Missing Dark Matter Decay Rules Out Sterile Neutrinos

Sterile neutrinos are the leading candidate for dark matter, these particles should decay and leave a signature emission which new research has failed to detect.

Decaying dark matter should produce a bright and spherical halo of X-ray emission around the centre of the Milky Way that could be detectable when looking in otherwise-blank regions of the galaxy. (Artistic rendering by Christopher Dessert, Nicholas L. Rodd, Benjamin R. Safdi, Zosia Rostomian (Berkeley Lab), based on data from the Fermi Large Area Telescope.)

As physicists continue to investigate dark matter — the substance that comprises over 80% of the matter in the known Universe, whose gravitational influence is responsible for holding galaxies themselves together — new research has ruled a mysterious x-ray emission as evidence of the decay of dark matter particles. In the process possibly ruling out the most favoured candidates for dark matter particles — sterile neutrinos.

Researchers including Benjamin Safdi, an assistant professor of physics at the University of Michigan (UM), Christopher Dessert a UM Physics PhD Student, and Nicholas Rodd, a physicist with the Berkley Lab theory group and the Berkley Center for Theoretical Physics, examined an unexplained X-ray signal originating from nearby galaxies. The electromagnetic signature had been speculated to be the result of the decay of particles that comprise dark matter, a theory the team say their paper, published in the journal Science, rules out.

“The leading hypothesis is that dark matter is made up of fundamental particles, just like the matter we see around us,” Rodd explains. He explains that as we know fundamental particles can decay — for example, a neutron taken out of a nucleus will decay after about 14 minutes into a

proton, electron, and neutrino — dark matter may do something similar.

“If it decays into photons, we can then search for dark matter lighting up the night sky.”

Looking for dark matter closer to home

The team focused on the idea that dark matter particles could be a more massive cousin of the neutrino — the sterile neutrino — which is unstable and decays to produce a 3.5 keV X-ray emission.

This 3.5 keV emission line has thus far been seen primarily in galaxy clusters — distant objects containing an enormous amount of dark matter — but they are millions of light-years away. “In essence, we searched for dark matter closer to home,” Rodd tells me. “We considered a source containing far less dark matter, but which is much closer by our own Milky Way.”

As the Milky Way exists in a giant cloud of dark matter, he says, meaning that wherever we look in the sky, we look through our local dark matter halo, and if dark matter decays, then some of that should be lighting up. “When you crunch the numbers, it turns out the Milky Way is actually a brighter source than these distant clusters, and we used this observation to put the 3.5 keV to the test.”

The team used 20 years worth of data to search for a link between a 3.5 keV emission and sterile neutrino decay, thus con=firming a key candidate for dark matter particles. (ESA)

The team performed a meta-analysis of raw data taken by the XMM-Newton space X-ray telescope, of objects in the Milky Way over a period of 20 years. As we know dark matter collects around galaxies, when previous analyses such as those performed by XMM-Newton, looked at nearby galaxies and galaxy clusters, each of those images should have captured decays from the Milky Way’s dark matter halo.

“To be honest the results were a bit disappointing!” says Rodd candidly. “My initial hope was that we would see a very bright signal, demonstrating that the 3.5 keV line is from dark matter, allowing us to finally unravel that mystery. But of course, it’s always important to know what is really going on, and in this case that appears to be that the 3.5 keV line is not due to dark matter.”

In addition to this, Rodd says that as the team used those images to look at the “darkest” part of the Milky Way, the sensitivity of their investigation technique was so strong that it is likely other dark matter candidates can be excluded as producing the line. “Nevertheless, our main focus was testing the 3.5 keV anomaly, we didn’t search more broadly,” he adds. “It’s possible sterile neutrino dark matter is decaying throughout the universe, and maybe that an extension of our analysis will reveal that.”

Finding the source of the 3.5 keV and future dark matter investigations

So, if the decay of sterile neutrinos comprising dark matter isn’t the source of this mysterious 3.5 keV emission, what is? “This is a very open and interesting question,” Rodd admits. “The line has been primarily observed in galaxy clusters, monstrous objects that are made of a collection of individual galaxies. The clusters are full of conventional matter, and so the discussion has been whether the physics of what we know could explain the line.”

Rodd believes that one possibility is that it could be an emission line associated with potassium, although he adds that there are a number of arguments suggesting this is unlikely. “A more the promising scenario is called ‘charge exchange,’ where lines can be produced in collisions between the cold gas and hot plasma that coexist in these clusters.”

And as for the search for dark matter and the determination of what it actually is, Rodd is clear, the search goes on, albeit with a new tool provided by their study. Further research promises to build upon the team’s approach of looking for the dark matter within our own galaxy, with this significantly expanding our understanding of whether or not dark matter can decay.

“Dark matter makes up 84% of the matter in the cosmos, and yet we have no idea what it is,” Rodd says. “For a field built on understanding the Universe, one could see this is an embarrassing situation. But, I don’t see it that way, for me, the most exciting discoveries relating to dark matter are yet to come.”

Rodd concludes: “I am excited about how else we can use our new strategy to continue the search for dark matter. Our work is an early application of this idea, my hope is that as we extend it we might finally see dark matter lighting up the sky.”