Search for sterile sub-eV neutrinos using two highly sensitive detectors

The standard model of particle physics represents only 20% of the matter in the universe. Physicists have theorized that the remaining 80% is made up of so-called dark matter, which consists of particles that do not emit, absorb or reflect light and therefore cannot be directly observed using existing instruments.

The existence of dark matter is indirectly inferred from astronomical observations of its gravitational effects. So far, researchers have not been able to directly observe this mysterious type of matter, but have introduced a number of theoretical models that outline possible “ traces ” that dark matter could leave behind when interacting with known standard model particles through forces. unknown, also called dark forces.

According to some of these theoretical models, dark matter could be observed indirectly by detecting the effects of its extremely rare interactions with normal matter. Recent astronomical data collected by the Planck telescope, launched into space over a decade ago and operated by the European Space Agency (ESA), have suggested the existence of an inactive (i.e. sterile) type of neutrino with a mass in the sub-eV-scale. , which could be promising candidates for dark matter.

The RENO (Reactor Experiment for Neutrino Oscillation) collaboration, a group of researchers from several institutes in South Korea, recently conducted a search for sterile sub-eV neutrino oscillations, based on data collected from two identical detectors located in South Korea over the course of 2200 days. Although they were unable to detect these fluctuations, their results, published in Physical Review Letters, could inform future searches for sterile neutrinos.

“Sterile neutrinos, if they exist, can be mixed with active neutrinos and thus leave observable effects in the data collected in the reactor neutrino experiments,” Soo-Bong Kim, one of the researchers who led the study, told Phys.org. “RENO’s experiments in Korea and Daya Bay in China, which use multiple and identical detectors in different locations, have high enough sensitivities to test Planck’s results.”

So far, experimental efforts aimed at detecting clues to sterile neutrino interactions on a sub-eV scale have failed to pick up signals from these elusive particles. The previous results therefore seem in part to exclude the validity of the recent hypothesis based on the data collected by the Planck telescope. To fully confirm or disprove this hypothesis, physicists will first have to conduct research covering the space of the remaining parameters, collecting very precise measurements.

In their study, Kim and his colleagues analyzed a large amount of data collected from two identical detectors positioned at ~ 300 m and ~ 1400 m from six reactors located at the Hanbit Nuclear Power Plant in Korea. This data was accumulated over the course of eight years, as part of the RENO experiment. The main objective of the RENO experiment is to measure or set a limit to the so-called parameter of the neutrino mixing matrix θ¹³, which is responsible for the oscillations that would derive from the mixing between different neutrino flavors.

“The large sample of data allows us to reduce uncertainties related to statistical fluctuations, and our configuration of two identical detectors is helpful in substantially reducing the uncertainties associated with measurement systems and methods,” explained Kim. “They substantially improved the accuracy of the neutrino energy spectrum measurement. Mixing with sterile, unobservable neutrinos causes the active neutrinos to disappear in the data, so we tried to probe the effects of sterile neutrinos by comparing the spectral shapes of the two detectors.”

Overall, the recent work by Kim and his colleagues confirms the feasibility of conducting dark matter research using artificial instruments that can measure oscillations with high levels of accuracy. So far, the researchers have not been able to detect any significant features that could result from the sterile interactions of neutrinos. Therefore, their results suggest that if these particles existed, their interactions with other particles would be extremely weak.

“The results collected by us and the Daya Bay Reactor Neutrino Experiment provide a road map for future precision measurements aimed at detecting sterile neutrino interactions,” said Kim. “We now plan to continue the search for sterile sub-eV-scale neutrinos. In addition, we recently reported the results of a search for sterile eV-scale neutrinos that showed an interesting indication of mixing with sterile neutrinos in the neutrino spectra. observed. to continue these efforts using the neutrino complex of the RENO reactor. ”

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