Adrien Hourlier at the Liquid Argon Test Facility (LArTF) works on the MicroBooNE detector. Image credit: Reidar Hahn

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.

Joshua Spitz

“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.

MicroBooNE’s detector is assembled at Fermilab. Image credit: Reidar Hahn

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.

The MiniBooNE detector had a particular limitation: It was unable to tell the difference between electrons and photons (particles of light) close to where the neutrino interacted with an atom inside of the detector. If MiniBooNE were truly seeing more electrons than predicted, it would indicate extra electron neutrinos causing the interactions. That would mean something unexpected was happening in the oscillations that researchers hadn’t accounted for: sterile neutrinos. But if photons were causing the excess, it would likely be a background process rather than oscillations gone wild and a new particle.

It was clear that researchers needed a more nuanced detector. In 2007, the idea for MicroBooNE was born.

MicroBooNE: Precision detector

The MicroBooNE detector is built on state-of-the-art techniques and technology. It uses special light sensors and more than 8,000 painstakingly attached wires to capture particle tracks. It’s housed in a 40-foot-long cylindrical container filled with 170 tons of pure liquid argon. Neutrinos bump into the dense, transparent liquid, releasing additional particles that the electronics can record. The method, pioneered by Spitz, an associate professor of physics at U-M, results in pictures that show detailed particle paths and, crucially, distinguish electrons from photons.

The installation team overlooks the finished MicroBooNE cosmic ray tagger system. Image credit: Reidar Hahn

Spitz’s group at U-M had a number of leading roles on the experiment. The group’s primary work involved calibrating the detector, developing software for making sense of the neutrino images collected and understanding how neutrinos interact with the argon atoms inside the detector. The group’s work helped produce the result found by the analyses.

MicroBooNE’s first three years of data show no excess of electrons—but they also show no excess of photons from a background process that might indicate an error in MiniBooNE’s data.

“We’re not seeing what we would have expected from a MiniBooNE-like signal, neither electrons nor the most likely of the photon suspects,” said Fermilab scientist Sam Zeller, who served as MicroBooNE co-spokesperson for eight years. “But that earlier data from MiniBooNE doesn’t lie. There’s something really interesting happening that we still need to explain.”

While the first analyses weighed in on the sterile neutrino, additional analyses could provide more information about exotic explanations, including dark matter, axion-like particles, the hypothetical Z-prime boson and beyond. There’s even a chance it could still be a sterile neutrino, hiding in even more unexpected ways.

Future neutrino exploration

Neutrinos are surrounded by mysteries. The anomalous data seen by the earlier MiniBooNE and LSND experiments still need an explanation. So, too, does the very phenomenon of neutrino oscillation and the fact that neutrinos have mass, neither of which is predicted by the Standard Model. There are also hints that neutrinos could help explain why there is so much matter in the universe, as opposed to a universe full of antimatter or nothing at all.

MicroBooNE is one of a suite of neutrino experiments searching for answers. Crucially, it’s also a long-running testbed for the liquid argon technology that will be used in upcoming detectors.

Spitz’s group at U-M working on MicroBooNE has included doctoral students Chris Barnes and Rory Fitzpatrick, as well as former postdoctoral fellow Joel Mousseau. Undergraduates, including those who have since graduated, include Polina Abratenko, Nick Kamp and Alexis Metzler.

MicroBooNE is supported by the U.S. Department of Energy, U.S. National Science Foundation, Swiss National Science Foundation, U.K. Science and Technology Facilities Council, U.K. Royal Society and European Union’s Horizon 2020.

Written by Tracy Marc, Fermi National Accelerator Laboratory