The MINOS+ and Daya Bay neutrino experiments combined results to produce the most stringent test yet for the existence of sterile neutrinos.

An international group of more than 260 scientists have produced one of the most stringent tests for the existence of sterile neutrinos to date. The scientists from two major international experimental groups, Daya Bay in China and the MINOS+ at the Department of Energy’s Fermilab, are reporting results in Physical Review Letters (Phys. Rev. Lett. 125, 071801 (2020)) ruling out oscillations into one sterile neutrino as the primary explanation for unexpected observations in the recent LSND and MiniBooNE experiments.

Neutrinos are elementary particles that, like electrons, cannot be broken down into smaller components. They are unlike any other particles known to exist since they are able to penetrate through extremely large amounts of matter without stopping.

There are three known types of neutrinos: electron, muon and tau. About two decades ago, scientists found that they can morph from one type into another through a phenomenon called “neutrino oscillation”, a discovery which was predicted by Bruno Pontecorvo, a scientist from the Laboratory of Nuclear Problems at the Joint Institute for Nuclear Research (Dubna), as far back as 1957. This discovery was awarded the 2015 Nobel Prize in physics. For instance, a neutrino created as an electron type traveling through space can later be identified as a muon type or tau type.

Even though the vast majority of accumulated data to date can be explained by three known neutrinos, a few experiments have reported anomalous observations suggesting the existence of additional types. Among these are the LSND experiment at the Los Alamos National Laboratory and the MiniBooNE experiment at Fermilab. Both exposed their detectors to a beam of muon neutrinos and reported an excess of electron neutrino candidate events beyond what would be expected from oscillations involving only the three known neutrinos, but possibly reconcilable if a new type of neutrino – a sterile neutrino – was involved. Sterile neutrinos would not be directly detectable for they do not interact with matter, but their oscillation with the three known neutrinos would provide a unique pathway to establish their existence. However, the new results from Daya Bay and MINOS+ question this possibility.

Comparison of the MINOS, MINOS+, Daya Bay, and Bugey-3 combined 90% CLs limit on sin²2θ_µe to the LSND and MiniBooNE 90% C.L. allowed regions. The regions excluded at 90% C.L. by the KARMEN2 Collaboration and the NOMAD Collaboration are also shown

Daya Bay uses eight detectors to precisely measure how electron reactor antineutrinos “disappear” or, in other words, morph into neutrinos of other types. Similarly, MINOS+ studies the disappearance of muon neutrinos produced by a Fermilab accelerator and propagating to an underground detector in northern Minnesota 735 kilometers away.

“The stakes are high. The interpretation is very tantalizing. If it were confirmed, a revolution in physics would ensue. Sterile neutrinos would become the first particles to be found outside the Standard Model, our current best theory of elementary particles and their interactions. They could also be candidates for dark matter and might have important consequences in cosmology,” said Daya Bay scientist Dmitry Naumov, Doctor of Physics and Mathematics, Deputy Director of DLNP at JINR.

“This close collaboration of MINOS+ and Daya Bay scientists enabled the combination of two complementary world-leading constraints on muon neutrinos and electron antineutrinos disappearing into sterile neutrinos”, explained Alexandre Sousa, associate professor of physics at the University of Cincinnati and one of the MINOS+ scientists who worked on the analysis. The disappearance of both particles needs to occur if electron (anti)neutrinos are to appear in a muon (anti)neutrino source via sterile oscillations with a single sterile neutrino. “So the combined result is a very powerful probe of the sterile neutrino hints we have to date.”

According to DLNP researcher Maxim Gonchar, Candidate of Physics and Mathematics, PI’s Deputy of the Daya Bay project, the neutrino disappearance measurements by Daya Bay and MINOS+ are now so precise that they essentially rule out explaining the combined anomalous observations from LSND, MiniBooNE and other experiments for the mass splitting range 10^-4 eV² ≲ Δm²₄₁ ≲ 10⁰ eV² solely through sterile neutrino oscillations.

“We would all have been absolutely thrilled to find evidence for sterile neutrinos, but the data we have collected so far do not support any kind of oscillation with these exotic particles,” noted Daya Bay scientist Pedro Ochoa-Ricoux, associate professor of physics and astronomy at UC Irvine.

The combined analysis reported by Daya Bay and MINOS+ not only ruled out the specific kind of sterile neutrino oscillation that would explain the anomalous results but also looked for other sterile neutrino signatures with never-before-achieved sensitivity, yielding some of the most stringent limits on the existence of these elusive particles to date.

“The two experiments use multiple detectors with well-understood uncertainties and have collected an unprecedentedly large number of events. Requiring consistency between the data sets of the two experiments provides a very rigorous test of sterile neutrino existence,” said Jenny Thomas, Head of the MINOS+ experiment, professor at University College London, and Karol Lang, professor at the University of Texas at Austin jointly in a statement.

“This joint effort very effectively tackles a fundamental problem in physics,” said Daya Bay spokespersons Kam-Biu Luk of Lawrence Berkeley National Laboratory and UC Berkeley and Jun Cao of the Institute of High Energy Physics in Beijing in a joint statement. “While there is still room for a sterile neutrino to be lurking in the shadows, we have significantly shrunk the available hiding space.”

Comparison of the MINOS, MINOS+, Daya Bay, and Bugey-3 combined 90% CLs limit on sin22θµe to the LSND and MiniBooNE 90% C.L. allowed regions. The regions excluded at 90% C.L. by the KARMEN2 Collaboration and the NOMAD Collaboration are also shown.

According to the joint press release of the MINOS+ and Daya Bay collaborations