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 techniques that will work over DARWIN’s much larger scales, says Baudis, a leading member of XENON and co-spokesperson for DARWIN.

Global experiment

The project is also on Switzerland’s national road map for future scientific infrastructure, and Germany’s research ministry has issued funding calls specifically for DARWIN-related research; these steps suggest that the nations are likely to contribute further cash in the future. And although DARWIN does not yet formally have a home, it could end up at Gran Sasso. In April, the laboratory formally invited the collaboration to submit a conceptual design report by the end of 2021. “It tells us very clearly that the lab is very interested in hosting such an experiment,” says co-spokesperson Marc Schumann, a physicist at the University of Freiburg in Germany. The team hopes to be taking data by 2026.

Although DARWIN is currently led by the XENON collaboration, Baudis is hopeful that Chinese colleagues, who this year are starting up an experiment called PandaX-4t, or the team involved in the US-based xenon experiment called Lux-Zeppelin, might join them in building a single ‘ultimate’ detector. These teams have also considered building experiments that would take them to the neutrino floor, but “the goal is, of course, to have one large global xenon-based dark-matter experiment”, says Baudis.

Physicists might have no choice but to club together because of the sheer quantity of xenon needed. The noble gas is difficult to obtain in large quantities owing to the energy-intensive process needed to extract it from the air and because of competing demand from electronics, lighting and space industries. One kilogram can cost more than US$2,500. Darwin’s 50 tonnes would be close to the world’s annual production of around 70 tonnes, meaning that — even if all 3 existing detectors combine their 25 tonnes — a future experiment would need to buy the rest in batches over several years. “We have to plan very carefully for it already now,” says Baudis.

Researchers behind similar experiments that use argon to look for dark matter also hope to build a detector to reach the neutrino floor. A 300-tonne experiment known as ARGO would likely begin operations around 2029 and could confirm any signal seen by DARWIN.

Why WIMPs?

WIMPS have been the focus of dozens of experiments because there is a strong theoretical case for their existence. They not only explain why galaxies seem to move as they do, but their existence also fits with theories in particle physics. A group of theories known as supersymmetry, devised in the 1970s to fill holes in physicists’ standard model of fundamental particles and their interactions, predict a WIMP-like particle. And when particle physicists model the early Universe, they find that particles with WIMP-like properties would survive the hot soup of interactions in just enough numbers to match the dark-matter abundance observed today.

But null results — from direct dark-matter detectors and from particle accelerators such as the Large Hadron Collider — mean that, if WIMPs exist, either the likelihood that they interact with matter or their mass must be at the lowest end of initial predictions. The failure to detect WIMPs has caused the physics community to “pause and reflect” on their status, says Tien-Tien Yu, a physicist at the University of Oregon in Eugene. Many in the physics community, including Yu, are now searching for other dark-matter candidates, including through smaller, cheaper experiments.

Still, WIMPs remain theoretically attractive enough to continue the decades-long hunt, says Yu. And the DARWIN team emphasizes that its supersensitive detector would have myriad uses — including addressing the pressing questions in neutrino physics, says Baudis. One mystery that DARWIN could help to solve is whether neutrinos are also their own antiparticle.

Whether a single experiment or many, “I would bet quite some money that a DARWIN-like detector gets built,” says Schumann.

This article is reproduced with permission and was first published on October 2 2020.