A space experiment may have identified a new particle that is the building block of dark matter, the mysterious stuff said to pervade a quarter of the universe that neither emits nor absorbs light.
The results are based on a small amount of data and are far from definitive, scientists said Wednesday. Yet, they provide a provocative hint that the puzzle of dark matter—a cosmic prize as eagerly sought as the now-discovered Higgs boson—may also be on its way to being solved.
The results are the first obtained by a $2 billion particle detector, known as Alpha Magnetic Spectrometer, or AMS, that is mounted on the exterior of the international space station. It collects and identifies charged cosmic rays arriving from the far reaches of space.
The experiment is sponsored by the U.S. Department of Energy. It is led by Nobel laureate Samuel Ting of the Massachusetts Institute of Technology and involves hundreds of scientists from all over the world. The latest data will be published in the journal Physical Review Letters.
The AMS findings are consistent with particles that could be formed "from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations," according to a statement by the European particle-physics laboratory, CERN, in Geneva, which assembled the particle detector.
Figuring out what makes up dark matter is a big prize because it is the key to understanding the shape, size and even the fate of the universe.
Knowing how much dark matter there is will tell us whether the universe will keep expanding; expand to a point and then collapse; or get bigger and bigger and then stop. It also can help predict how Earth's neighborhood, the Milky Way galaxy, formed and how it might evolve.
Dark matter is invisible, yet its presence is felt by the immense gravitational tug it exerts on stars, galaxies and other cosmic bodies. What could this mysterious substance be made of? One of the leading candidates is a WIMP, or weakly interacting massive particle.
WIMPs are elusive. They rarely interact with normal matter such as atoms; indeed, billions of WIMPs may be darting right through the Earth every second without hitting anything.
About 25% of the universe is believed to be dark matter, about 70% is the little-understood dark energy, and about 5% is ordinary matter made of atoms. Scientists have been looking for WIMPs in deep mines; in particle smash-ups in colliders; and, now, with detectors in space.
"This is the decade of the WIMP," said Michael Turner, a cosmologist at the University of Chicago. "All of these experiments are zeroing in on the outrageous idea that most of the matter in the universe is" made up of WIMPs, a new form of matter.
In 1990, Dr. Turner and a colleague suggested a way in which WIMPs might be discovered in galaxies, which provided the theoretical underpinning for the AMS experiment. The idea is that when WIMPs crash into each other, they annihilate and produce two particles—electrons and positrons—which are much easier to detect than the WIMPs themselves.
The theory predicts two outcomes: that the annihilations should produce a large number of positrons; and that after the excess, there should be a sudden decline in positron production.
In its first 18 months, AMS analyzed 25 billion primary cosmic ray events. It identified more than 400,000 positrons. The positron numbers then start to flatten out—a possible sign that the hoped-for plunge in positron numbers could come next.
So far, though, there isn't enough data to confirm that expected plunge. Physicists also need to ensure that the positrons they are seeing don't emanate from a pulsar, a type of star; that wouldn't be a finding about dark matter.
"What's tantalizing is that the positrons are leveling-off," said Dr. Turner, who wasn't involved in the AMS experiment. "But we're not there yet" because not enough data have been crunched.