The Alternate View
The Rise and Fall of Sterile Neutrinos
by John G. Cramer
The present Standard Model of Particle Physics (SMPP) is rather a paste-up job, with about two dozen adjustable parameters (masses, mixing angles, etc.) that lack any good theoretical basis, cannot be derived from any more fundamental theory, and must be “put in by hand” to fit experimental results. Therefore, the identification of possible areas of failure of the SMPP is important, because this can serve as a guide to finding the “more fundamental theory” that must be hiding somewhere behind our present theoretical understanding.
The SMPP tells us that in our universe there are only four fundamental forces, the short-range strong and weak forces, and the long-range (1/r2) electromagnetic and gravitational forces. These forces all have associated spin-1 boson mediating particles: respectively, the gluon, Z0, W±, and photon, plus the hypothetical spin-2 graviton, that mediate their interactions. There is also one additional mediating boson in the SMPP, the spin-0 electrically neutral Higgs boson or H0, which gives all the other fundamental particles their characteristic rest masses.
The SMPP also tells us that in our universe there are twelve fundamental spin-½ fermions, which includes six quarks (up, down, strange, charmed, top, and bottom) that interact with all four of the fundamental forces. There are also three charged leptons, e±, m±, and t±, that ignore the strong force but interact with the other three forces, and there are also the electrically neutral leptons, the neutrinos ne, nm, and nt, that ignore both the strong and electromagnetic forces and interact only with the weak force and gravity. All of the quarks and leptons, as spin-½ fermions, obey the Pauli exclusion principle (i.e., only one particle is allowed in each quantum state) and have both matter and antimatter forms. The boson particles, on the other hand, have no antimatter forms and obey Bose-Einstein statistical behavior in which they prefer to clump together, encouraging many boson particles to pile into the same quantum state (See AV 77 on Bose-Einstein condensates).
In this column, we will focus on whether there are more than three neutrino flavors. The SMPP assumes that all three matter neutrinos are “left-handed,” violating parity symmetry with their spin vector always pointing in the opposite direction from their direction of propagation, while antimatter neutrinos are right handed, with spin and motion parallel. Many extensions of the SMPP include prediction of additional right-handed neutrinos (and left-handed anti-neutrinos) that are “sterile,” in that they ignore the weak force and interact only with gravity.
The three known neutrino particles are distinguished from each other by their very small but slightly non-zero rest masses. The rest masses of the three flavors of neutrino are not known at present, but data from the cosmic microwave background tells us indirectly that the sum of the three neutrino rest masses cannot be any larger than about 0.15 eV (electron volts), while direct data from the KATRIN experiment tells us that the rest mass of the electron neutrino (ne) is less than 0.45 eV. Unlike the other particles of the SMPP, the three neutrinos can “oscillate”, changing from one flavor to another as they travel through space. Measurements of these neutrino oscillations (flavor profile vs. travel distance) tell us that the mass-squared-difference of the lightest two neutrinos is about 7.4 × 10⁻⁵ eV² and that of the heaviest two neutrinos is about 2.5 × 10⁻³ eV².
However, in 1998, the physics world was shaken by the seemingly SMPP-breaking results from the LSND experiment (Liquid Scintillator Neutrino Detector), performed at Los Alamos, using a ~20-53 MeV beam of electron and muon neutrinos from the LAMPF proton linac accelerator. The LSND data provided statistically strong (~3.8σ) evidence from accelerator-produced neutrino oscillations that showed an excess of electron antineutrinos over what was predicted by application of the SMPP. This was taken as an indication that there might be a fourth neutrino flavor, a “sterile” neutrino that ignored the weak force and interacted only with gravity. Such light (mass ∼6 eV) sterile neutrinos should affect neutrino oscillation results, while heavier sterile neutrinos (keV to GeV) could account for the dark matter in the universe, might be related to the universe’s mysterious matter-antimatter asymmetry, and would be very difficult to detect directly due to their lack of interactions.
A research group at Fermilab set out to test the LSND results using a higher-energy neutrino beam of muon neutrinos at ~0.5-1 GeV. They designed MiniBooNE (Mini Booster Neutrino Experiment), a Cherenkov detector that was filled with 800 tons of ultra-pure mineral oil. Neutrino interactions in the oil produce charged particles, some of which move faster than the local speed of light in the oil medium (~0.68c, which corresponds to an electron with a kinetic energy greater than 186 keV). These make Cherenkov light flashes that were detected by a large number of photo-multiplier tubes in the MiniBooNE detector.
The group collected data at Fermilab from 2002 to 2019. After analysis, they reported observing an excess of electron-like events in both neutrino and antineutrino modes. This excess was consistent with LSND’s findings, strengthening the sterile neutrino hypothesis. However, the group cautioned that their reported signal could also be explained by other effects (e.g., photon misidentification, nuclear interactions,. . .), leading to a somewhat ambiguous result. Subsequently, a number of other neutrino-based experiments (GALLEX, SAGE, BEST, DANSS, PROSPECT, STEREO, and Neutrino-4) have reported relevant results, some of which provided minor support for the existence of sterile neutrinos, while others did not. Accurately interpreting these mixed results would require the daunting task of fully understanding the systematic uncertainties and backgrounds of each experiment, and would be subject to controversy. That has been the ambiguous situation with sterile neutrinos until very recently.
Now, however, two new experimental results provide convincing evidence that low-mass sterile neutrinos do not exist and that there are only three neutrino flavors, consistent with the orthodox SMPP. The first experiment we will discuss is the MicroBooNE experiment at Fermilab, an improvement over the previous experiment MiniBooNE. The new experiment uses a large,170-ton liquid-argon-filled time projection chamber, providing direct tracking and particle identification information on neutrino-derived charged particles instead of just Cherenkov light flashes. MiniBooNE could not distinguish between a single electron (the signal for a standard neutrino) and a closely packed electron-positron pair or a photon. MicroBooNE’s superior imaging can tell these apart.
For a neutrino source MicroBooNE simultaneously used two different pulsed Fermilab muon neutrino (nm) beams that could be distinguished by arrival time, one (BNB) with 0.57% ne contamination and the other (LuMI) with 4.6% ne contamination.
Detailed analysis of the resulting MicroBooNE data found no anomalous excesses that would indicate the presence of sterile neutrinos. The experiment directly tested the LSND/MiniBooNE anomaly and ruled out the simplest 3+1sterile-neutrino model to a confidence level of 95%. The MicroBooNE results significantly narrow the parameter space for the possible existence of sterile neutrinos and strengthen the SMPP’s basic three-neutrino scenario.
The other experiment that contributes to resolving the sterile neutrino conundrum is the KARlsruhe TRItium Neutrino (KATRIN) experiment, primarily designed to find the electron neutrino mass using tritium beta-decay by precisely measuring the shape of the distribution of the highest energy decay-emitted electrons. The 70-meter-long KATRIN setup consists of three main modules: a high-luminosity windowless gaseous tritium source (WGTS) supplying up to 1011 beta decays per second, a very large high-resolution magnetic adiabatic collimation and electrostatic high-pass filter, and a final electron detector. KATRIN presently holds the record for establishing that the rest mass-energy of the electron neutrino is less than 0.45 eV.
KATRIN is designed to accurately measure the detailed shape of the endpoint distribution of the 18,580 ± 7 eV tritium beta decay. Its principal source of error comes from the use of diatomic tritium molecule as the emitting source, rather than single atoms of tritium. The consequence of this is that a small and poorly defined part of the decay energy may be diverted to rotational states of the tritium di-molecule as it transforms to a tritium-helium-3-ion molecule, distorting the shape of the emitted electron’s maximum-energy endpoint.
For compatibility with the LSND/MiniBooNE results and the so-called see-saw mechanism, the hypothetical sterile neutrino is expected to be more massive than the other neutrinos and specifically to have a rest mass in the vicinity of 6.3 electron-volts. Theoretical models allowing some of the beta decays H3 → He3(ion) + e— + n to proceed with sterile neutrinos predict a “kink” (or break in slope) in the endpoint distribution. This is because the sterile decay electron endpoint must terminate is a few eV before the normal decay endpoint, and superposition of the two decay processes produces the kink.
The observed absence of such a kink in the KATRIN data is one of the principal factors that allows the group to exclude the presence of sterile neutrinos over a very broad region of parameter space, including that favored by the LSND/MiniBooNE results.
Have some extensions of the SMPP been falsified or eliminated by these results from MicroBooNE and KATRIN? Such theories are slippery and difficult to rule out completely, but there has been significant progress in that direction. The recent claim of the Neutrino-4 experiment, suggesting evidence for a sterile neutrino with a large mixing angle and amass around 4 eV, was specifically targeted by KATRIN, with a focus on the possible “kink” in the tritium beta-decay spectrum. They achieved a clean, model-independent measurement that falsified the Neutrino-4 claim and excluded large swaths of the parameter space suggested by the “Gallium anomalies” (neutrino deficits from radioactive decays).
Some post-SMPP theories had proposed that a sterile neutrino might rapidly decay into another particle (like a photon and a lighter neutrino) before it could be detected or could participate in neutrino oscillations. MicroBooNE specifically looked for such “decaying sterile neutrino” signatures. The models were ruled out at 99% confidence for the most likely ranges that would have explained the short-baseline anomalies.
What remains of sterile neutrino ideas now lies mainly in the “Dark Sector,” the candidate particles that might explain the cosmological evidence for the galactic dark matter that seems to provide the extra gravitation for galactic formation and anomalous rotation curves. Heavy sterile neutrinos with masses in the keV or MeV range and scenarios involving two flavors of sterile neutrinos have not been ruled out, provide dark-matter candidate particles, and pose new challenges for future experiments.
Hard SF Novels: John’s new 3rd hard SF novel, Fermi’s Question, and its prequel, Einstein’s Bridge, are available as eBooks from Baen Books at: https://www.baen.com/einstein-s-bridge.html. His 1st hard SF novel Twistor is available online at: https://www.amazon.com/Twistor-John-Cramer/dp/048680450X.
Non-Fiction Books: John’s new book How to Live Much Longer: The Mitochondrial DNA Connection (Springer Copernicus) will be available in early 2026. His book on his transactional interpretation of quantum mechanics, The Quantum Handshake: Entanglement, Nonlocality, and Transactions, (Springer, January 2016) is available online at: https://www.amazon.com/dp/3319246402.
Alternate View Columns Online: Electronic reprints of 243 or more of “The Alternate View” columns written by John G. Cramer and previously published in Analog are currently available online at: http://www.npl.washington.edu/av.
References: The MicroBooNE Collaboration, “Search for light sterile neutrinos with two neutrino beams at MicroBooNE,” Nature 648, 64–69 (2025); DOI: 10.1038/s41586-025-09757-7.
The KATRIN Collaboration, “Sterile-neutrino search based on 259 days of KATRIN data,” Nature 648, 70–75 (2025); DOI: 10.1038/s41586-025-09739-9.