An international team of physicists, including Rutgers researchers, has concluded that a hypothesized fourth type of neutrino, known as the sterile neutrino, likely does not exist. Using the MicroBooNE experiment at Fermilab, they analyzed data from two neutrino beams over ten years and found no evidence for it with 95% certainty. The findings, published in Nature, challenge previous explanations for unusual neutrino behavior.
The MicroBooNE experiment, conducted at the U.S. Department of Energy's Fermi National Accelerator Laboratory in Batavia, Illinois, employed a large liquid-argon detector to track neutrino interactions. Neutrinos, tiny particles that pass through matter with minimal interaction, come in three known flavors—electron, muon, and tau—according to the Standard Model of particle physics. These can oscillate, or change types, during travel.
Earlier observations of neutrino anomalies led scientists to propose a sterile neutrino, which would interact only via gravity and evade standard detection. To test this, the MicroBooNE team collected data from two beams: one from the Booster source and another from the NuMI (Neutrinos from the Main Injector) beam. After a decade of measurements, they detected no signs of sterile neutrino production or oscillation, effectively ruling out this hypothesis at a 95% confidence level.
Andrew Mastbaum, an associate professor of physics at Rutgers University and a MicroBooNE leadership member, highlighted the implications. "This result will spark innovative ideas across neutrino research to understand what is really going on," he said. "We can rule out a great suspect, but that doesn't quite solve a mystery."
Rutgers graduate students contributed significantly: Panagiotis Englezos managed data processing and simulations, while Keng Lin validated the NuMI beam's neutrino flux. Mastbaum coordinated analysis tools, addressing systematic uncertainties like neutrino-nucleus interactions and detector responses.
The discovery narrows searches for physics beyond the Standard Model, which fails to account for dark matter, dark energy, and gravity. It also refines liquid-argon detection techniques for upcoming projects like the Deep Underground Neutrino Experiment (DUNE). As Mastbaum noted, "With careful modeling and clever analysis approaches, the MicroBooNE team has squeezed an incredible amount of information out of this detector." These methods will probe deeper questions about matter and the universe's origins.