The final results of the STEREO experiment reject the sterile neutrino hypothesis

The final results of the STEREO experiment reject the sterile neutrino hypothesis

Create a stereo experience. credit: nature (2023). DOI: 10.1038/s41586-022-05568-2

After several years of operation, the STEREO collaboration has published the final results of their antineutrino studies. With their data, the researchers ruled out hints of sterile neutrinos, an extra neutrino state expected in many theories. The result, which appears in the January 11th issue of naturehas important implications for many areas of fundamental physics.

In modern particle physics they are all known Elementary particles And their interactions are described in the so-called Standard Model of particle physics. The model includes neutrinos, which are particles invented by Wolfgang Pauli in 1930 in order to comply with the universal laws of conservation of energy. Neutrinos are very light, electrically neutral, and interact only via the electroweak force. Therefore, it is very difficult to detect them. Direct experimental detection did not occur until 1956.

Today, three different types of neutrinos are known. These neutrinos can change their identity between these different states because of their very small, but non-zero mass. The so-called neutrino oscillations were created about two decades ago.

In 2011, increased resolution created an anomaly between the observed and predicted antineutrino flux emitted from nuclear reactors. This triggered the hypothesis of a complementary neutrino state that is sterile, that is, does not interact via the weak interaction. It can explain this particle as well physical phenomena which is still not fully understood, such as dark matter.

To unequivocally test this hypothesis of sterile neutrinos and characterize them, the STEREO experiment was designed and commissioned in 2017 at the high-flow nuclear research reactor at ILL Grenoble. A detector consisting of six identical elements was placed just 10 meters from the reactor core.

The project made use of experience gathered over several generations of reactor neutrino experiments. protected from external environmentthe detector cells are ideally positioned to search with unprecedented precision for the signature of sterilized neutrinos: position-dependent distortions in their energy distribution should appear at a short distance from the reactor.

Now, the STEREO collaboration, consisting of researchers from the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg, Germany and the French institutes CEA Saclay, CNRS, Universities of Grenoble Alpes and Savoie Mont Blanc as well as the Institut Laue-Langevin (ILL), have published their latest findings. That compiled the full data set: Physicists have confirmed that there is an anomaly in the neutrino flux emitted by nuclear reactors, but that sterile neutrinos are not the cause.

“We were able to detect a total of more than 100,000 neutrinos in the years 2017 to 2020, but we were unable to identify any trace of possible sterile neutrinos within these measurements,” explains Christian Buck, one of the experiment’s principal investigators from MPIK. . “The observed anomalies most likely result from underestimated uncertainties in the nuclear data from the radioactive decay used in flux prediction rather than from the neutrino experiments themselves.”

While this result strongly rejects the sterile neutrino hypothesis, it serves as additional support for the Standard Model and its neutrino content.

Besides searching for sterile neutrinos, the STEREO experiment also provides the most accurate measurement to date of the antineutrino spectrum of uranium-235 fission. It is intended to be used as a reference spectrum for future high-resolution reactor experiments, such as determining the neutrino mass hierarchy or lower-energy tests of the Standard Model. In addition, accurate measurements of this type may help a better understanding of phenomena that occur during a reactor shutdown, for example.

The researchers at MPIK made significant contributions, both in building the STEREO detector and in analyzing the data. For example, very special liquids in the reagent have been developed, produced and labeled at MPIK. In particular, a gadolinium-laden liquid scintillator, which forms the detector core, originates from the MPIK.

Another crucial contribution is the very special packing systems and light sensors for measuring the light signals after the neutrino interaction in the detector. In the area of ​​analysis, the MPIK group has been active in energy reconstruction, efficiency determination, and analysis coordination.

more information:
David Lhuillier, STEREO neutrino spectrometer of fission 235U rejects the sterile neutrino hypothesis, nature (2023). DOI: 10.1038/s41586-022-05568-2.

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