Dark matter is the mysterious substance that comprises about 23 percent of all the mass in our universe, but thus far it has eluded physicists' many attempts to directly detect it. Maybe instead of looking for a dark matter particle, they should be looking for something more akin to a wave—a hypothetical dark matter candidate known as an axion.
In that case, perhaps we should be "listening" for the dark matter. Physicists at Stockholm University and the Max Planck Institute of Physics have proposed a novel design for an "axion radio" that employs cold plasmas (gases or liquids of charged particles) to do just that in a recent paper in Physical Review Letters.
"Finding the axion is a bit like tuning a radio: you have to tune your antenna until you pick up the right frequency," said co-author Alexander Millar, a postdoc at Stockholm University. "Rather than music, experimentalists would be rewarded with 'hearing' the dark matter that the Earth is traveling through."
Much of the hunt for dark matter thus far has focused on so-called weakly interacting massive particles, or WIMPs. There was very good theoretical reason to focus on that mass range, most notably the concept of supersymmetry, whereby every particle in the Standard Model should have a "super-partner" that is heavier and in the opposite class (fermion or boson). For example, an electron (in the fermion class) would have a boson super-partner called the "selectron." One or more of those super-partners might make up the dark matter.
But all the experiments hunting for dark matter have repeatedly come up empty, and the available parameter space within the WIMP-y mass range is shrinking rapidly. Physicists know they are getting very close to the "neutrino floor," where the detection technology will become so sensitive, and will be picking up so many random neutrinos, that picking out a dark matter signal in all the noise will become much harder.
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