As searches come up empty, some are thinking of new ways to look for dark matter.
Ars Technica said:Countless experiments around the world are hoping to reap scientific glory for the first detection of dark matter particles. Usually, they do this by watching for dark matter to bump into normal matter or by slamming particles into other particles and hoping for some dark stuff to pop out. But what if the dark matter behaves more like a wave?
That’s the intriguing possibility championed by Asimina Arvanitaki, a theoretical physicist at the Perimeter Institute in Waterloo, Ontario, Canada, where she holds the Aristarchus Chair in Theoretical Physics—the first woman to hold a research chair at the institute. Detecting these hypothetical dark matter waves requires a bit of experimental ingenuity. So she and her collaborators are adapting a broad range of radically different techniques to the search: atomic clocks and resonating bars originally designed to hunt for gravitational waves—and even lasers shined at walls in hopes that a bit of dark matter might seep through to the other side.
“Progress in particle physics for the last 50 years has been focused on colliders, and rightfully so, because whenever we went to a new energy scale, we found something new,” says Arvanitaki. That focus is beginning to shift. To reach higher and higher energies, physicists must build ever-larger colliders—an expensive proposition when funding for science is in decline. There is now more interest in smaller, cheaper options. “These are things that usually fit in the lab, and the turnaround time for results is much shorter than that of the collider,” says Arvanitaki, admitting, “I’ve done this for a long time, and it hasn’t always been popular.”
While most dark matter physicists have focused on hunting for weakly interacting massive particles, or WIMPs, Arvanitaki is one of a growing number who are focusing on less well-known alternatives, such as axions—hypothetical ultralight particles with masses that could be as little as ten thousand trillion trillion times smaller than the mass of the electron. The masses of WIMPs, by contrast, would be larger than the mass of the proton.
Cosmology gave us very good reason to be excited about WIMPs and focus initial searches in their mass range, according to David Kaplan, a theorist at Johns Hopkins University (and producer of the 2013 documentary Particle Fever). But the WIMP’s dominance in the field to date has also been due, in part, to excitement over the idea of supersymmetry. That model requires every known particle in the Standard Model—whether fermion or boson—to have a superpartner that is heavier and in the opposite class. So an electron, which is a fermion, would have a boson superpartner called the selectron, and so on.
Physicists suspect one or more of those unseen superpartners might make up dark matter. Supersymmetry predicts not just the existence of dark matter, but how much of it there should be. That fits neatly within a WIMP scenario. Dark matter could be any number of things, after all, and the supersymmetry mass range seemed like a good place to start the search, given the compelling theory behind it.
But in the ensuing decades, experiment after experiment has come up empty. With each null result, the parameter space where WIMPs might be lurking shrinks. This makes distinguishing a possible signal from background noise in the data increasingly difficult.
“We’re about to bump up against what’s called the ‘neutrino floor,’” says Kaplan. “All the technology we use to discover WIMPs will soon be sensitive to random neutrinos flying through the Universe. Once it gets there, it becomes a much messier signal and harder to see.”
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