They exist. They don't exist. They exist. They don't exist. It is probably not there with certainty that they really do not exist. We are talking about sterile neutrinos. In 1996 their existence was postulated for the first time to explain a neutrino surplus that had occurred in measurements at the Los Alamos National Laboratory in the USA. Detectors who trace the elementary particles in the area of nuclear power plants have also repeatedly detected less of their anti -particles than theoretically expected. The deviation is known as a reactor-antino-anomaly. The stereo collaboration now confirms the anomaly in an article in the specialist magazine "Nature", but reports that this discrepancy cannot be explained by the existence of a sterile neutrino.
Neutrinos are among the most common particles in the universe – and also among the most mysterious. They have mass, but how much exactly, you do not know. So far, only one upper limit could be set. They can be divided into three families or "flavors": electron, tau and muon neutrinos. But there could also be more. Because they interact so little with matter, we do not feel that they are now – at this second – flying through us billions of times. Therefore, they cannot be detected directly, but only by other particles that are produced during their interaction with atomic nuclei.
In addition, they continuously convert into one another: physicists say they oscillate.
The effect arises because each neutrino is a mixture of the three neutrinotypes - interference between them ensures that a different neutrinotype dominates the mixture at different times.
Today, neutrino-oscillation experiments can "disappear" and "appear" to measure neutrinos very precisely.
Irregularities that have not been predicted by the theory, so-called anomalies, could indicate that a fourth, exotic neutrino is hidden in the neutrino mix.
This would be sterile, that is, it interacts exclusively through gravity with its surroundings and otherwise does not feel any strength.
The stereo experiment, short for »Sterile Reactor Neutrino Oscillations«, was installed in 2016 at the research reactor of the Laue-Langevin (Ill) institute in Grenoble, French and measures the Antineutrino energy spectrum associated with the division of uranium-235, In short distances with high precision. Because in nuclear rescue reactions, electron anti-tinos are generated in large quantities. In order to be able to use the electron antinutrinos generated in the reactors to examine neutrino oscillation, you have to know the flow of the emitted antineutrinos and the number of antineutrinos generated in certain energies (the energy spectrum). In 2011, a research team from the French research center for nuclear energy found that the average anti -neutrino flow, which can be detected in these experiments, is about 6 percent below the predicted value.
If these electron antineutrinos were to transform into sterile antineutrinos, one would have to expect that the antineutrino flux deficit of 2011 would be confirmed. And because the particles gradually transform into the other varieties on their way, the antineutrino energy spectrum should vary with the distance from the reactor. Although the flow measured by the authors was actually 5.5 percent lower than the model prediction, the expected oscillation pattern did not show up in their results.
"The measured antineutrino energy spectrum indicates that the anomalies can be attributed to distortions in the nuclear reaction data used for the predictions," says the research team. The stereo experiment has thus confirmed the already known antineutrino anomaly, but the cause of this remains a mystery. "The impressive precision of the experiment has ensured that the sterile neutrino is no longer an explanation," writes the physicist Jun Cao from the Chinese Academy of Sciences, which was not involved in the experiments . "This allows the long -cherished hypothesis to put Ad Acta and open the field for new theories." The search for an explanation continues.
