Dark matter scientists are doubling down on efforts to catch the elusive particles thought to constitute most of the matter in the universe. These theorized particles make themselves felt through gravity: They appear to tug on the normal matter throughout the universe but they otherwise can’t be seen or touched. Experiments aiming to observe the rare occasions when dark matter particles interact with normal atoms have been operating for decades without success and have already ruled out many of the most basic explanations for dark matter. Rather than give up the search, however, three of the largest experiments recently won approval to make big upgrades, potentially allowing them to reach the sensitivities needed to finally pin down these cagey missing particles.
The three experiments moving forward—the Super Cryogenic Dark Matter Search–SNOLAB (SuperCDMS); LZ Dark Matter Experiment, or LUX–ZEPLIN, (LZ); and the Axion Dark Matter eXperiment (ADMX–Gen2)—are among the largest and longest-running projects in the field. They beat out around 20 other bids submitted to the U.S. Department of Energy and the National Science Foundation, which jointly announced their selections for the next-generation dark matter experiments last week. Both SuperCDMS and LZ will search for dark matter candidate particles called weakly interacting massive particles (WIMPs) whereas ADMX-Gen2 will look for alternative candidates called axions.
The selection of two WIMP projects came as a relief amid fears that the current federal budget crunch might narrow the field. “A number of us were concerned the tight funding was going to collapse the next generation to one experiment, and for a number of reasons that doesn’t make a lot of sense,” says Stanford University physicist Blas Cabrera, spokesperson for SuperCDMS. One reason it is wise to continue both projects, he says, is because they use different materials—SuperCDMS will employ a combination of germanium and silicon detectors, whereas LZ uses liquid xenon. If a WIMP collides with the atoms in any of these targets, it will release a small amount of energy detectable by the experiment. Some dark matter theories suggest that WIMPS might interact with different elements at different rates. “Nature may be more complicated than we initially thought, and we should be thinking more broadly,” Cabrera says.
It also makes sense, he adds, to continue searching not just for WIMPs but also for axions, which theorists believe are much lighter and interact even less frequently than WIMPs. Both types of particles could explain the apparent abundance of dark matter in the universe but there is no proof that either exists. (It is also possible that both exist, or that that some other combination of sources is responsible for dark matter.)
A final verdict on axions appears to be within reach. “At very high confidence, this generation-2 experiment can either detect the axion or reject the hypothesis,” says ADMX spokesperson Leslie Rosenberg of the University of Washington in Seattle. The project uses a powerful magnet to coax axions, if they exist, into decaying into photons. Such decays would produce a tiny amount of electromagnetic power detectable by the machine, which must be kept very cold to block out contaminating radiation. The upgraded version of the experiment will switch to a new cooling technology that will lower its temperature from 1.5 kelvins (or –271.6 degrees Celsius, already extremely chilly) to just 100 millikelvins. Within three years ADMX-Gen2 should have either discovered axions or proved they don’t exist—at least the ones proposed in current theoretical models.
Other dark matter detection experiments not selected during this round involved lasers and so-called bubble chambers, which search for bubbles created when dark matter particles interact with atoms in a chamber of superhot carbon, fluorine and iodine. One bubble chamber project called PICO plans to move forward anyway and apply for funding down the line. “We got excellent reviews but we are not ready; we are in the process of addressing a background issue,” says PICO team member Juan Collar of the Kavli Institute for Cosmological Physics at the University of Chicago. “We are happy with the outcome. It has taken a lot of pressure off of us.”
Dark matter has already proved harder to find than many physicists expected when they began the search. Nevertheless, many see a light at the end of a long tunnel. The next-generation experiments might finally have what it takes to find the missing particles once and for all. And if they don’t, we’ll know that much more about what dark matter isn’t. “In science the ruling out is as important as discovery,” Cabrera says, “but of course discovery is always more fun.”