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Pondering a world without WIMPs

31 March 2017

Physicists at a recent workshop were encouraged to think broadly to identify the universe’s dark matter.

Despite impressive sensitivity, dark-matter detection experiments such as Large Underground Xenon (detector array above) have not found evidence of weakly interacting massive particles. Credit: C. H. Faham/LUX

For three days last week, more than 100 physicists took over the University of Maryland student center, which had been left abandoned by spring breakers. The topic at hand: rethinking the future of dark-matter searches.

The workshop, titled US Cosmic Visions: New Ideas in Dark Matter, represented a recalibration for the dark-matter community. For decades physicists have been fixated on the WIMP, a particle with a mass–energy in the GeV range and a tendency to intermittently mingle with ordinary matter via the weak force. Theorists weaved WIMPs—weakly interacting massive particles—into elegant supersymmetric models. Experimentalists built vast, exquisitely sensitive underground detectors to nab the occasional extroverted WIMP amid the constant deluge that’s thought to stealthily pass through the planet.

But at least for the physicists at the workshop, the era of hoping for a WIMP miracle is all but over. Cutting-edge experiments such as XENON in Italy and LUX in South Dakota have so far come up empty in their searches. Researchers have failed to spot signs of missing energy at CERN’s Large Hadron Collider that would flag the existence of a WIMP or a related supersymmetric particle.

Those null results motivated the US Department of Energy’s high-energy physics division to pledge up to $10 million to fund a new breed of dark-matter experiment. The workshop was not meant as a competition—that comes later—but as a means of identifying compelling but overlooked candidates and experimental tactics. The message, which was stated repeatedly and was often said with a hint of desperation: Go broad.

Neal Weiner, a theorist at New York University, set the tone in one of the opening plenary sessions. He introduced the “priorhedron,” a shape whose dimensions are determined by the strengths of the cosmological and field theory assumptions (priors) that motivate a given dark-matter candidate. Until now physicists have largely hung their hats on strong but not foolproof assumptions that support the existence of WIMPs, such as the particles’ ability to fit in supersymmetric models and address the hierarchy problem.

Weiner urged the physicists in the room to shift the dimensions of their priorhedrons and usher in a new era of moderate assumptions for guiding dark-matter searches. “You can’t say that dark matter is made out of dragons,” he warned, but you can, for example, remove some of the restrictions placed by standard cosmology in formulating a new model. And even if such broad thinking doesn’t identify dark matter, Weiner said, it could lead to other new physics or the resolution of outstanding questions such as the energy scale of inflation.

The Axion Dark Matter Experiment is already looking beyond WIMPs. Credit: ADMX

Over the course of the workshop, the attendees largely heeded the call to broadness. Presenters dove into the dark-matter implications of a potential new boson found in the decay products of beryllium nuclei and the ongoing discrepancy concerning the radius of the proton. They spoke of axions and other dark-matter candidates so lightweight that they would be detected as waves, and of particles so heavy that they would clump together and encounter Earth only occasionally as a vast invisible glob. There was talk over lunch and coffee of dark forces, dark photons, and dark neutrons. The attendee list included experimentalists who are new to the dark-matter field and scientists who aren’t in the field but decided it was worth the $130 registration fee to hear ideas get thrown around.

Many of the ideas that were presented were not new: The axion was proposed in the 1970s, and researchers with the upgraded Axion Dark Matter Experiment in Washington State have been busy since last fall trying to tune a microwave antenna to the broadcast frequency of dark matter. In fact, the axion search was treated as old hat by many of the attendees, who at least for one afternoon flocked to other focus sessions in cramped quarters while a dozen or so axion insiders gathered in the largest room of the workshop. “Axions are on the rise, yet here we are in an empty room,” said one perplexed attendee.

Perhaps the lack of axion interest reflected the mood of the workshop: If you’re going to go broad, then why not sample more exotic fare? “There’s a feeling that this is a scary place to be,” Weiner said. “But it’s also an opportunity.”

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