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Scientists Find Signal in Space That Could Be Dark Matter

Mar 26, 2010
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A search for the collision of matter and antimatter in our galaxy has turned up a signal that might be the best direct evidence of dark matter to date.

These are the first findings to come from the Alpha Magnetic Spectrometer experiment, which was installed on the International Space Station in 2011 during the second-to-last shuttle mission ever. AMS has since recorded more than 25 billion particle events streaming in from all over the universe, including 400,000 positrons, the antimatter doppelganger of the electron. This is the largest collection of antimatter ever seen in space.

The data show “unexpected new phenomena,” though whether these have their origin in dark matter or some more mundane explanation is not yet known, said Nobel-prize-winning physicist Samuel Ting of MIT during a talk at CERN today. These findings will be published Friday in Physical Review Letters.

The physics community has been eagerly awaiting the results, especially after Ting teased that he had “big news” coming from his team during a conference in February.

Dark matter is an as yet unknown material that pervades the universe. In total, it outweighs ordinary matter, like protons and electrons, by a ratio of six to one. Astronomers know that dark matter exists even though they haven’t identified it because they see its gravitational pull in every galaxy and cluster in the cosmos. Exactly what sort of particles make up dark matter is one of the biggest mysteries in modern physics.

Though they rarely interact, scientists think dark matter particles should occasionally hit one another, annihilating into positrons and electrons, which AMS detects. A dark matter signal would see the ratio of positrons relative to electrons rise at higher energies and then sharply drop off. The problem is that the universe is complex and full of other sources that could produce almost exactly the same signal. Pulsars — neutron stars that shoot out a beam of electromagnetic energy — should act as gigantic particle accelerators, also creating a positron signal that rises in this same way, but with a gradual falloff.

The AMS experiment has observed that the positron excess, whatever its source, seems to come uniformly from all parts of the sky. This indicates that the signal has one particular source and is not many different phenomena. And AMS sees a rise in its data but the drop-off, if it exists, seems to happen at higher energies than the experiment has so far searched, above 350 GeV (roughly 350 times the mass of a proton). After 250 GeV, AMS sees the spectrum appearing to plateau but needs more data to definitively say anything about dark matter.

The results mesh well with those of previous experiments that searched for electron-positron annihilation events in the galaxy. In 2008, the PAMELA satellite noticed an excess of energy that could be a signal of dark matter, a finding later confirmed by NASA’s Fermi Gamma-Ray Space Telescope. Unfortunately, neither test could determine whether the excess came from dark matter or pulsars. AMS is able to probe at higher energies with much better precision than any previous experiments, so physicists were hopeful that it could discover something more definitive.

“The new AMS results agree beautifully with what PAMELA observed, thus reinforcing the trend that the positron fraction rises with energy, but this time, with unprecedented statistics and background controls,” wrote physicist Stephane Coutu from Pennsylvania State University in an accompanying viewpoint article in the same issue of Physical Review Letters. But it may take a long time to resolve exactly where the signal comes from, he added.

May 14, 2009
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