big deal? Well, the sterile nutrino is probably the simplest possible extension of the standard model. And so the anomalies observed in earlier experiments made its existence feel very plausible. Microbon has basically eliminated any of the evidence for the sterile nutrino. That sends the particle back into the realm of pure speculation.
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We have no empirical evidence whatsoever to believe it exists. But it may still exist. Microboot is sensitive to a mass range of around 0.1 to 10 electron volts. But if the sterile nutrino has a much higher mass, it could still be out there. And that means that the particle remains a solid prospect for explaining the tininess of the other nutrino masses and for the existence of dark matter. For those, the serial nutrino would need to be many orders of magnitude
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larger than the sensitivity of microbon. In fact, the non-existence of light sterile nutrinos could even be seen as a boost for this particle's role in those mysteries. But to detect massive sterile nutrinos, 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 Fermy Lab
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for sorting out this strange anomaly and for their continuing effort in understanding the nutrino, the most mysterious particle in the standard model. And if you want to learn more about their nutrino program, maybe watch that episode when Don Lincoln and Fermy Lab actually hosted PBS Spacetime. Thank you to Babubble for supporting PBS. If you spend any time online, you might have found that today's world is highly international. That's why Babel teaches you how to
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