Earth-bound positrons could come from dark matter: study

Source: Xinhua| 2017-11-18 03:49:45|Editor: Mu Xuequan
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WASHINGTON, Nov. 17 (Xinhua) -- Using new data from the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory in Mexico, researchers have ruled out two nearby pulsars as the sources of a mysterious excess of anti-matter particles coming to the Earth.

That raises the possibility of an alternative explanation that sees dark matter, a hypothetical type of matter that we have yet to observe, as the origin of positrons, or anti-electrons, according to the study published in the Friday issue of the journal Science.

"Our measurement doesn't decide the question in favor of dark matter," Jordan Goodman, professor of physics at the University of Maryland and the lead investigator and U.S. spokesperson for the HAWC collaboration, said in a statement.

"But any new theory that attempts to explain the excess using pulsars will need to account for what we've found," Goodman said.

The mystery started back in 2008, when a space-borne detector measured an unexpectedly high number of positrons -- the anti-matter cousins of electrons -- in orbit.

Ever since, two competing theories have been put forth to explain its origin.

Some suggested the extra particles might come from nearby collapsed stars called pulsars, which spin around several times a second and throw off electrons, positrons and other matter with violent force.

Others speculated that the extra positrons might come from processes involving dark matter -- the invisible but pervasive substance seen so far only through its gravitational pull.

In the new study, researchers used HAWC to measure gamma rays made by positrons streaming from two of the nearby pulsars previously identified as possible sources of the excess, which helped them calculate how far positrons generated by the stellar neighbors could diffuse through space.

"We see the positrons are not moving fast enough to make it to Earth," said Hao Zhou, a scientist at Los Alamos National Laboratory, who was responsible for developing the particle diffusion model and calculating the gamma-ray emission morphology around the two pulsars in HAWC data.

The results indicated that the two pulsars are very unlikely to be the origin of the positron excess, despite being the right age and the right distance from Earth to contribute.

"After excluding two of the main source candidates, we are closer to understanding the origin of the positron excess," said lead author Francisco Salesa Greus, a scientist at the Polish Academy of Sciences Institute of Nuclear Physics in Krakow, Poland.

"Dark matter is everywhere, but we don't know what it is," Dingus said. "If one could prove that these positrons are from dark matter, then that would be a major step to figuring out the nature of dark matter."