A $2-billion cosmic ray detector on the International Space Station has… (NASA / ESA )
Let that long-held breath out, folks. The Alpha Magnetic Spectrometer has picked up a lot of mysterious antimatter in low Earth orbit – but that doesn’t necessarily mean it’s a sign of dark matter.
In fact, even with the 400,000 positrons picked up by the cosmic-ray experiment -- the largest number of such particles ever analyzed in space -- it's unclear whether those positrons result from decaying dark matter, or simply from pulsars sending particles into the universe.
"What you have probably seen from the data is a significant new measurement," said Brown University physicist Richard Gaitskell, a lead scientist on a different dark matter detector called the Large Underground Xenon experiment. "Unfortunately, the data wasn’t that conclusive."
All the stuff that we can see – stars, galaxies, everything on Earth – makes up just 4.9% of the universe. More than five times as much, 26.8%, is made of up dark matter. It doesn’t interact with normal matter and can’t be seen or felt, but scientists can see its powerful gravitational effects on galactic scales.
Scientists have been trying to detect dark matter in a number of ways – in high-energy particle accelerators, in underground detectors and in space – but have thus far failed to come up with conclusive signs of it.
The ambitious $1.6-billion Alpha Magnetic Spectrometer is one space-borne experiment, and it’s roughly 10 times more sensitive than its predecessors. The detector, which was ferried on the space shuttle Endeavour to the International Space Station in 2011, has picked up billions of particle events since then, said lead scientist Samuel Ting, a Nobel laureate and MIT particle physicist.
Ting’s findings have been hotly anticipated ever since he left scientists hanging at a science meeting in February, promising that the results would be ready for release in the coming weeks. The physicist discussed the findings at a NASA conference Wednesday.
The Alpha Magnetic Spectrometer works essentially by tracking ratios of electrons and their positively charged antimatter counterparts, positrons. Scientists think that an excess of positrons could be a sign of two dark matter particles crashing into each other and decaying into a shower of particles.
The more energetic the positrons, the more massive their "parent" dark matter particles must be. They look at the positrons in higher and higher energy brackets, waiting for the populations to suddenly drop off. That drop-off point would mark the maximum possible mass of the dark matter particles.
“That would be the smoking gun for a dark matter annihilation signal,” said Gaitskell. “Unfortunately, with this current result there is no indication of that.”
Other teams have looked for the long-theorized drop-off. Results published in 2009 showed the Russian-European PAMELA spacecraft found no drop-off through 100 gigaelectron volts. Further work with the Fermi Gamma-ray Space Telescope in 2011 saw no steep decline even up to 200 gigaelectron volts. The results baffled scientists.
Ting’s results, Gaitskell said, reveal essentially the same thing: that they found no clear drop-off in the positron-to-electron ratio even through energy ranges of 350 gigaelectron volts. This could mean that dark matter particles are simply bigger than the researchers thought, and they’ll have to look at even higher energy brackets to find out.
It could also mean that positrons caused by pulsars – rotating neutron stars that hurl radiation into space – are drowning out any dark matter signal, if such a signal exists.
But the Alpha Magnetic Spectrometer is in only its second year of operation, and is just getting started, Ting noted. With a little – or maybe a lot – more data, “we should be able to solve this problem,” he said.
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