Rutgers researchers help close the door on a decades-old physics mystery
After collecting and analyzing data for a decade, a group of scientists, including a team from Rutgers, have debunked a decades-old theory about a mysterious particle.
Their findings, published in Nature, come from the MicroBooNE experiment at the U.S. Department of Energy's Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. (The acronym MicroBooNE stands for "Micro Booster Neutrino Experiment."
Using a powerful liquid-argon detector and data from two neutrino beams, researchers ruled out the existence of a single sterile neutrino with 95% certainty.
Andrew Mastbaum, an associate professor in the Department of Physics and Astronomy in the Rutgers School of Arts and Sciences and a member of the MicroBooNE leadership team, said the result is a turning point.
"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."
Neutrinos are tiny subatomic particles that barely interact with matter. They can pass through planets without stopping. The Standard Model, the main theory of particle physics, states there are three types of neutrinos: electron, muon and tau. These particles can change from one type to another, a process called oscillation.
But in past experiments, researchers saw neutrinos that appeared to change in ways that didn't fit the Standard Model. To explain this, scientists proposed a fourth type: the sterile neutrino. Unlike the others, it would be even more difficult to detect because it wouldn't interact with matter at all, except through gravity.
MicroBooNE scientists tested this idea by observing neutrinos from two different beams and measuring how they oscillate. After ten years of data collection and analysis, the team found no sign of sterile neutrinos, closing the door on one of the most popular explanations for strange neutrino behavior.
Mastbaum helped lead the experiment's analysis program as co-coordinator for analysis tools and techniques, overseeing how scientists turned raw data into meaningful physics results. He previously led the team that worked out what the research team refers to as systematic uncertainties, which are the possible sources of error in the measurements. This includes understanding how neutrinos interact with atomic nuclei, how many neutrinos are in the beam and how the detector responds.
Getting these uncertainties right is critical because it allows scientists to make strong, reliable statements about what the data really shows, Mastbaum said.
Panagiotis Englezos, a doctoral student in the Department of Physics and Astronomy at the Rutgers School of Arts and Sciences, served on the MicroBooNE Data Management Team, helping to process data and produce supporting simulations. Keng Lin, also a doctoral student in the department, helped validate the neutrino flux from Fermilab's NuMI (Neutrinos from the Main Injector) beam, one of the two neutrino beams used in this analysis. These efforts ensured the accuracy and reliability of the experiment's findings.
This result is important, Mastbaum said, because it rules out a major theory about new physics. The Standard Model doesn't explain everything, including dark matter, dark energy or gravity, he said, so scientists are searching for clues that point beyond the model. Eliminating one possibility helps focus the search on other ideas that could lead to breakthroughs in understanding the universe.
Rutgers scientists played a crucial role in analyzing the data and improving techniques for measuring neutrino interactions in liquid argon. These advances will help future experiments, including the Deep Underground Neutrino Experiment (DUNE).
"With careful modeling and clever analysis approaches, the MicroBooNE team has squeezed an incredible amount of information out of this detector," Mastbaum said. "With the next generation of experiments, such as DUNE, we are already using these techniques to address even more fundamental questions about the nature of matter and the existence of the universe."
