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Supermaterials Offer New Means of Searching for Superlight Dark Matter

Two new papers offer new methods of searching for the most evasive forms of dark matter.
September 19, 2016, 1:00pm
Liquid helium. Image: Wiki

No one said detecting dark matter would be easy. We didn't even know about the stuff until a couple of decades ago, after all, despite the fact that it represents some 85 percent of all of the mass in the universe and is what's responsible for giving structure to the cosmos. We see its effects across the universe, but we have yet to see it. We're not even sure what exactly we're looking for—there are many theories as to the exact properties of a dark matter particle. Some aren't even all that dark.


The leading candidate for dark matter is a particle known as a WIMP, or weakly-interacting massive particle. These are really heavy, classically "dark" particles. They interact with other matter via only the gravitational force, crucially evading electromagnetic interactions, which are what most of the interactions we see out in the world are based on: from a baseball whapping into a catcher's mitt to the nanoscale electrical circuits enabling the machine you are now staring at.

WIMP detection is premised on WIMPs having sufficient mass to smack into an atomic nuclei with enough force to create a bit of light or heat, which can then be registered by the detector. A problem then arises when we start trying to imagine dark matter particles that maybe aren't so heavy and, as such, may result in interactions below the sensitivity of current detectors. This is where the work of Kathryn Zurek and colleagues at the Lawrence Berkeley National Laboratory comes in—bagging superlight dark matter may require supermaterials.

Zurek and co.'s work is described in two papers published last week in the Physical Review Letters. One looks at the possibility of using helium superfluids to detect dark matter while the other considers superconducting metals. Both possibilities offer detector sensitivities below current offerings of around 10 million electronvolts. The lower limit for light, "warm" dark matter is around 1,000 electronvolts.

The idea behind the helium superfluid detector is that the incoming superlight dark matter particle doesn't just couple to one collision event. Instead, the one incoming particle is able to yield several interactions—enough to be detected. This is part of quirkiness of superfluidic materials, which are able to flow without losing kinetic energy thanks to their complete absence of viscosity. Somewhat famously, superfluidic helium has the bizarre capability of creeping up the walls of its container and leaking away. So, the incoming superlight DM particles can couple to the superfluid without burning off kinetic energy—that is, without stopping.

A similar sort of thing happens in a metallic superconductor, which is the basis of the detector described in Zurek's second paper. Here, current is able to flow without resistance in much the same way that the superfluid is able to flow without viscosity. The difference detector-wise is that the superlight dark matter particles interact with pairs of electrons rather than atomic nuclei. The pairs are split and the result are particle-like sound waves called phonons. Because the electrons in a superconductor are already moving at velocities faster than those expected of a superlight dark matter particle, the momentum of the incoming DM particles isn't "lost" in the collision event and they can be observed as particles in the superconductor recoil from the collision.

The superconductor method will only work with dark matter particles that are able to couple to electrons, which means that they interact to some extent electromagnetically and are thus not so dark. As odd as it sounds, light-based dark matter hasn't been excluded from the hunt. The superfluid-based detector would presumably be bagging more conventional dark-dark matter particles, albeit very light ones. In any case, as dark matter searches across the globe continue to come up dry, it's clear that we need more tools.