A gentle reminder before we begin: you're allowed to be interested in particle physics. 😉

Neutrinos are among the most mysterious particles in physics. They are extremely light, electrically neutral, and interact so weakly with matter that trillions of them pass through your body each second without leaving a trace. They are produced in the Sun, nuclear reactors, the atmosphere, and by cosmic explosions. In fact neutrinos are everywhere — yet they're almost invisible.

Despite their elusiveness, they have already upended physics. In the late 20th century, scientists discovered that neutrinos can oscillate, changing from one type to another as they travel, which is something that the simplest version of the Standard Model of particle physics — the prevailing theory of elementary particles — doesn't predict. Because oscillations require neutrinos to have mass, this discovery revealed new physics. Today, scientists study neutrinos for what they might tell us about the universe’s structure and for possible hints of particles or forces yet unknown.

However, detecting neutrinos is very hard. Because they rarely interact with matter, experiments must build massive detectors filled with dense material in the hopes that a small fraction of neutrinos will collide inside with atoms. One way to detect such collisions uses Cherenkov radiation, a bluish glow emitted when a charged particle moves through a medium like water or mineral oil faster than light does in that medium.

(This is allowed. The only speed limit is that of light in vacuum: 299,792,458 m/s.)

The MiniBooNE experiment at Fermilab used a large mineral-oil Cherenkov detector. When neutrinos from the Booster Neutrino Beamline struck the atomic nuclei in the mineral oil, the interaction released charged particles, which sometimes produced rings of Cherenkov radiation (like ripples) that the detector recorded. In MiniBooNE’s data, the detection events were classified by the type of light ring produced. An "electron-like" event was one that looked like it had been caused by an electron. But because photons can also produce nearly identical rings when they strike the nuclei, the detector couldn’t always tell the difference. A "muon-like" event, on the other hand, had the distinctive ring pattern of a muon, which is a subatomic particle like the electron but 200-times heavier, and which travels in a straighter, longer track. To be clear, these labels described the detector’s view; they didn’t guarantee which particle was actually present.

MiniBooNE began operating in 2002 to test an anomaly that had been reported at the LSND experiment at Los Alamos. LSND had recorded more electron-like” events than predicted, especially at low energies below about 600 MeV. This came to be called the "low-energy excess" and has become one of the most puzzling results in particle physics. It raised the possibility that neutrinos might be oscillating into a hitherto unknown neutrino type, sometimes called the sterile neutrino — or it might have been a hint of unexpected processes that produced extra photons. Since MiniBooNE couldn't reliably distinguish electrons from photons, the mystery remained unresolved.

To address this, scientists built the MicroBooNE experiment at Fermilab. It uses a very different technology: the liquid argon time-projection chamber (LArTPC). In a LArTPC, charged particles streak through an ultra-pure mass of liquid argon, leaving a trail of ionised atoms in their wake. An applied electric field causes these trails to drift towards fine wires, where they are recorded. At the same time, the argon emits light that provides the timing of the interaction. This allows the detector to reconstruct interactions in three dimensions with millimetre precision. Crucially, it lets physicists see where the particle shower begins, so they can tell whether it started at the interaction point or some distance away. This capability prepared MicroBooNE to revisit the "low-energy excess" anomaly.

MicroBooNE also had broader motivations. With an active mass of about 90 tonnes of liquid argon inside a 170-tonne cryostat, and 8,256 wires in its readout planes, it was the largest LArTPC in the US when it began operating. It served as a testbed for the much larger detectors that scientists are developing for the Deep Underground Neutrino Experiment (DUNE). And it was also designed to measure the rate at which neutrinos interacted with argon atoms, to study nuclear effects in neutrino scattering, and to contribute to searches for rare processes such as proton decay and supernova neutrino bursts.

(When a star goes supernova, it releases waves upon waves of neutrinos before it releases photons. Scientists were able to confirm this when the star Sanduleak -69 202 exploded in 1987.)

Initial MicroBooNE analyses using partial data already challenged the idea that MiniBooNE’s excess was due to the anomaly. However, the collaboration didn’t cover the full range of parameters until recently. On August 21, MicroBooNE published results from five years of operations, corresponding to 1.11 x 10²¹ protons on target, which was about a 70% increase over previous analyses. This complete dataset together with higher sensitivity and better modelling has provided the most decisive test so far of the anomaly.

The MicroBooNE detector recorded neutrino interactions from the Booster Neutrino Beamline, a setup that produces neutrinos, using its LArTPC detector, which operated at about 87 K inside a cryostat. Charged particles from neutrino interactions produced ionisation electrons that drifted across the detector and were recorded by the wire. Simultaneous flashes of argon scintillation light, seen by photomultiplier tubes, gave the precise time of each interaction.

In neutrino physics, a category of events grouped by what the detector sees in the final state is called a channel. Researchers call it a signal channel when it matches the kind of event they are specifically looking for, as opposed to background signals from other processes. With MicroBooNE, the team stayed on the lookout for two signal channels: (i) one electron and no visible protons or pions (abbreviated as 1e0p0π) and (ii) one electron and at least one proton above 40 MeV (1eNp0π). These categories reflect what MiniBooNE would've seen as electron-like events while exploiting MicroBooNE's ability to identify protons.

One important source of background noise the team had to cut from the data was cosmic rays — high-energy particles from outer space that strike Earth’s atmosphere, creating particle showers that can mimic neutrino signals. In 2017, MicroBooNE added a suite of panels around the detector. For the full dataset, the panels cut an additional 25.4% of background noise in the 1e0p0π channel while preserving 98.9% of signal events.

In the final analysis, the MicroBooNE data showed no evidence of an anomalous excess of electron-like events. When both channels were combined, the observed events matched the expectations of the Standard Model of particle physics well. The agreement was especially strong in the 1e0p0π channel.

In the 1eNp0π channel, MicroBooNE actually detected slightly fewer events than the Model predicted: 102 events v. 134. This shortfall, of about 24%, is however not enough to claim a new effect but enough to draw attention. But rather than confirming MiniBooNE’s excess, this result suggests there's some tension in the models the scientists use to simulate how the neutrinos and argon atoms will interact. Argon has a large and complex nucleus, which makes accurate predictions challenging. The scientists have in fact stated in their paper that the deficit may reflect these uncertainties rather than new physics.

The new MicroBooNE results have far-reaching consequences. Foremost, the results reshape the sterile-neutrino debate. For two decades, the LSND and MiniBooNE anomalies had been cited together as signs that the neutrino was oscillating into a previously undetected state. By showing that MiniBooNE's excess was not due to extra electron-like interactions, MicroBooNE shows that the 'extra' events were not caused by excess electron neutrinos. This in turn casts doubt on the simplest explanation, of sterile neutrinos.

As a result, theoretical models that once seemed straightforward now face strong tension. While more complex scenarios remain possible, the easy explanation is no longer viable.

Second, they demonstrate the maturity of the LArTPC technology. The MicroBooNE team successfully operated a large detector for years, maintaining the argon's purity and low-noise electronics required for high-resolution imaging. Its performance validates the design choices for larger detectors like DUNE, which use similar technology but at kilotonne scales. The experiment also showcases innovations such as cryogenic electronics, sophisticated purification systems, protection against cosmic rays, and calibration with ultraviolet lasers, proving that such systems can deliver reliable data over long periods of operation.

Third, the modest deficit in the 1eNp0π channel points to the importance of better understanding neutrino-argon interactions. Argon's heavy nucleus produces complicated final states where protons and neutrons may scatter or be absorbed, altering the visible event. These nuclear effects can lead to mismatches between simulation and data (possibly including the 24% deficit in the 1eNp0π signal channel). For DUNE, which will also use argon as its target, improving these models is critical. MicroBooNE’s detailed datasets and sideband constraints will continue to inform these refinements.

Fourth, the story highlights the value of complementary detector technologies. MiniBooNE’s Cherenkov detector recorded more events but couldn’t tell electrons from photons; MicroBooNE’s LArTPC recorded fewer events but with much greater clarity. Together, they show how one experiment can identify a puzzle and another can test it with a different method. This multi-technology approach is likely to continue as experiments worldwide cross-check anomalies and precision measurements.

Finally, the MicroBooNE results show how science advances. A puzzling anomaly inspired new theories, new technology, and a new experiment. After five years of data-taking and with the most complete analysis yet, MicroBooNE has said that the MiniBooNE anomaly was not due to electron-neutrino interactions. The anomaly itself remains unexplained, but the field now has a sharper focus. Whether the cause lies in photon production, detector effects or actually new physics, the next generation of experiments can start on firmer footing.