Physics + Math

19
Mar

Pursuing Dark Matter, Hundreds of Miles Above Earth

“That’s a cosmic ray.” A few seconds later: “And that’s another cosmic ray.”

Samuel Ting, a Nobel Laureate and professor of physics at MIT, is standing before a monitor in his office above a high-ceilinged control room outside Geneva, Switzerland, calling out cosmic rays striking a detector orbiting hundreds of miles above the Earth. The monitor shows a schematic of the detector picking up the events. Lines draws themselves across the screen, turning green where they hit the instrument.

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The AMS detector aboard the International Space Station

The control room below is dominated by a map of the world that covers an entire wall. Unfurling across it in lazy S-curves are a series of trajectories—the future paths of the International Space Station, the orbital home of the Alpha Magnetic Spectrometer (AMS-02), a 7.5-ton, $2 billion device that’s tasked with looking for dark matter. Computers manned 24 hours a day by researchers monitoring AMS sit in a horseshoe shape before the massive map. Ting, lead researcher on AMS, has shepherded the project from the its conception nearly 20 years ago, and he looks at the detector schematic with a certain pride.

AMS control room
Inside the AMS control room at CERN

Elsewhere in the sprawl of buildings that makes up CERN, physicists tend experiments that happen beneath their feet in the Large Hadron Collider. Here, the AMS team looks up instead, waiting for signs of dark matter via cosmic rays, the high-energy particles that whip through the solar system from regions beyond. Dark matter is one of the items on CERN researchers’ to-find list.

When the LHC comes back online in 2015 after maintenance is completed, physicists will continue searching for signs of dark matter in its more powerful collisions. But for the AMS team, the hunt for dark matter is happening now. As you read this, data from the detector is streaming down from the ISS to computers on Earth, each a tiny pixel in the very large picture that is the universe.

Eighty Years

The first suggestion of dark matter’s existence was in 1933, when Swiss astronomer Fritz Zwicky was studying the Coma galaxy cluster. He calculated the mass of those galaxies in two ways: first, by judging their speed, which is related to the pull of gravity they exert on each other, and then by judging from the light they released. Far from lining up, the two numbers were terrifically different. The gravitational mass was ten times more than the mass implied by the galaxies’ light. The best explanation, the one that drives the search for dark matter today, was that there was a whole lot of matter in those galaxies that telescopes couldn’t detect—matter that didn’t produce radiation or light of any kind.

Cosmologists and astrophysicists have since built simulations of how dark matter might be distributed around the universe, producing beautiful three-dimensional mazes of clumps and strands. And research has established that there is much, much more dark matter—more than five times as much—than the kind of matter that makes up both us and the stars. But what, exactly, it consists of is a puzzle that experimental particle physicists and theorists alike have been spinning stories about for decades.

One leading idea is that dark matter is a particle that almost never interacts with anything else, called a weakly interacting massive particle, or WIMP. That would fit nicely into existing theories about the complement of particles that populate the universe. But first, researchers have to find them.

Three Possibilities

Sitting at a table in the CERN cafeteria, Heidi Sandaker, a professor at University of Bergen in Norway and a physicist on the ATLAS experiment in the Large Hadron Collider, sketches diagrams on a sheet of graph paper. She lays out the three types of experiments you can do that would show signs of dark matter.

For one of them, physicists place detectors at the bottom of a mine, where the earth filters out a lot of noise, to look for signs of an extremely rare interaction between a dark matter particle and a standard matter particle. The results of these types of interactions would be very subtle, perhaps just a few electrons out of place. The second option involves ramming particles together in an accelerator and checking afterward to see whether there’s less energy released than you’d expect, which could be a sign that a dark matter particle was created and went off on its merry way. And the third requires sending a detector up into space, far from the interference of the atmosphere, to see whether certain particles bouncing around out there are more or less numerous than you would expect, which could be the result of dark matter particles colliding with each other. The last, of course, is why AMS is up on the International Space Station.

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The blue hues in this composite image of the "bullet cluster" show where dark matter is distributed. The image was created by gravitational lensing of galaxies in the background.

These different approaches complement each other because the search for dark matter has a lot in common with looking for your missing keys. It’s all about ruling out places they might be—or in this case, what traits dark matter might have. But unlike looking for your keys, coming up empty-handed at the end of one trawl for information can actually be cause for celebration.

Last year, the LUX experiment turned up no sign of dark matter at all, and scientists reported it with a certain sense of glee, according to Dennis Overbye, who wrote about the “failure” wryly in the New York Times. That’s because the LUX experiment, one of the underground variety, would only detect signs of dark matter if the WIMPs were especially light and numerous. That it found nothing means that they must be on the heavier and rarer side, if they exist. (It’s not in the couch, everyone—onward, to the shelf in the bathroom!) Given the wide variety of error margins and detector types, lining up the results from all the different experiments isn’t a simple task. But that’s the idea.

In the years before the maintenance shutdown, experiments at the LHC already ruled out a swath of lighter solutions. But when the collider starts up again in 2015, it will be accelerating particles at higher energy levels than ever before. That means the collisions will be more intense, and in the traces that remain, researchers will be able to look for signs of dark matter candidates in the heavier, yet-to-be-ruled-out range. Physicists also intend to ramp up the number of collisions to many times more than when the LHC was working at lower energy levels, so that means there will be plenty of chances to see something, Sandaker says.

Shower of Particles

AMS, for its part, has been gathering data continuously since its launch in 2011. And the team has already seen some suggestive tentative results. As cosmic rays pass through the detector up on the space station, researchers on the ground can tell certain things about the particles. The detector has a cylindrical magnet at its heart, explains physicist Mike Capell, a senior research scientist on AMS, and when cosmic rays pass through the magnet, their paths curve ever so slightly. This curve, along with myriad other measurements taken by a suite of devices, tells researchers the particles’ momentum, charge, and classification—for instance, if it’s an electron versus a proton. Anywhere from a few hundred to a couple thousand particles whiz through every second, depending on where the space station is in its commute. (The display in Ting’s office, with one ray every few seconds, is a kind of “greatest hits” from that shower.)

AMS is taking a census of those particles, making a note of whether they’re, say, a stray nucleus from helium atom, one from a carbon atom, an electron, a positron (a particle with the same mass as an electron but opposite charge), or any number of other things. The idea is to see whether that census corresponds to what current theories about cosmic rays imply should be there. If there’s a discrepancy, that could be a signature of dark matter. For instance, when two dark matter particles collide, they are thought to produce an electron and a positron. If there are more positrons at high energies than expected, then that could be a clue that dark matter is out there, colliding with itself.

CERN AMS control building
The AMS control center at CERN

In 2013, the AMS team announced that they had, in fact, seen just such an excess. “You expect a fraction of positrons from garden-variety physics, but it’s not nearly as many positrons as we found,” Capell says. “So why would you have an excess of positrons? You would have them if you didn’t have garden-variety physics happening, if you had dark matter colliding.” Earlier experiments with the Fermi Large Area Telescope and the satellite-borne PAMELA had reported similar results, but not with AMS’s precision.

Still, it’s not proof that dark matter is producing these positrons. Unlike in LHC experiments, where researchers can control every input and predict outcomes in detail, AMS is just recording what nature produces. There could be other sources that explain the positron bump. One possibility among many is that the positrons are being emitted by a pulsar, a dense left-over from the death of a gigantic star that sprays out a plume of radiation. But if a pulsar were the source, the positrons would all be coming from one particular direction, and the AMS team has reported that was not the case.

Work to clarify those results goes on because there are still plenty of other things that could be yielding these results. “If this was dark matter, depending on the properties of the dark matter, we would expect to see deviations in other spectra and ratios,” Capell says. “For example, anti-proton to proton ratios would have something goofy. And we’re also working on that.”

Twelve Million to Go

Clearer answers will emerge as AMS continues to collect data. Yes, these extra positrons are predicted by theory, but so is a dearth of positrons at even higher energies, where the picture provided by AMS is still fuzzy. It’s a measurement that’s only possible with more time and more events recorded. “It agrees with the dark matter model, but it doesn’t prove it,” Ting says of the current data. “It’s necessary but not sufficient.”

Ting estimates it will take a few years to gather enough data to fill in the picture of the positrons, about 12 million more collisions between cosmic rays and the detector. For now, Ting says, “Something unusual is happening. We still do not know what it is.”

One the way out, I stop in the lobby outside the AMS control room, where the view from a camera looking out from AMS is projected on a screen. An array of solar cells looms and then drifts into shadow, leaving just the bright line of the edge of the Earth. Invisible to my eye, thousands of particles strike the detector and send their messages, via a string of satellites, down to this building. Here, they’ll be added to the great puzzle of experimental and observational results that is our sense dark matter and, in a way, of the universe.