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Diversifying the Dark Matter Portfolio

American Physical Society

Originally Posted: July 25, 2018

In 2012, CERN coaxed the long-sought Higgs boson into making an appearance, and in 2015, the Laser Interferometer Gravitational-Wave Observatory directly observed an elusive space-time wiggle. Both phenomena were theorized about for decades before their eventual discovery.

So perhaps it is high time for dark matter, the mysterious stuff that makes up around 27 percent of the universe, to finally reveal itself. But directly detecting particles that don’t reflect, absorb, or emit light is no easy task, especially without knowing what kind of particles they are — or how they interact with regular matter.

One of the prevailing hypotheses for many years has been that dark matter consists of weakly interacting massive particles (WIMPs), which are possibly 100 times more massive than a proton. Over the past ten years, direct dark matter detection experiments searching for WIMPs have improved significantly, reaching better and better sensitivities, but, so far, the hypothesized particles continue to evade even the best detectors.

“I think this has been a little bit surprising — that no one has had any indication of detecting dark matter,” says Jodi Cooley, a physics professor at Southern Methodist University and collaborator on SuperCDMS. “This is challenging theorists and experimentalists to start looking in new directions.”

So does this absence of WIMPs — and a subsequent uptick in experiments exploring alternatives — mean the WIMP hypothesis has fallen by the wayside?

Not exactly: “What’s really going on is a diversification of ideas,” says Dan Hooper, a senior scientist at Fermi National Accelerator Laboratory. “WIMPs aren’t going away as an idea; people are still very interested in them. But I think we’re much more open minded now to a greater diversity of theories.”

The secret could be that dark matter particles are much less massive than the hypothesized WIMP range, making it difficult for many detectors to catch their more subtle interactions. “Light” dark matter could help explain why traditional WIMP experiments have failed to capture their prey, but it has also opened up the field of dark matter research to a host of novel experiments.

One dark matter alternative that’s been generating excitement recently is the axion — theoretically much lighter than a WIMP, and something that wouldn’t be detected in the particle-scattering type experiments typical for direct dark matter detection. “In contrast to WIMPs, axions are very light,” says Daniel Bowring, a scientist on the Axion Dark Matter Experiment (ADMX). “People talk about WIMPs having masses between 10 and 1000 GeV — for the axion, we’re looking for masses all the way down to μeV.”

While axions aren’t a new concept, they’ve gained some traction, partially thanks to recent ADMX improvements that might finally make it possible to spot the tiny particles. READ MORE