LEADER OF THE PARTICLE PACK: CMS, one of two general-purpose particle physics experiments at the Large Hadron Collider in Europe, could soon tell astronomers what dark matter is made of. Image: © CERN
BALTIMORE—Dark matter pervades the universe, giving shape to the cosmos on the grandest scales. So perhaps it is fitting that physicists are turning to a large-scale physics experiment to uncover what dark matter is made of.
Dark matter helps mold galaxy formation and accounts for five times the mass of all the ordinary, visible matter in the universe, but it has eluded direct detection for decades. Astronomers can see its gravitational effects—galaxies and galaxy clusters behave as if they have far more mass than ordinary matter alone can provide—but the particle nature of the stuff remains a mystery.
At a recent dark matter symposium at the Space Telescope Science Institute here, hopes for a solution via the Large Hadron Collider loomed large. The LHC, essentially a 27-kilometer particle racetrack buried 100 meters belowground near Geneva, started up in 2009 and quickly became the most powerful particle collider in the world. At a series of controlled impact points, protons boosted to near the speed of light collide head-on, and physicists sift through the outflying debris to look for hints of new physics.
Astronomers and cosmologists have their fingers crossed that a positive ID on a dark matter particle will be among the new phenomena that should soon come streaming out of the LHC. After all, astronomical probes that have sought out the signature of dark matter particles have come up empty, as have experiments on the ground designed to detect the stuff.
"I'm feeling a little pessimistic," astronomer Sandra Faber of the University of California, Santa Cruz, said during a panel discussion at the symposium. "As every speaker in the last two days gave their talk, I thought to myself, 'How can I make a case here that astronomy is going to answer the nature of dark matter?' I think it's pretty thin. I think we have to turn to the physicists to actually discover this particle—or particles—and tell us what it is."
One ongoing possibility is that one of many specialized dark matter detectors—such as Xenon100 in Italy or CoGeNT in Minnesota—could catch a whiff of the stuff as Earth passes through an ambient haze of dark matter. But many researchers hold out more hope for the LHC, which could produce dark matter in its particle collisions.
That would light the way toward other complementary detections. Many researchers, for instance, have used the Fermi Gamma-Ray Space Telescope to look for the signature of dark matter particles crashing into one another, mutually annihilating, and giving off gamma rays. Dan Hooper of Fermilab and Lisa Goodenough of New York University published a study in Physics Letters B in March showing that Fermi observations of the center of the Milky Way Galaxy seem to show a gamma-ray excess indicative of dark matter annihilations there.
But the data are somewhat ambiguous, U.C. Santa Cruz physicist and Fermi team member Robert Johnson said in a talk at the dark matter symposium. "You don't see any residuals at the galactic center that would suggest something extra going on there," he said. "I think it's an open question whether there is a dark matter source at the galactic center. It's very difficult to disentangle it from the other stuff going on there."
In an interview Johnson noted that the LHC should be able to clarify things considerably. "We're kind of looking blind at the data now," he said. If the LHC could pin down the attributes of a promising dark matter particle, the Fermi data would come into much tighter focus. "The LHC has a much greater reach in the parameter space for looking for something like supersymmetric dark matter," Johnson said. Supersymmetry is a popular hypothetical model for particle physics that posits that each elementary particle—quarks, electrons and so on—has a hidden counterpart particle just waiting to be discovered. One such supersymmetric partner might provide an ideal candidate for the dark matter particle.
Looking for supersymmetry is one of the LHC's primary tasks, along with discovering the long-sought Higgs boson, which is theorized to lend other elementary particles mass. And depending on the traits of the supposed supersymmetric particles, the LHC may be close to finding them. "By this summer or this winter we may have something to say if there are supersymmetric particles living out there," physicist Albert de Roeck of CERN, the European particle physics lab that operates the LHC, said at the symposium.
Even if it takes a bit longer than that—as it may if supersymmetry resides in a regime less accessible to the LHC's detectors—astronomers are willing to wait. "Give us about three years for the LHC to reach maximum luminosity," astrophysicist Joe Silk of the University of Oxford said in the panel discussion. "If they find evidence of supersymmetry, I think this would give a fantastic boost to the field."
Astronomers are counting on the LHC to break new ground in the dark matter search, but in an interview de Roeck said their expectations were not putting significant added pressure on CERN. "Supersymmetry is good—it will satisfy many customers," he said. But compared with the race to find the Higgs and the political pressure to deliver returns on such an ambitious and expensive experiment, the added pressure to find dark matter is "peanuts," de Roeck said.