Seeing Dark Matter in the Ice

Buried beneath more than a kilometer of ice in the Antarctic may lie answers, or least insights, into one of the great questions confronting 21st century astrophysics: What is dark matter?

In the dense centers of the dwarf spheroidal galaxies that live in the dark matter halo of the Milky Way there are, perhaps, particles known as WIMPs that are colliding with each other and “annihilating” into other kinds of particles, which shoot out into the void.

Here on earth, buried beneath more than a kilometer of ice in the Antarctic, is a vast array of digital optical sensors known as IceCube. It’s a very special kind of telescope designed to detect neutrinos, which are one of the products—perhaps—of WIMP annihilation.

In the convergence of these two things may lie answers, or least insights, into one of the great questions confronting 21st century astrophysics: What is dark matter? What’s the nature of this matter, which we can’t see, that makes up about five-sixths of the universe’s mass?

“Mainly we know what it's not,” says Pearl Sandick, a post-doctoral fellow in the theory group of physics professor Steven Weinberg. “It's not any of the particles in the Standard Model. It's not protons or neutrons or electrons.The best guess is that most of the dark matter in the universe is made up of some particle we have yet to discover.”

The leading candidate, says Sandick, is what’s known as a weakly interacting massive particle, or WIMP. The existence of WIMPs is predicted by many theories of particle physics beyond the Standard Model. One example is supersymmetry.

“Supersymmetry proposes a whole slew of new particles, which are the super-partners of the standard particles we know about,” she says, “and one of them is going to be the lightest. It just happens to turn out that in most supersymmetric theories, the lightest particle has roughly the properties that a dark matter particle should have.”

The problem, however, is that if WIMPs do indeed exist, they would be incredibly hard to see (if they were easy to see, they wouldn’t be so “dark”). They would be electrically neutral, and would have very weak interactions with normal matter. For decades, experiments have been searching for collisions of WIMPs with normal matter, yet there is currently no conclusive evidence that any such collisions have been observed. WIMPs are also expected to annihilate with each other, but annihilations of the WIMPs that make up the dark matter halo in which our galaxy resides have also not yet conclusively been observed.

In order to see WIMPs, or see evidence of them, physicists have had to turn to fairly elaborate means. For example, physicists are currently trying to produce WIMPs in the Large Hadron Collider (LHC) in Switzerland by smashing together protons at energies high enough that WIMPs might be produced in the collisions. Although the newborn WIMPs wouldn’t be directly detectable at the LHC, the hope is that their existence could be inferred by the signatures of other particles produced in the same collision.

Sandick and her collaborators are taking a more indirect—and much longer distance—approach. They propose using the IceCube neutrino detector to look at dwarf spheroidal galaxies at the margins of the Milky Way. And they’re looking not for WIMPs directly, but for signs of the last ricochet in a kind of intergalactic billiards shot that began with WIMPs crashing into each other and annihilating.

“In the early universe, WIMPs annihilated a lot,” says Sandick. “Today, the density of WIMPs is much lower, so they less frequently find each other to annihilate. In some regions of the universe, however, the density might be high enough. In the center of the sun, for instance, or in these galaxies, they might still be annihilating. So basically what we're looking for are the annihilation products.”



One product of the annihilation of those WIMPS, says Sandick, might be neutrinos. Those neutrinos then travel across space, pass through the earth and, finally, hit the ice where IceCube is deployed. Although neutrinos typically just pass through matter, from time to time one will collide with a particle in the ice and produce another kind of subatomic particle, a muon. And if that muon continues on through the ice at a speed faster than the speed of light in the ice, the superluminal muon will create a cone of electromagnetic radiation known as Cherenkov radiation (much like a supersonic aircraft generates a sonic boom). And it’s photons from that radiation that—at last—the IceCube sensors can register.

“That’s the signature we're looking for,” says Sandick, “and when you see the radiation cone, you can infer the direction the neutrino came from.”

Even if IceCube is able to “see” these neutrinos, and differentiate them from neutrinos that might come from other sources, and reverse engineer their trajectory back to the dwarf galaxies, it wouldn’t prove that WIMPs exist. It would, however, be solid evidence in that direction, and a foundation for more experiments and more refined theory. And it would be a testament to the extraordinary ingenuity of particle physicists, as well as to their rather eccentric eagerness to cast skyhooks out into the unseen.