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Last Chance for WIMPs: Physicists Launch All-Out Hunt for Dark-Matter Candidate

Physicists are hatching a plan to give a popular but elusive dark-matter candidate a last chance to reveal itself. For decades, physicists have hypothesized that weakly interacting massive particles (WIMPs) are the strongest candidate for dark matter — the mysterious substance that makes up 85% of the Universe’s mass. But several experiments have failed to find evidence for WIMPs, meaning that, if they exist, their properties are unlike those originally predicted. Now, researchers are pushing to build a final generation of supersensitive detectors — or one ‘ultimate’ detector — that will leave the particles no place to hide.

“The WIMP hypothesis will face its real reckoning after these next-generation detectors run,” says Mariangela Lisanti, a physicist at Princeton University in New Jersey.

Physicists have long predicted that an invisible substance, which has mass but doesn’t interact with light, permeates the Universe. The gravitational effects of dark matter would explain why rotating galaxies don’t tear themselves apart, and the uneven pattern seen in the microwave ‘afterglow’ of the early Universe. WIMPs became a favourite candidate for the dark matter in the 1980s. They are typically predicted to be 1–1,000 times heavier than protons and to interact with matter only feebly — through the weak nuclear force, which is responsible for radioactive decay, or something even weaker.

Supercooled xenon

Over the coming months, operations will begin at three existing underground detectors — in the United States, Italy and China — that search for dark-matter particles by looking for interactions in supercooled vats of xenon. Using a method honed over more than a decade, these detectors will watch for telltale flashes of light when the nuclei recoil from their interaction with dark-matter particles.

Physicists hope that these experiments — or rival WIMP detectors that use materials such as germanium and argon — will make the first direct detection of dark matter. But if this doesn’t happen, xenon researchers are already designing their ultimate WIMP detectors. These experiments would probably be the last generation of their kind because they would be so sensitive that they would reach the ‘neutrino floor’ — a natural limit beyond which dark matter would interact so little with xenon nuclei that its detection would be clouded by neutrinos, which barely interact with matter but rain down on Earth in their trillions every second. “It would be sort of crazy not to cover this gap,” says Laura Baudis, a physicist at the University of Zurich in Switzerland. “Future generations may ask us, why didn’t you do this?”

The most advanced of these efforts is a planned experiment called DARWIN. The detector, estimated to cost between €100-million (US$116-million) and €150 million, is being developed by the international XENON collaboration, which runs one of the 3 experiments starting up this year — a 6-tonne detector called XENONnT at the Gran Sasso National Laboratory near Rome. DARWIN would contain almost ten times this volume of xenon. Members of the collaboration have grants from several funding agencies to develop detector technology, including precise detection