Is there a fourth neutrino? New results from Fermilab say no

microboone detector at Fermilab

The MicroBooNE detector being installed at Fermilab. 

This story was adapted from a Fermilab news release. 

New results from the MicroBooNE experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory deal a blow to a theoretical particle known as the sterile neutrino. Colorado State University researchers from the Department of Physics were among the 200 scientists around the world who contributed to this new, crucial insight into how our universe works.

For more than two decades, this proposed fourth neutrino, the sterile neutrino, has remained a promising explanation for anomalies seen in earlier physics experiments. Finding a new particle would be a major discovery and a radical shift in our understanding of the universe.

However, four complementary analyses released by the international MicroBooNE collaboration earlier this month show no sign of the sterile neutrino. Instead, the results align with the Standard Model of Particle Physics, scientists’ best theory of how the universe works. The data is consistent with what the Standard Model predicts: three kinds of neutrinos ­– no more, no less.

CSU contributions

Michael Mooney, an assistant professor in the Department of Physics, has been part of the MicroBooNE collaboration for several years, serving as run coordinator during the first operation of the MicroBooNE detector in neutrino beam in 2015 and 2016. He has since supervised two CSU graduate students who both contributed to the recent results.

Graduate student Ivan Caro Terrazas worked on electron neutrino reconstruction, needed for selection of neutrino signal events inside the detector, for one of three analyses looking for an excess of low-energy electron neutrino events beyond expectations.

Ryan LaZur, a former graduate student in Mooney’s lab, worked on muon neutrino reconstruction. This work was necessary for providing a constraint on systematic uncertainties for the same analysis ­– searching for an excess of low-energy electron neutrino events in the detector.

The results represent years of intense work by many people, including students, postdocs, technicians, engineers and faculty, Mooney said.

“Having been so heavily involved with operations when the detector was first brought online, it is deeply meaningful to see MicroBooNE finally release these results,” Mooney said. “While it seems that the most simple explanation for the previous anomaly is disfavored, these results have significantly advanced our understanding of neutrinos; they will be a guiding light for future short-baseline neutrino experiments looking to explain an anomaly that is not yet resolved.”

The experiment

MicroBooNE is a 170-ton neutrino detector roughly the size of a school bus that has operated since 2015. The international experiment has close to 200 collaborators from 36 institutions in five countries. They used cutting-edge technology to record spectacularly precise 3D images of neutrino events and examine particle interactions in detail – a much-needed probe into the subatomic world.

Neutrinos are one of the fundamental particles in nature. They’re neutral, incredibly tiny, and the most abundant particle with mass in our universe — though they rarely interact with other matter. They’re also particularly intriguing to physicists, with a number of unanswered questions surrounding them. These puzzles include why their masses are so vanishingly small and whether they are responsible for matter’s dominance over antimatter in our universe. This makes neutrinos a unique window into exploring how the universe works at the smallest scales.

MicroBooNE’s new results are a turning point in neutrino research, according to Fermilab. With sterile neutrinos further disfavored as the explanation for anomalies spotted in neutrino data, scientists are investigating other possibilities. These include things as intriguing as light created by other processes during neutrino collisions or as exotic as dark matter, unexplained physics related to the Higgs boson, or other physics beyond the Standard Model.

Several CSU researchers are also involved in the Deep Underground Neutrino Experiment, also hosted at Fermilab.

To read more about the MicroBooNE results and why the experiment began in the first place, visit the Fermilab website.