Step Aside, WIMPs!

It seems the search for particles of dark matter has come up short once again, leading some scientists to question whether we should be looking for particles at all. Two of the world’s most massive detector projects—China’s PandaX-II collaboration and the US’s LUX group—have ended up empty-handed in their search for weakly interacting massive particles (or WIMPs), long considered one of the most plausible explanations for our galaxy’s surprising rotational behavior.

In a new paper from the LUX lab (Large Underground Xenon experiment), slated for publication today in Physical Review Letters, the collaboration of scientists report that their detector found no trace of particles that match the expected behavior of WIMPs. The experiment involved keeping a close eye on a tank of xenon buried nearly a mile underground. That kind of shielding—along with a 70,000-gallon water tank surrounding the xenon—is necessary to prevent cosmic rays or solar radiation from leaking in and creating a false signal. Among the only things that can penetrate that far into the ground are neutrinos and, theoretically, particles of dark matter.

This is the interior of LUX’s water shielding tank—the metal canister at the center of the image holds both liquid and gaseous forms of xenon, which acts as the detector’s scintillator material.
Image Credit:Wikipedia user Gigaparsec, (CC BY 3.0)

If the galaxy truly is filled with massive particles streaming through us at all times, as WIMP theories of dark matter predict, they ought to interact at least occasionally with ordinary matter. When that happens, we should see a physical reaction occur seemingly out of nowhere—an atom of xenon suddenly accelerating as though it’s been struck by something highly energetic, producing UV radiation in the process. But that hasn’t happened—the LUX collaboration’s enormous tank of liquid xenon has remained, for the most part, quiet and dark.

This comes just a few months after a similar null-result report from the PandaX-II collaboration, short for (P)article (AND) (A)strophysical (X)enon detector. Yes, the acronym’s a bit of a reach—but just look how cute their logo is!

Image Credit:PandaX Dark Matter Experiment

PandaX’s detector has the curious honor of being located in the deepest lab in the world, China Jinping Underground Laboratory, nearly 8,000 feet underground. The rock from which the lab’s space was carved is almost pure marble, which means extremely low signal contamination rates—you can dig as far down as you want, but if you accidentally set up shop near a vein of radioactive minerals, your data’s going to be full of noise.

Going to all that trouble to find a quiet place has paid off, though—the extremely low levels of background radiation have allowed scientists to place some of the tightest constraints yet on the possible properties of particle dark matter. If it’s out there, we’re closing in on a firm idea of what it must look like, just based on process of elimination.

The hope is that soon, detector projects like LUX and PandaX will either finally spot the WIMPs we’ve been searching for, or they’ll rule out their existence entirely with a thorough enough search. Either possibility would, in its own way, come as a huge relief—direct detection experiments aren’t cheap, and some scientists wonder if having multiple countries running nearly-identical experiments simultaneously is the best use of often scant research funds.

Alternative possibilities to WIMP dark matter abound, and although each has its own problems, ideas like Modified Newtonian Dynamics—which posits that gravity works differently at large enough scales—are gaining traction again, partly driven by the continued stream of no-signal results from direct detection experiments.

Whatever the true nature of this mystery turns out to be, it will almost certainly come as a surprise.

—Stephen Skolnick
Title suggested by Eran Moore Rea

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