The decades-long search for dark matter could ultimately end in an impasse.
This mysterious substance that is thought to hold galaxies together should also surround and even stream through our bodies. Yet we do not see or feel it because dark matter infrequently interacts with normal matter. In the hopes of detecting rare collisions between supposed dark matter particles and atomic nuclei, physicists have built increasingly large detectors that pick up on faint signals and have buried them deep underground, far away from cosmic rays and most forms of interference. Now the detectors have picked up on something else altogether: solar neutrinos, tiny, ghostly particles which sail through normal matter and may mask dark matter signals.
Catching a glimpse of what physicists have dubbed the “neutrino fog” underscores the remarkable sensitivity of current detectors but also sets harsh limits on the possible futures of current dark matter search methods. Evidence for the milestone comes from two competing experiments at the forefront of the field: XENONnT at the Gran Sasso National Laboratory in L’Aquila, Italy, and PandaX-4T (part of the Particle and Astrophysical Xenon Experiments, or PandaX) at the China Jinping Underground Laboratory. Each project uses a giant tank of extremely cold liquid xenon as a detector. Both teams unveiled their results in separate presentations earlier this month at a conference in L’Aquila, with the PandaX-4T team expanding on its findings in a preprint paper posted on July 15.
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“This is a result where we really stretch the capabilities of the instrument,” says Knut Morå, a statistician working on XENONnT. Members of the PandaX-4T team expressed similar excitement about their finding in an e-mail to Scientific American, noting that it “demonstrates the richness of the current and future PandaX physics program.” Both teams stress that the neutrino fog will not disrupt their current dark matter searches, which are slated to run for a few more years.
The neutrino fog arises from a fateful cosmic coincidence. The thermonuclear fusion reactions that allow our sun to shine also churn out fast-moving and featherweight neutrinos that can strike xenon nuclei with the exact same momentum that has been predicted for much slower and heavier hypothetical particles of dark matter.
“The events are indistinguishable one at a time, but [in aggregate], the expected neutrino flux has a different spectral shape than [what’s predicted by] many dark matter models,” Morå says, explaining how both teams flagged the fog in the first place. Additionally, the number of candidate collisions observed by both projects was consistent with forecasts based on well-established solar physics, bolstering the case that the events were linked to solar neutrinos rather than some unexpected variety of dark matter.
The statistical power associated with XENONnT’s and PandaX-4T’s two-year findings falls just short of the field’s stringent thresholds for “evidence,” never mind “discovery.” But researchers who had no hand in the experiments say there’s little cause for doubt. In addition to almost uncannily matching scientists’ models, the results come with a less than 0.4 percent chance of false alarm. “In any other field of science, [this] is like a 100 percent guaranteed discovery,” says Ben Carew, a master's student in physics who formerly worked at the Australian Research Council Center of Excellence for Dark Matter Particle Physics’ node at the University of Sydney.
For Morå, seeing the experimental result brought relief—and then excitement. The findings confirmed that the XENONnT detector succeeded at the difficult task of blocking out almost all other unwanted signals—a boon for a field that is historically starved of wins. “It means we didn’t see any weird gremlins,” he says.
Over the past four decades, generations of dark matter detectors have worked to “see nothing but better,” says Ciaran O’Hare, a dark matter researcher at the University of Sydney. “Now we’re moving into a new era where these detectors can actually do some valuable discovery science, although it’s the kind of science that [might] prevent them from doing what they were initially built to do.”
The neutrino fog is not currently stifling the search for dark matter, nor is it expected to in the next generation of detectors. Solar neutrinos mask rather low-mass varieties of putative dark matter particles, which are as yet weakly probed by both experiments. Progress will likely not be stymied by neutrinos until after 10 to 15 years.
In theory, current detectors could begin identifying subtle distinctions between neutrino signals and dark matter signals if they collected thousands of additional data points. Yet doing so would require resources both teams lack. Over two years, the XENONnT detector sensed around 40 neutrinos, and PandaX-4T sensed around 75. To “power through the fog,” O’Hare says, the researchers would need to build a detector that was 100 times larger or run an experiment that was 100 times longer. Researchers could save time by comparing results from similar experiments using detectors based on argon or germanium. Or they could factor annual variations in the number of solar neutrinos into their models. But ultimately the search would still rely on “brute force,” he says.
O’Hare is himself a member of an international collaboration called CYGNUS that is developing a backup approach—a new dark matter detection device that tracks the celestial sources of incoming particles. Solar neutrinos arrive from the sun, whereas dark matter particles are expected to come from the direction of the collaboration’s namesake: the constellation of Cygnus. Although most theorists suspect dark matter to be evenly distributed throughout the Milky Way’s starry disk, the solar system is currently moving through the disk toward Cygnus, meaning detectors should register a “headwind” of dark matter particles from that general region of the sky.
Such a device would likely require several years of development, however. Building a sufficiently sensitive and accurate detector is already challenging as is, Carew points out, without adding on extra capabilities.
O’Hare believes recent technological advances, as well as heightened attention to the fog, could offer CYGNUS more momentum. “[This alternative approach] historically hasn’t had the same push behind it,” he says. “I think that will slowly start to change.”
Change could also come from physicists rethinking what form dark matter might take, O’Hare points out. Like most experiments to date, XENONnT and PandaX-4T assume dark matter manifests as WIMPs, or weakly interacting massive particles. As the efforts continue to rule out various WIMP candidates, interest is surging in another idea: that dark matter consists of axions. These hypothetical particles are much lighter than WIMPs and can be detected via other means.
If dark matter detectors do succumb to the fog, scientists could repurpose the instruments to study neutrinos. A handful of dedicated experiments identify and observe these peculiar particles, but interactions like those reported by XENONnT and PandaX-4T—collisions between solar neutrinos and entire atomic nuclei—have never been captured before. More data could not only strengthen existing findings but also give rise to new sorts of measurements. Of particular interest to the PandaX-4T team is a hypothetical form of radioactive decay—neutrinoless double beta decay—which, if observed, could settle the debate on whether neutrinos have or are their own antiparticle. “The physics opportunities for the next generation PandaX are much richer than just dark matter search,” members of the team told Scientific American.
Aided by xenon-based tools or not, the search for dark matter particles will likely endure. Answering the question of what exactly makes up the universe’s unseen matter—or what else besides dark matter explains certain cosmic quirks—has the potential to revolutionize the laws of physics, O’Hare emphasizes. “It’s possible that nature gave us this particle which doesn’t do anything other than [help] form galaxies,” he says. “But we would never know unless we tried everything.”