MINOS and Daya Bay join forces to narrow the window on sterile neutrinos

Do sterile neutrinos—hypothetical particles that do not interact with matter except through gravity—really exist? If so, this would solve some of today’s major mysteries in particle physics and cosmology. For two decades, researchers around the globe have sought evidence that would prove or disprove the reality of sterile neutrinos, with inconclusive outcomes.

Now, a new result has all but ruled out the possible existence of a light sterile neutrino in a regime suggested by an earlier experiment. Researchers from two major international collaborations—the Main Injector Neutrinos Oscillation Search (MINOS) at Fermi National Laboratory and the Daya Bay Reactor Neutrino Experiment in the south of China—joined forces, each contributing years of data that, taken together, paint a nearly complete picture. The joint result published in Physical Review Letters has significantly shrunk the hiding space for a light sterile neutrino.

Hints of a new type of neutrino beyond the well-known three types—electron, muon, and tau—first surfaced in the 1990s, when scientists at the Los Alamos National Laboratory were looking for neutrino oscillations, the morphing of one type of neutrino into another type. Members of the Liquid Scintillator Neutrino Detector (LSND) experiment announced evidence of muon neutrinos oscillating into electron neutrinos. However, the oscillation occurred much faster than the oscillations discovered by Super-Kamiokande that led to the 2015 Nobel Prize in Physics.

If the LSND results are correct and due to neutrino oscillations, the only explanation is the existence of a fourth type of neutrino. But this new neutrino would have to be much stranger than anything seen before. Being “sterile,” it would not interact with matter except through gravity. Light sterile neutrinos are also among the leading candidates to resolve some outstanding puzzles in astrophysics and cosmology.

Over the last twenty years, a number of experiments have tried to confirm or refute the LSND findings, but the results have been inconclusive. The new result released by the MINOS and Daya Bay experiments strongly suggests that the ghost-like sterile neutrinos do not explain the LSND result after all.

The LSND experiment saw muon-type antineutrinos turning into electron-type antineutrinos, so to address the LSND observations, scientists must look at both types of neutrinos simultaneously. That’s why the collaboration between Daya Bay and MINOS was crucial, according to En-Chuan Huang, a postdoctoral fellow at Los Alamos National Laboratory who participated in the Daya Bay research as a graduate student at the University of Illinois at Urbana-Champaign Department of Physics, working under Professor Jen-Chieh Peng.

“Neither the MINOS nor the Daya Bay disappearance results alone can be compared to the LSND appearance measurements,” Huang explains. “Looking at multiple types of neutrinos together gives us a much stronger handle on sterile neutrinos.”

The Daya Bay experiment looks at electron antineutrinos coming from a nuclear power plant in the Guangdong province of China. Daya Bay observed that some of these antineutrinos disappear and measured for the first time one of the parameters governing neutrino oscillations, a result garnering the 2016 Breakthrough Prize in Fundamental Physics. A sterile neutrino would affect the rate these electron antineutrinos disappear, but the Daya Bay scientists have seen no evidence for this.

The MINOS experiment uses an intense beam of muon neutrinos that travels 735 km from the Fermi National Accelerator Laboratory in Chicago to the Soudan Underground Laboratory in northern Minnesota. MINOS has made world-leading measurements to study how these neutrinos disappear as they travel between the two detectors. The existence of a sterile neutrino could cause some of these muon neutrinos to disappear at a faster rate than one would expect if sterile neutrinos do not exist. Scientists working on the MINOS experiment have shown that this does not happen. 

But these two results from MINOS and Daya Bay are not sufficient by themselves to address the puzzle that LSND set out almost twenty years ago.

“It’s not common for two major neutrino experiments to work together this closely,” comments Adam Aurisano of the University of Cincinnati Department of Physics. “But to really make a statement about the LSND evidence for sterile neutrinos, we must take Daya Bay’s electron-antineutrino data and the MINOS muon-neutrino data and put them both together into a single analysis.”

The result is a publication that very strongly excludes most of the possible sterile neutrino oscillation scenarios that could explain the LSND result. Both the MINOS and Daya Bay experiments are continuing to analyze additional data, and an even more sensitive search for the sterile neutrino is planned.

“It is difficult enough to detect ordinary neutrinos which hardly interact, and it is much more challenging to search for sterile neutrinos which may not exist,” concludes Peng. “And neutrinos may continue to surprise us in the future.”