Science

Another twist in the history of dark matter

US researchers say we can rule out that dark matter is responsible for the mysterious electromagnetic signals that have previously been observed by nearby galaxies.

There was high hopes that these signals would provide hard evidence for dark matter identification, but new work by a team from the University of Michigan (UM), the Lawrence Berkeley National Laboratory, and the University of California, Berkeley, suggests otherwise.

Dark matter cannot be observed directly because it does not absorb, reflect or emit light, but we do know that it exists due to the effect on other matter. Physicists have suggested that dark matter is a closely related neutrino cousin called a sterile neutrino.

Neutrinos – subatomic particles without charge that rarely interact with matter – are released during nuclear reactions within the sun. They have a tiny amount of mass, but this is not explained by the standard model of particle physics.

Physicists suggest that the sterile neutrino, a hypothetical particle, could explain this mass and could also be dark matter.

Researchers should be able to recognize the sterile neutrino because it is unstable and breaks down into ordinary neutrinos and electromagnetic radiation, says Ben Safdi of UM, co-author of a paper that is now available on the preprint server arXiv.

To detect dark matter, physicists scan galaxies to look for this electromagnetic radiation in the form of X-ray emissions. In 2014 researchers discovered Excessive X-ray emission from nearby galaxies and clusters of galaxies, which, according to Safdi, matches that which would result from the decay of sterile dark neutrino matter.

But he and his colleagues say that their meta-analysis of the raw data from the XMM Newton The space X-ray telescope of objects in the Milky Way over 20 years found no evidence that the sterile neutrino encompasses dark matter.

“This 2014 paper and follow-up work confirmed that the signal aroused great interest in astrophysics and particle physics, because for the first time it was known exactly what dark matter is at the microscopic level, “says Safdi.

“Our finding does not mean that dark matter is not a sterile neutrino, but it does mean that, contrary to the 2014 claim, there is as yet no experimental evidence to suggest its existence.”

Dark matter hunters look for weak electromagnetic radiation in the form of X-ray signals. In the 2014 work, the X-ray emission was referred to as a 3.5 keV line (keV for kiloelectron volts) because the signal appeared on X-ray detectors.

In the new work, the research team examined data from the “darkest” part of the Milky Way, which according to Safdi has significantly improved the sensitivity of previous analyzes.

UM’s co-author, Christopher Dessert, says that galaxy clusters where the 3.5 keV line was observed have large background signals that act as noise during observations and can make it difficult to locate certain signals, that can be associated with dark matter.

“The reason we look through the halo of our Milky Way’s galactic dark matter is because the background is much lower,” said Dessert.

The researchers say that if sterile neutrinos were dark matter and their decay would result in an emission of the 3.5 keV line, they should have observed this line in their analysis. However, they found no evidence of sterile dark neutrino matter.

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