New results from physics experiment at ORNL show no sign of sterile neutrinos

Neutrinos, elusive fundamental particles, can act as a window into the center of a nuclear reactor, the interior of the Earth, or some of the most dynamic objects in the universe. Their tendency to change "flavors" may provide clues into the prominence of matter over antimatter in the universe or explain the existence of dark matter.

Physicists are particularly interested in proving the existence of "sterile" neutrinos. Their discovery would reveal a new form of matter that interacts only with gravity and could influence the universe's evolution.

In a new study published in Physical Review Letters, a team of researchers from U.S. universities and national laboratories has set stringent limits on the existence and mass of sterile neutrinos. While they have yet to find the particles, they now know where not to look.

The collaboration analyzed data from the PROSPECT-I detector, stationed near the High Flux Isotope Reactor, or HFIR, at the Department of Energy's Oak Ridge National Laboratory, or ORNL. In the nuclear reactor core, the fission process releases electron antineutrinos.

Neutrinos and antineutrinos come in three known "flavors," and strangely, they can switch between them as they propagate through space. This oscillation phenomenon shows neutrinos have a tiny, but nonzero, mass. They interact through the weak nuclear force and gravity. In contrast to these three known flavors, sterile neutrinos would only interact via gravity. Some theories predict their existence, and persistent hints in anomalous experimental data may support their presence.

"If these new sterile neutrino types exist, then the neutrinos generated by the reactor will have some probability to transform into this sterile type as they propagate from the reactor to the detector," said Bryce Littlejohn, a professor at Illinois Tech and one of the paper's 47 authors. "If that were to occur, PROSPECT would detect fewer reactor-produced neutrinos than expected, since a sterile neutrino would not interact in the detector."

PROSPECT is unique because it is close to a compact nuclear reactor core. It can search for sterile neutrinos with high mass values relative to other experiments that are further from larger reactors. In doing so, it has placed the strongest limits of any reactor experiment in a high-mass region and disfavors the possibility that anomalous results in recent Russian reactor and radioactive source neutrino experiments are due to sterile neutrinos.

"These results, tapping the full potential of the dataset from PROSPECT, show no unusual signs of neutrinos disappearing on their journey to the detector," Littlejohn said.

"The PROSPECT experiment has been very productive, even though it is a relatively small detector and collaboration," said author Russell Neilson, a professor at Drexel University. "Unique features of the experiment have resulted in scientific papers on the sterile neutrino, characterizing antineutrino emissions from reactors, and searching for dark matter." Another recent study even used the detected antineutrino signal to locate the position of the HFIR reactor core, he added.

This paper is the 10th physics publication based on data collected by PROSPECT in 2018. It presents a marked improvement in rejecting background noise and efficient data use compared to earlier results probing the existence of sterile neutrinos.

"The PROSPECT-I detector experienced some technical problems that limited earlier results. At LLNL, we led the development of a technique to extract more information from the data, greatly improving background rejection," said author Nathaniel Bowden, a physicist at the Department of Energy's Lawrence Livermore National Laboratory, or LLNL. "Studies like these give us important insights that also advance our work on national security - for example, building sensitive neutron detectors or using antineutrinos to monitor nuclear reactor operations."

"The UTK/ORNL analysis group, led by my former graduate students Diego Venegas-Vargas, Xiaobin (Jeremy) Lu and Blaine Heffron, has been instrumental in developing and optimizing the dataset used in this result," said Alfredo Galindo-Uribarri, a distinguished scientist in the Physics Division of ORNL and an adjunct professor in the Department of Physics and Astronomy of the University of Tennessee, Knoxville, or UTK. "They also explored and implemented innovative analysis methods that have resulted in a final dataset demonstrating the full potential of the PROSPECT detector."

He added, "In addition to co-leading the current findings on sterile neutrinos, the UTK/ORNL group has spearheaded efforts in producing the most precise measurement of the uranium-235 energy spectrum, developing machine learning-driven methods for antineutrino event reconstruction and introducing novel calibration techniques for future antineutrino detectors."

The collaboration is joining forces with two other experiments to extend the search for sterile neutrinos into other mass regimes. The team is also working on an upgrade to the PROSPECT detector that will retain its excellent performance while improving robustness, allowing for a large increase in collected data.

PROSPECT is supported by the Department of Energy Office of Science and the Heising-Simons Foundation. The researchers also received support from, Drexel University, the Illinois Institute of Technology, the University of Hawai'i, Yale University, Brookhaven National Laboratory, the Laboratory Directed Research and Development program at Lawrence Livermore National Laboratory, the National Institute of Standards and Technology and Oak Ridge National Laboratory. The collaboration also benefits from the support and hospitality of the High Flux Isotope Reactor, a DOE Office of Science User Facility.

UT-Battelle manages ORNL for DOE's Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit https://science.energy.gov/.

- Abridged and adapted from a story by Ashley Piccone of Lawrence Livermore National Laboratory, with additional reporting by Dawn Levy of Oak Ridge National Laboratory