First evidence of 'Dark Matter' found

A massive particle detector mounted on the International Space Station may have detected elusive dark matter at last, scientists announced today (April 3).

The detector, the Alpha Magnetic Spectrometer (AMS), measures cosmic-ray particles in space. After detecting billions of these particles over a year and a half, the experiment recorded a signal that may be the result of dark matter, the hidden substance that makes up more than 80 percent of all matter in the universe.

AMS found about 400,000 positrons, the antimatter partner particles of electrons. The energies of these positrons suggest they might have been created when particles of dark matter collided and destroyed each other.

NASA will hold a press conference detailing the AMS science results at 1:30 p.m. EDT (1830 GMT) today. You can watch the AMS science results live on SPACE.com, via NASA TV.

Elusive matter

Dark matter emits no light and can't be detected with telescopes, and it seems to dwarf the ordinary matter in the universe. [Gallery: Dark Matter Throughout the Universe]

Physicists have suggested that dark matter is made of WIMPs, or weakly interacting massive particles, which almost never interact with normal matter particles. WIMPs are thought to be their own antimatter partner particles, so when two WIMPS meet, they would annihilate each other, as matter and antimatter partners destroy each other on contact. The result of such a violent collision between WIMPs would be a positron and an electron, said study co-author Roald Sagdeev, a physicist at the University of Maryland.

The characteristics of the positrons detected by AMS match predictions for the products of dark-matter collisions. For example, based on an overabundance of positrons measured by a satellite-based detector called the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), scientists expected that positrons from dark matter would be found at energy levels higher than 10 gigaelectron volts (GeV), said study co-author Veronica Bindi, a physicist at the University of Hawaii.

And the positrons found by AMS increase in abundance from 10 GeV to 250 GeV, with the slope of the increase reducing by an order of magnitude over the range from 20 GeV to 250 GeV — just what scientists expect from positrons created by dark-matter annihilations.

Furthermore, the positrons appear to come from all directions in space, and not a single source in the sky. This finding is also what researchers expected from the products of dark matter, which is thought to permeate the universe.

Intriguing signal

The $2 billion AMS instrument was delivered to the International Space Station in May 2011 by the space shuttle Endeavour, and installed by spacewalking astronauts on the orbiting laboratory's exterior backbone.

In just its first year and half, the AMS detector has measured 6.8 million positrons and electrons. As the instrument continues to collect data, scientists will be better able to tell whether the positron signal really does come from dark matter.

If the positrons aren't created by annihilating WIMPs, there are other possible explanations. For example, spinning stars called pulsars spread out around the plane of our Milky Way galaxy.

But even with more AMS data, "we will still not be completely able to figure out if it's really a dark-matter source or a pulsar," Bindi told SPACE.com. To understand dark matter thoroughly, scientists hope to detect WIMPs directly via underground experiments on Earth, such as the Cryogenic Dark Matter Search and XENON Dark Matter projects.

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BlahISuck said:

With Higgs you can rule stuff out, because you predict its mass and spin. I remember learning about radioactive decay wayy back in the day, which can produce positrons as well. I suppose the energies of the positron match predicted values and that's a plus. But unlike the Higgs, where we isolated a particle that said here it is, right now we have graffiti that says "dark matter was here". We can't see dark matter, that's for sure, but my instincts tell me that one piece of indirect evidence isn't enough, as we haven't ruled out other possible sources. I'll abstain from celebrating until there's some excluding evidence. More testing!

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The Standard Model has been the mainstay of physics for decades, and it has been quite successful — it predicted the existence of the Higgs boson, for example, evidence for which was finally found last year by teams of physicists working with the Large Hadron Collider (LHC). (It's still not certain that the discovered Higgs is the same kind that one might expect from the Standard Model, though.)

There are still some problems, though. For example, astrophysicists know that a large chunk of the universe is made up of something called dark matter, an invisible substance that only interacts with other matter via gravity. The Standard Model has trouble accounting for it, since making dark matter out of particles that we know about wouldn't get the same thing. [8 Baffling Astronomy Mysteries]

Another unanswered mystery is called the hierarchy problem. Gravity is 10^32 times weaker than the weak nuclear force, which governs phenomena such as radioactivity. It still isn't clear why, and supersymmetry theories might be an answer to that problem.

Supersymmetry (or SUSY) is a theory that says the particles that make up matter, called fermions, and those that carry forces, called bosons, all have "superpartners." The superpartners would all have the same quantum properties except one, which describes their spins. Fermions — electrons, for instance — have half-integer spins whereas bosons have so-called integer spins.

But so far nobody has found the supersymmetric partners to known elementary particles — at least not yet. Lee said the LHC is just now approaching energies where some of those particles might be found.

In that vein, Santiago Folgueras of the University of Oviedo in Spain said the recent work has given scientists a better idea of where to look for SUSY particles, but it is hard to do because there aren't many "events," or particle decays, that yield data. Most of the progress has been in setting lower limits on the energies at which supersymmetric partners are likely to be observed.

That doesn't mean there aren't skeptics of theories such as supersymmetry. Mikhail Shifman, a professor at the University of Minnesota, wrote an essay on ArXiv, a website where physicists post their research, in October 2012, saying there's a good chance supersymmetry theories might be a dead end. He noted that the discovery of the Higgs boson was a solid confirmation of the Standard Model (at least so far), but none of the supersymmetric partners of elementary particles has been found yet.

Matt Strassler, a former professor of physics at Rutgers, said Shifman was a bit premature. The LHC work has ruled out many kinds of SUSY theory, though no broad class of theories has been completely excluded yet.

Lee said a lot more work is still required to narrow down the possibilities. "It's like you lost your wedding ring on a beach and have to find it. It's a big area to look in."

That's why it's important for scientists from many institutions to be doing this kind of work, he added. "If you have your friends help you look you have a much better chance of finding it."

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