Professor Joshua Spitz

A decades-long experiment to search for a new particle called the sterile neutrino has been dealt a blow, according to four analyses conducted by a bevy of scientists, including University of Michigan physicists.

New results from the MicroBooNE experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory show no sign of the sterile neutrino, a proposed particle that 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, the four complementary analyses released by the international MicroBooNE collaboration and presented during a seminar today all did not find evidence 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.

“We have had hints that there’s a fourth or more type of neutrino, and the discovery of a new particle has enormous implications for how the universe evolved, how galaxies clump together, how stars explode, and why we live in a matter-dominated universe,” said U-M physicist Joshua Spitz, one of the founding collaborators of the MicroBooNE experiment. “In all the different ways we looked, we didn’t see evidence for an excess, which would be indicative of a new type of fundamental particle.”

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 an exciting turning point in neutrino research. 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.

First hints of sterile neutrinos

Neutrinos come in three known types—the electron, muon and tau neutrino—and can switch between these flavors in a particular way as they travel. This phenomenon is called “neutrino oscillation.” Scientists can use their knowledge of oscillations to predict how many neutrinos of any kind they expect to see when measuring them at various distances from their source.

Neutrinos are produced by many sources, including the sun, the atmosphere, nuclear reactors and particle accelerators. Starting around two decades ago, data from two particle beam experiments threw researchers for a loop.

In two previous experiments, including one called MiniBooNE, scientists also saw more particle events than calculations predicted. These strange neutrino beam results were followed by reports of missing electron neutrinos from radioactive sources and reactor neutrino experiments.

Sterile neutrinos emerged as a popular candidate to explain these odd results. While neutrinos are already tricky to detect, the proposed sterile neutrino would be even more elusive, responding only to the force of gravity. But because neutrinos flit between the different types, a sterile neutrino could impact the way neutrinos oscillate, leaving its signature in the data.

Please see the Michigan News website to read the rest of the story.

Written by Tracy Marc, Fermi National Accelerator Laboratory

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Joshua Spitz