What is a sterile neutrino

End of sterile neutrinos?

According to current knowledge, around eighty percent of the matter in the cosmos consists of dark matter - an invisible and previously puzzling form of matter. Various hypothetical elementary particles are traded as possible candidates for dark matter. One of these particles, called a sterile neutrino, now appears to be excreting. Because although a team of astronomers looked very closely, it was unable to detect the characteristic radiation that would have to be produced when these elementary particles decay in our galaxy. So the dark matter apparently does not consist of sterile neutrinos, according to the team in the journal "Science".

In the past few years, several research groups had detected a signal in distant galaxies that could have come from sterile neutrinos. This is because the observed X-rays have an energy of 3.5 kiloelectron volts and may have arisen from the decay of sterile neutrinos with an energy of 7 kiloelectron volts. This means that these elementary particles lie in an energy range that qualifies them as possible candidates for dark matter. If the previously unknown form of matter were actually composed of sterile neutrinos, the dark matter in the Milky Way system would also have to generate the characteristic X-rays.

Christopher Dessert from the University of Michigan and his colleagues went to search for this radiation with the European X-ray satellite XMM-Newton. Not an easy task as the expected signal is weak and many other cosmic X-ray sources interfere with the observations. The researchers therefore looked far away from all known X-ray sources in regions - and did so for an extremely long period of time: the total exposure time was a whole year. With these measures, it should have been possible to detect the radiation from sterile neutrinos even if it were ten times weaker than assumed. But the astronomers received nothing.

For the X-rays measured in other galaxies, there must be another explanation than the decay of sterile neutrinos, conclude the scientists around Dessert. The existence of these particles is predicted by some theories with which physicists try to unite gravity with the other natural forces in a unified model. But sterile neutrinos are difficult to detect because they hardly interact with ordinary matter.