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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.

Â鶹ÒùÔºicists 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 evolution of the universe.

In a new study in Â鶹ÒùÔºical 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 (HFIR) at Oak Ridge National Laboratory. In the nuclear reactor core, the fission process leads to the release of electrons and antineutrinos.

Neutrinos and antineutrinos come in three known "flavors," and strangely, they can switch between them as they propagate through space. In contrast to these three known flavors, sterile neutrinos only interact via gravity. Some theories predict their existence, and there are persistent hints in anomalous experimental data that may indicate 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 author Bryce Littlejohn, a professor at Illinois Tech. "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 stationed close to a compact reactor core—it can search for sterile neutrinos with high mass values relative to other experiments that are farther 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," said Littlejohn.

"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 was even able to use the antineutrino signature to point back to the HFIR reactor core."

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 Lawrence Livermore National Laboratory (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."

Going forward, the collaboration is joining forces with two other experiments to extend the search for sterile neutrinos into other mass regimes. They are also working on an upgrade to the PROSPECT detector that retains its excellent performance while improving robustness to allow for a large increase in collected data.