Mining for Missing Matter

On an early summer morning in northern Minnesota, a crew of about a dozen waits by the top of mine shaft No. 8. Donning hard hats, the engineers and physicists pile into a creaky, double-decker elevator cage. It is pitch black for most of the three-minute descent. Ears pop, the cage floor vibrates and a giant motor dating from 1925 thunders overhead.

When the cage door slides open, the team is 713 meters below the surface. Directly ahead lies a maze of tunnels — an abandoned mine where laborers once extracted iron ore of uncommon purity. But the scientific crew takes a U-turn into a huge and unexpectedly spacious two-room cavern known as the Soudan Underground Laboratory.

The workers have journeyed deep into the Earth to plumb the darkest depths of the cosmos, hunting for the missing material believed to account for 83 percent of the universe’s mass.

That material, known as dark matter, must exist, astronomers say, because the cosmic allotment of ordinary, visible matter doesn’t provide enough gravitational glue to hold galaxies together. Although the missing material shouldn’t be any more prevalent in the underworld than above ground, dark matter hunters have good reason to frequent Soudan and other subterranean lairs. Because dark matter particles would interact so weakly, experiments designed to detect the dark stuff could easily be overwhelmed by the cacophony of other particles. So scientists at Soudan and elsewhere use Earth’s crust to filter out cosmic rays — charged particles from space that bombard Earth’s atmosphere.

Physicists have been directly searching for dark matter for more than two decades. But until recently, only one experiment, beneath a mountain in central Italy, had consistently reported evidence of the invisible particles. Now two more experiments have found similar hints. When taken together, the findings suggest that the most popular models for dark matter may not be correct — the particles pegged have a lower mass than many physicists had proposed.

“Any discovery of dark matter would be a major revolution,” says theorist Neal Weiner of New York University. “But if these results are right, I think it’s even more exciting than that.”

If the low-mass measurements are confirmed, a second revolution is in the making: In addition to dark matter, a new force may be needed to explain the workings of the universe. Favorite particle physics theories may require revision or may even have to be discarded.

But not so fast, some scientists say. Other recent work questions whether researchers have actually spotted low-mass dark matter particles. And with so much at stake, including the likelihood of a Nobel Prize for whoever discovers dark matter first, rival teams have resorted to name-calling. One team has twice publicly ridiculed the results of a second, while the team whose analysis has come under fire has likened the attacks to the Spanish Inquisition.

It’s an exciting but confusing time, Weiner says.

Catching some WIMPs 

Physicists have long had a leading model for dark matter. They believe that it consists of a proposed particle left over from the Big Bang called the WIMP, for weakly interacting massive particle. WIMPs sense only gravity and the weak force, the interaction that governs radioactive decay. Particle physicists like WIMPs because they fit neatly into a theory known as supersymmetry, which unifies the two basic types of elementary particles — force carriers and matter particles.

Supersymmetry requires that every force carrier has a heavier matter particle for a partner, and every matter particle has a heavier force-carrying partner, doubling the number of particles in nature. The lightest supersymmetric partner would be stable, making it an ideal candidate for the WIMP that physicists propose.

Astronomers favor WIMPs as much as physicists do because of an intriguing cosmic coincidence. The predicted abundance of WIMPs in the universe today would account for the amount of dark matter needed to keep rotating galaxies from flying apart and galaxy clusters intact. The heavenly match between WIMPs and dark matter is known as the WIMP miracle.

Miracles are all well and good, but actually detecting the stuff is another matter.

At Soudan and other underground laboratories, scientists use ultrapure solid crystals or liquefied noble gases to try to record the rare collision of a WIMP with an atomic nucleus. Like a struck billiard ball, the jostled nucleus travels a short distance in the detector, hitting neighboring nuclei or electrons. In a solid, like a germanium crystal, the motion of the nucleus generates sound waves that reverberate throughout the detector and cause a tiny but detectable rise in temperature. The interaction of the nucleus with its neighbors also ionizes atoms, liberating some of their outer­most electrons. Some experiments measure a different signal, the emission of light generated by a collision.

But ordinary matter can jostle a detector’s atomic nuclei too. Going underground helps by shielding experiments from the shower of high-energy particles that cosmic rays produce when they hit the atmosphere. In some cases, researchers also have to cool experiments to a few hundredths of a degree above absolute zero to reduce the constant motions of atoms and molecules in the detectors.

And the experiments need additional shielding because of natural radioactivity in the walls of the mine and the cooling apparatus. For instance, the current version of the Cryogenic Dark Matter Search experiment, called CDMS II, at Soudan is made up of five stacks of germanium and silicon wafers, all sheathed in five nested copper containers. Those containers are then surrounded by lead and polyethylene.

These efforts win only part of the battle. Some ordinary particles still make it through. Neutrons, most notably, are the bane of dark matter experiments because the particles mimic the interaction of WIMPs, confounding scientists. So experimenters analyze the collected signals — sound, light, ionization — to try to discriminate a WIMP collision from an interaction between a nucleus and a spurious background particle.

After accounting for every possible background source, scientists currently expect to find only a few candidate WIMP events a year, says Dan Bauer of the Fermi National Accelerator Laboratory in Batavia, Ill. “So if you get fooled by even one event per year, you’ve failed,” he says.

A recent analysis of data recorded by CDMS II in 2007 and 2008 identified two interactions that might be attributed to WIMPs, scientists reported late last year (SN: 1/2/10, p. 8). The detection was not definitive, as radioactive decay of ordinary material could be responsible for about 0.8 events during the same time period.

In a clean room accessible only by researchers dressed in full protective regalia — white jumpsuit, booties and gloves — the team is now building a new experiment at the Soudan mine, SuperCDMS. With nearly four times the mass of the old experiment and many times the sensitivity, SuperCDMS has the capability to find more WIMPs per year than its predecessor.

Going lower

Tucked away in another part of the Soudan cavern, a smaller experiment, consisting of a single hockey puck of germanium encased in a cooler, has recently made its own mark in the dark matter game.

The Coherent Germanium Neutrino Technology experiment, COGENT, began operating at Soudan in 2009 and is designed to record lower-mass WIMPs than CDMS II can. In February, Juan Collar of the University of Chicago and his collaborators reported a few hundred collisions that his team says could be explained by dark matter interactions. The new finding, which Collar emphasizes must be confirmed by analyzing more data, points to a WIMP weighing between about seven and 11 times the mass of a proton, about one-tenth as massive as particles in many of theorists’ WIMP models.

Though the preliminary result is based on only two months of data, the COGENT finding has taken on added importance because it dovetails with results recorded over the past decade by an experiment named DAMA/LIBRA, say Weiner and theorist Dan Hooper of Fermilab. DAMA/LIBRA, short for Dark Matter Large Sodium Iodide Bulk for Rare Processes, hunts for dark matter at Gran Sasso, an underground laboratory in central Italy, and was the first to consistently report evidence of the particles.

The experiment sidesteps the problem of contamination from ordinary background particles by looking for a variation in the number of particle collisions it records over a year. A collision rate that is higher in summer than in winter could be a sign that Earth is plowing through a WIMP wind during half its annual trip around the sun. DAMA/LIBRA has seen just such a seasonal increase for more than a decade (SN: 5/10/08, p. 12); researchers recently reported the latest results online at arXiv.org/abs/1007.0595.

No other experiment has ever seen such a seasonal modulation, but scientists might not spot it if dark matter particles are very light, only a few times the mass of a proton — the same mass range that may have been detected by COGENT, notes Weiner. That’s because most other detectors feature nuclei that are too heavy to be jostled by a light WIMP.

Yet another experiment at Gran Sasso has also found possible evidence of a lighter-than-expected WIMP. At the annual Identification of Dark Matter meeting in Montpellier, France, physicists working on an upgraded version of an experiment called the Cryogenic Rare Event Search with Superconducting Thermometers, CRESST-II, reported data consistent with a lighter-than-expected WIMP. But another team that reanalyzed data from a different experiment at Gran Sasso reported evidence to the contrary.

CRESST-II consists of ultracold crystals of calcium tungstate (CaWO4). The experiment detects both the tiny amount of light and heat generated in each collision with any of the nuclei in the crystals — calcium, tungsten or oxygen.

So far, the signals recorded by CRESST-II suggest that dark matter particles collided with the lightest nuclei in the crystals, oxygen, instead of tungsten, the heaviest. That further suggests that the WIMPs striking the CRESST-II detectors had a low mass and therefore couldn’t jostle the heavy tungsten nuclei, the team reported July 26.

But Peter Sorensen of the Lawrence Livermore National Laboratory in California and colleagues presented a new analysis of data taken in 2006 with XENON10, a predecessor to an ongoing experiment that uses liquid xenon to search for WIMPs.

Sorensen and colleagues looked back at just one signal, ionization, to search for the particles, rather than the two signals that the experiment was originally designed to detect. Although this strategy made background interference more of a problem, it also made the analysis more sensitive to low-mass WIMPs.

The XENON10 results are incompatible with those from DAMA/LIBRA, COGENT and CRESST-II, Sorensen says. “If these experiments were in fact seeing dark matter, that would imply that we should also be seeing a lot, which we don’t,” he notes. “At some level it’s a bummer, because it would be nice to see something finally.”
Leo Stodolsky of the Max Planck Institute for Physics in Munich, Germany, a CRESST-II team member, says he and his colleagues have to make sure they aren’t being fooled by neutrons or another background source before making any definite claim.

Nonetheless, says dark matter theorist Katherine Freese of the University of Michigan in Ann Arbor, “the low-mass region — that’s the one that everyone is excited about, up, down and sideways.”

Hooper, who has been in the field for a decade, says it is hard to explain why XENON10 is not seeing a signal, but he still has hope that some of the hints of low-mass WIMPs will be credible.

“This is the first time in my career that I’m prepared to make bets with my colleagues that we’re actually seeing something,” he says.

New physics, hot debate

There’s a delicate balance in trying to make sense of the existence of a light WIMP, explains Weiner. On the one hand, a low-mass WIMP would have to interact with ordinary matter in an unusually weak way — otherwise, atom smashers would have already revealed hints of its existence. On the other hand, WIMPs that interact so weakly would tend to have too large an abundance in the universe today to fit with the WIMP miracle. They would have been less likely to be destroyed in the dense, early universe, when collisions between particles were more common.

One solution proposed by Weiner and other researchers is that nature harbors not only a low-mass WIMP but also a new force in the dark sector. The low-mass WIMP would account for the results of the experiments, and the dark force could have depleted enough WIMPs in the early universe so that there would not be too many around today.

“It would also mean that all these simple supersymmetric models that we’ve been thinking about all these years are going to be thrown out the window,” says Hooper. “We’re going to have to think about new kinds of theoretical frameworks, symmetric and otherwise.”

Weiner would still like to better understand why different underground experiments don’t seem to agree with each other. Though he went to the July meeting enthusiastic about a low-mass WIMP, now he says he is less so. “It’s a very confusing picture at the moment.”

Researchers are hoping that new results expected this fall from XENON100, the newer and larger version of the XENON10 experiment, will provide some clarity.

“For the next year, it’s no question that the experiment to watch is XENON100,” Freese says. That experiment, which began operating last fall at Gran Sasso, features the most massive detector now in operation, with 160 kilograms of liquid xenon.

The liquid sits in a stainless steel cylinder surrounded by lead and polyethylene to provide shielding from background particles. One set of sensors records the light emitted when a particle strikes one of the nuclei; another set records the signal generated if the colliding particle ionizes the xenon atoms. Together, the signals help discriminate WIMPs from other particles and pinpoint the location of the collision.

To further distinguish background particles from WIMPs, only the central 40 kilograms of xenon is used as the active detector. Any particle that interacts with both the outer 120 kilograms of liquid and the xenon in the central volume will automatically be rejected because it would be extraordinarily unlikely for a bona fide dark matter particle to interact in both places.

In May, XENON100 team leader Elena Aprile of Columbia University and her colleagues posted results from the first 11 operating days of the experiment at arXiv.org/abs/1005.0380. The initial data appear to rule out the preliminary findings from the much smaller COGENT experiment, which has led to a war of words between the XENON100 and COGENT teams (“A dark debate,” SN Online: 5/13/10).

In two scathing articles posted online, Collar and colleagues accused Aprile and her team of conducting a sloppy, misleading analysis.

“You have experimentalists essentially trying to get blood out of a turnip,” Collar said in an interview. “I’m not even convinced that everybody signing that paper believes what they wrote.”

Says Aprile of Collar’s reaction: “The Inquisition comes to mind.”

At the heart of the debate is an unknown: how much light xenon generates in response to collisions with particles that have very low energies or masses. Aprile acknowledges that further measurements, which colleagues in her lab in Irvington, N.Y., are now conducting, are needed to settle the question.

That brouhaha aside, most dark matter researchers are awaiting the results from XENON100’s first 100 days of data. Aprile says that the findings will be unveiled this fall in a dramatic protocol known as an unblinding, in which all members of the international team gather around their computers as preprogrammed software analyzes the results.
“We will be able to make a very big statement about dark matter,” Aprile predicts.

Like other teams, she and her collaborators are already planning a larger version of their experiment. This successor to XENON100 would use 2,300 kilograms of liquid xenon.

“My hope is that the next generation of experiments will discover dark matter” within the next few years, says CDMS II researcher Jodi Cooley of Southern Methodist University in Dallas. After making their initial discovery, researchers could then tailor experiments to unveil dark matter’s detailed nature, she says.

By then, the deepest physics laboratory ever built — a proposed facility at the Homestake mine in South Dakota — may be ready. There, some 2,200 meters underground, modern-day miners will attempt to unveil the darkest secrets of the universe as never before.