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NMSU physicist among researchers that may have evidence of the elusive sterile neutrino

A New Mexico State University professor is part of an international research team that recently released the results of an experiment that may support a theory for a fourth fundamental particle – the mysterious sterile neutrino that passes through matter without interacting with it.


Circles of round brown cylinders
This interior view of the MiniBooNE detector tank shows the array of photodetectors used to pick up the light particles that are created when a neutrino interacts with a nucleus inside the tank. (Fermilab photo by Reidar Hahn)
Head and shoulders of a man
Robert Cooper, in the Department of Physics at New Mexico State University, is part of a research team whose research may support evidence for a fourth fundamental particle: the sterile neutrino. (NMSU photo by Billy Huntsman)
Aerial shot of a lab
Aerial view of the U.S. Department of Energy’s Fermi National Accelerator Laboratory near Chicago. (Courtesy photo by Reidar Hahn/Fermilab)

Robert Cooper, assistant professor in NMSU’s Department of Physics, is one of a group of scientists from 17 universities and labs, which conducted their experiments collectively known as MiniBooNE at the Fermi National Accelerator Laboratory near Chicago.

“We are presenting very compelling evidence that could support the sterile neutrino hypothesis,” Cooper said. “It will take other types of experiments that look at this phenomenon from a slightly different ‘angle’ to claim discovery.”

But they are not jumping to conclusions. Cooper emphasized the research team is not claiming the discovery of a fourth neutrino.

“Besides the sterile neutrino theory, another explanation could be that there is a new or underestimated source of background events,” Cooper said. “Clearly, sterile neutrinos could rewrite our understanding of neutrino and particle physics, but either way, something new is occurring.”

MiniBooNE has been conducting experiments for over 20 years. The anomaly that resulted in the sterile-neutrino hypothesis was first detected in the 1990s at Los Alamos National Laboratory.

“Not much was done differently this time around,” Cooper said. “The detector was run twice as long, and as a result, MiniBooNE doubled the number of neutrino events. Some have argued that the excess of electron events could have been due to an ‘unlucky’ statistical fluctuation. The increased number of events has improved our ‘statistical sensitivity’ and shows that this result is significant and very unlikely to be due to a statistical accident.”

MiniBooNE specifically researches neutrino oscillations. Currently there are three kinds, or flavors, of neutrinos known: an electron neutrino, a muon neutrino, and a tau neutrino. Neutrino oscillation is the phenomenon in which neutrinos change into different flavors.

“MiniBooNE was designed to search for electron neutrinos,” Cooper said. “It identifies electron neutrinos by searching for interactions that produce electrons in the final state. We use the Booster Neutrino Beamline at Fermilab to produce a pure beam of muon neutrinos to search for these.”

“An electron neutrino can produce an electron when it interacts with matter, and the muon neutrino can produce a muon when it interacts with matter,” Cooper said. “At the neutrino energies we produce in our experiments, more than 800 million electron volts, and the distance scales of our experiment – half a kilometer – we expect to see almost no electrons in the final state.”

But at the conclusion of their experiments, the researchers saw an excess of final-state electrons beyond what they expected.

“We have measurements and models that constrain this number to a small amount,” Cooper said. “Another source of ‘false’ electrons is gamma rays. Our detector does an extraordinary job of distinguishing background gamma ray events from electrons, but some may slip through the cracks of our analysis. Just like contamination, we have multiple measurements of these backgrounds and extensive simulation and modeling. Therefore, our excess of electron events seems significant, validated, and real.”

The existence of a sterile fourth neutrino would mean a total rethinking of particle physics would be necessary.

“Our understanding of particle physics, cosmology, and possibly dark matter would be affected,” Cooper said. “Discovering new particles is a very big deal. Supposing sterile neutrinos exist, this would have deep consequences about the types of future experiments we perform to explore the boundaries of our particle physics knowledge.”

Sterile neutrinos could answer a number of questions about dark matter, which physicists theorize makes up most of the universe and about which very little is known.

Cooper said there could even be two or more types of sterile neutrinos.

A formal discovery will require more work and the MiniBooNE team is continuing its research.

“I’m working on multiple new efforts to validate this possible discovery,” Cooper said. “All that being said, we have very significant evidence, but it is not absolute proof yet.”

The preprint article has been submitted to a peer-reviewed journal and is available at: https://arxiv.org/abs/1805.12028 .