In a bid to spot the elusive fundamental particle, the neutrino, a team of researchers from various universities and national laboratories in the US has tightened the limits on its existence and mass. While the particle remains elusive, the team is hopeful since they know exactly where to look for sterile neutrinos – at the center of nuclear reactors.
Neutrinos are subatomic particles with nearly zero mass while also carrying no charge. The particle interacts very weakly with matter, allowing it to travel to large amounts of matter without being detected. This is why they are also known as ghost particles.
Nevertheless, physicists are certain that neutrinos offer us a window into understanding some of the most dynamic objects in the universe, including our planet’s interior. We also know that neutrinos occur due to supernova explosions, radioactive decay, or even when cosmic rays interact with our atmosphere.
However, instead of relying on these occurrences to observe them, scientists have spotted a reliable source of their existence – the nuclear reactor.
The PROSPECT-I detector
The researchers studied data collected by the PROSPECT-I detector at the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (ORNL). The nuclear fission reaction in the reactor core releases electrons and antineutrinos.
The antineutrino is the antiparticle version of the neutrino, with the same properties but opposite quantum numbers. As they propagate through space, they can switch into antineutrinos. This holds for three types of neutrinos: electrons, muons, and tau. However, sterile neutrinos do not interact with matter but are only predicted to interact with gravity.
Researchers hypothesize that if sterile neutrinos do exist, neutrinos generated during a nuclear reaction will also transform into sterile neutrinos. This would reduce the number of neutrinos detected after the reaction since the detector will not pick up sterile neutrinos.
The PROSPECT-I detector in operation at Oak Ridge National Laboratory’s High Flux Isotope Reactor. Image credit: PROSPECT collaboration
Tightening limits for neutrinos
Since the PROSPECT detector is close to the reactor core, it is more likely to detect sterile neutrinos. Researchers looked for sterile neutrinos with high mass values compared to other detectors placed further from the reactor.
“The PROSPECT experiment has been very productive, even though it is a relatively small detector and collaboration,” said Russell Neilson, a professor at Drexel University who was also involved in the work.” 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.”
Researchers at the Lawrence Livermore National Laboratory (LLNL) also contributed to the project by helping solve some of the technical problems faced with the initial results.” We led the development of a technique to extract more information from the data, greatly improving background rejection,” added Nathaniel Bowden, a physicist at LLNL in the press release.
The researchers now plan to focus on extending the search for sterile neutrinos into other mass regimes. This is likely to help us understand more about nuclear reactions and the existence of dark matter.
The research findings were published in the journal Physical Review Letters.