What happens when two dark matter particles collide? We don’t know the answer to that question because we don’t know what dark matter is. A whole host of possible particles could constitute dark matter, and some of them allow dark matter to “self-interact.” Here, when two dark matter particles collide, they would decay into other particles that we could potentially observe. This should happen often in regions of the Universe densely populated with dark matter, and it’s possible that some of the resulting particles are bombarding us all the time—we just don’t know their origin.
Our atmosphere is constantly bombarded by particles from space of varying kinds, collectively known as cosmic rays. Cosmic rays come from sources such as supernovae and active galactic nuclei (exceptionally bright cores of galaxies). Could some of them also come from collisions of dark matter particles? Sorting that one out has been challenging.
The Alpha Magnetic Spectrometer (AMS-02) aboard the International Space Station has recently provided new data on the flux of antiprotons reaching the Earth’s atmosphere. (antiprotons are the antiparticles of protons). Because of the unprecedented precision of that data, two new teams have separately published papers, without knowledge of each other, using the new data to make an argument that some of the antiprotons are being produced in dark matter particle collisions.
This is not the first time research has attempted to find evidence for dark matter among cosmic rays. But thanks to the precision of AMS-02, this is the strongest such case yet. If borne out by future research, this could mark the beginning of the end of our quest to identify dark matter particles.
The challenge faced by both teams involves correctly modeling the background created by all the other sources of cosmic rays. If they underestimate that flux, for example, then they’d overestimate the number of particles coming from dark matter collisions.
One of the studies was lead by Alessandro Cuoco of RWTH Aachen University, Germany. His researchers ran computer simulations for two scenarios—one with and one without the mysterious dark stuff. They concluded that the scenario with dark matter was the best fit to the AMS-02 data. Specifically, they concluded that it’s a dark matter particle with a mass near 80 GeV, or roughly 143 thousand-trillion-trillionths of a kilogram (1.43*10-25 kg). That’s about 85 times the mass of a proton or antiproton.
The other team, lead by Ming-Yang Cui of the Chinese Academy of Sciences, arrived at similar results. Ming-Yang and his colleagues made use of prior data on the boron-to-carbon ratio among the cosmic rays. Since boron radioactively decays into carbon as it travels through space to reach us, the ratio between these elements can give us an estimate of how long the rays have been traveling and, therefore, how far away their source is. The farther the source, the less boron will remain.
They incorporated this data into a Bayesian analysis to determine the likelihood of antiprotons originating in dark matter collisions. They came to the same conclusion as the European paper: there is a dark matter “signal,” and some of the antiprotons probably originate in dark matter collisions. They find a mass for the dark matter particle that’s somewhere between 20 and 80 GeV, broadly in agreement with the other work.
This mass range means the particles would be WIMPs, or Weakly Interacting Massive Particles, the currently favored version of dark matter. “The dark matter model that can explain the antiproton data is a kind of WIMPs,” Qiang Yuan of the Key Laboratory of Dark Matter and Space Astronomy, Chinese Academy of Sciences, told Ars. “However, at the current stage, we cannot say too much about its identity. It might be something new to us, [meaning] some kind of particles beyond the known particle family. But we do not know right now.”
For some time, researchers have expected that dark matter is something outside the Standard Model, as none of the known particles quite fit all the requirements to be dark matter.
Another important characteristic of dark matter is its cross-section, or the likelihood of two dark matter particles colliding. That’s determined by a lot of factors, such as how closely packed dark matter particles are within a galaxy, as well as the speed they’re traveling. So learning the cross-section would help researchers model the effect that dark matter has on galaxies and on the Universe on larger scales. Cuoco et al. estimate a cross section of about 3×10-26 cubic centimeters per second, while Cui et al. estimate a range between .2×10-26 and 5×10-26. Again, the two papers are in agreement.
Ars previously reported evidence that gamma rays coming from certain dwarf galaxies could be the result of dark matter particles colliding; other papers have hypothesized that we might be seeing gamma rays originating from dark matter in other places as well. Both new papers are in agreement with this earlier research and make similar predictions about the identity of dark matter. Not only that, but they significantly improve on the limits placed on dark matter particles’ characteristics.
“The very accurate recent measurement of the antiproton flux by the AMS-02 experiment allows us to achieve unprecedented sensitivity to possible [dark matter] signals, [about four times] stronger than the limits from gamma-ray observations of dwarf galaxies,” Cuoco et al. write in their paper.
While these new papers strengthen the conclusion that dark matter exists and is self-interacting, neither is the end of the story. Both make certain simplifying assumptions in their modeling of the cosmic-ray sources that may not be accurate. “We should also keep in mind that the dark matter explanation of the data is just a hypothesis,” Yuan told Ars. “More works are needed to confirm or falsify it.”