Two major international collaborations have joined forces to publish a paper that sheds new light on one of the most pressing questions in particle physics: "do sterile neutrinos exist?
Hints of a new type of neutrino beyond the well-known three types (electron, muon, and tau neutrinos) first surfaced in the 1990s when scientists at the Los Alamos National Laboratory were looking for neutrino oscillations - the ability of one type of neutrino to morph into another type. The LSND experiment at Los Alamos 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 new, fourth type of neutrino. But this neutrino would have to be much stranger than anything seen before, being sterile, meaning that it does 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 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. 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 at which these electron antineutrinos disappear, but the Daya Bay scientists have seen no evidence for this. However, these two results from MINOS and Daya Bay, by themselves, are not enough to address the puzzle that LSND set out almost twenty years ago.
"Neither the MINOS nor Daya Bay disappearance results alone can be compared to the LSND appearance measurements", says En-Chuan Huang of Los Alamos Laboratory and the University of Illinois at Urbana-Champaign, one of the scientists working on the Daya Bay experiment. "Looking at multiple types of neutrinos together gives us a much stronger handle on sterile neutrinos." 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. This is where the collaboration between Daya Bay and MINOS comes in.
"It's not common for two major neutrino experiments to work together this closely", says Adam Aurisano of the University of Cincinnati, one of the MINOS scientists who worked on the result. "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.
The joint result has significantly shrunk the hiding space for a light sterile neutrino. Both the MINOS and Daya Bay experiments are continuing to analyze more data, and an even more sensitive search for the sterile neutrino is anticipated. "The neutrino is one of the most enigmatic particles we have encountered", says Aurisano, "and history suggests that surprises may await us".