Unprecedented high-energy particle measurements collected with the Alpha Magnetic Spectrometer (AMS) mounted on the International Space Station (ISS) add new weight to the theory that there is invisible “dark matter” in the Universe that affects the way visible matter behaves.
Long-duration measurements enabled by the ISS should nail down the answer in the next year or two, scientists say.
The international AMS Collaboration published its first scientific paper in Physical Review Letters Wednesday, reporting that analysis of 25 billion primary cosmic ray events between May 19, 2011, and Dec. 10, 2012, found 6.8 million were identified “unambiguously” as electrons and their antimatter counterparts, positrons.
Those are believed to be the byproducts generated when two dark matter particles collide in space and are annihilated. But they also can be produced by pulsars, which means making the distinction definitively will require more detected “particle events” for reliable statistics.
“Our evidence supports the existence of dark matter, but cannot rule out the origin comes from pulsars,” said Samuel Ting, a Nobel laureate from the Massachusetts Institute of Technology who is the principal investigator on AMS. “But I’m confident, with enough time because we will be on the space station for the lifetime of the space station, we should be able to solve this problem.”
The evidence to date is tantalizing, given accuracy to within 1% achieved by the 15,251-lb. AMS, which carries eight primary instruments and downlinks to Earth through 300,000 data channels. So far the signals suggestive of dark matter appear to come from all directions equally, while pulsars in theory would be clustered.
“The positron-to-electron ratio shows no anisotropy, indicating the energetic positrons are not coming from a preferred direction in space,” the AMS Collaboration states.
However, to date only about 10% of the expected data in the 0.5-350 GeV range have been reported, and more data points are needed to refine the statistics.
“The exact shape of the spectrum …, extended to higher energies, will ultimately determine whether this spectrum originates from the collision of dark matter particles or from pulsars in the galaxy,” the collaboration stated. “The high level of accuracy of this data shows that AMS will soon resolve this issue.”
Ting said a definitive answer may be available in “a year or two.” He was speaking from the European Organization for Nuclear Research (CERN) near Geneva, where he and his colleagues presented their first findings.
The AMS can be thought of as a spaceborne version of the Large Hadron Collider, a particle accelerator at CERN. But while particle collisions in ground-based accelerators have yet to produce definite evidence of dark matter, the AMS is open to the sky from its perch on the starboard truss of the ISS, and its detectors receive a flood of particles around the clock for analysis on the ground.
The AMS is sponsored by the U.S. Department of Energy, withhandling the ISS deployment and operation. Among nations providing instruments and other hardware are Italy, Germany, Spain, France, Taiwan, China and Switzerland. It was delivered to the station by space shuttle Endeavour during STS-134, which launched May 16, 2011.
“Space station is a really nice platform, well suited for this instrument,” said William Gerstenmaier, associateadministrator for human exploration and operations. “It allows for a long, long duration of observation time, which is important to this instrument. We can essentially use decades of observation time, which will help produce statistics in certain low-frequency events.”