Even after decades of searching, scientists have never seen a particle of dark matter. Evidence for the substance’s existence is close to incontrovertible, but no one yet knows what it is made of. For decades physicists have hoped dark matter would prove to be heavy—consisting of so-called weakly interacting massive particles (WIMPs) that could be straightforwardly detected in the lab.
With no definitive sign of WIMPs emerging from years of careful searching, however, physicists have been broadening the scope of their quest. As new, more precise experiments ramp up data collection, researchers are reassessing theories about how dark matter particles lighter than a proton might appear in their detectors. Two papers posted on the preprint server arXiv.org earlier this year are emblematic of these shifting sensibilities. They are the first to propose that a detector could find plasmons—aggregates of electrons moving together in a material—produced by dark matter.
The first study was conducted by a group of dark matter researchers at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., the University of Illinois at Urbana-Champaign and the University of Chicago. They propose that low-mass dark matter could produce plasmons—which they claim some detectors may already be seeing. Inspired by that first paper, physicists Tongyan Lin and Jonathan Kozaczuk, both at the University of California, San Diego, calculated how likely low-mass dark matter is to generate plasmons in a detector.
“We are screaming, ‘Plasmon, plasmon, plasmon!’ because that’s a compelling, existing phenomenon that we think might be relevant for interpreting dark matter experiments,” says Gordan Krnjaic, a dark matter theorist at Fermilab and the Kavli Institute for Cosmological Physics at the University of Chicago and a co-author of the first study. Particle physicists and astrophysicists have been speculating about how to detect low-mass dark matter for nearly a decade. But they had not previously considered seeking plasmons—which are more familiar to chemists and material scientists—as its signature.
“I think it’s great,” says Yonit Hochberg, a theoretical physicist at the Hebrew University of Jerusalem, who provided feedback to Krnjaic’s team but was not directly involved in either paper. “The fact that there are [plasmons] that could be having an impact that haven’t been taken into account is, I think, an extremely important point that really warrants further investigation.”
Other researchers are more dubious about the first paper. That study is “not at all convincing to me,” says Kathryn Zurek, a dark matter theorist at the California Institute of Technology, who was not involved with either paper. “I just don’t see how this works.”
Noah Kurinsky, a co-author of the first paper and a dark matter experimentalist at Fermilab and the Kavli Institute for Cosmological Physics, takes criticism from physicists in stride. “We’ve challenged them to prove us wrong, which I think is superhealthy for this field. And that's exactly what they should be trying to do,” he says.
The hunt for an invisible, nearly traceless substance usually goes something like this: To detect dark matter particles, physicists get a material, put it somewhere deep underground, hook it up to instruments and hope to see a signal. Specifically, they hope dark matter will strike the detector, producing electrons, photons or even heat that their instruments can observe.
The theory behind dark matter detection dates back to a 1985 paper that considered how a neutrino detector could be repurposed to look for particles of the substance. The study proposed that an incoming dark matter particle could hit an atomic nucleus in the detector and give it a kick—similar to one billiard ball crashing into another. This collision would transfer momentum from the dark matter, walloping the nucleus hard enough to make it spit out an electron or a photon.
At high energies, this picture is essentially fine. Atoms in the detector can be thought of as free particles, discrete and unconnected to one another. At lower energies, however, the picture changes.
“Your detectors are not made of free particles,” says Yonatan (Yoni) Kahn, a dark matter theorist at the University of Illinois at Urbana-Champaign and a co-author of the first paper. “They’re just made of stuff. And you have to understand the stuff if you want to understand how your detector actually works.”
Within a detector, a particle of low-mass dark matter would still transfer momentum. But instead of breaking a rack of billiard balls, it might cause them to wobble. In others words, it would act more like a Ping-Pong ball.
“As we go to lower dark matter masses. There are other more subtle effects that start to kick in,” Lin says. These subtle effects include what physicists like to call “collective excitations.” When several particles move at once, they can be described as a single entity, just as a sound wave is composed of multitudinous vibrating atoms.
Plasmons occur when a group of electrons experience such motions. When a group of atomic nuclei vibrate, their collective excitation is instead called a phonon. Such phenomena are typically seen as irrelevant by astrophysicists and high-energy physicists studying dark matter.
But as the late Nobel laureate physicist Philip Anderson once quipped, “More is different”—a nod to the fact that novel effects emerge at different scales. A droplet of water, for example, obeys different rules than an individual molecule of H2O. “I have totally drunk that Kool-Aid,” Kahn says.
Both papers take slightly different approaches to plasmon production. They come to the same conclusion, however: we should really be on the lookout for such signals. In particular, Lin and Kozaczuk calculated that low-mass dark matter would create plasmons at about one ten-thousandth the rate of directly producing an electron or photon. This figure may sound infrequent, but it is more than enough for physicists looking to be precise.
Shot in the Dark
Until recently, the most sensitive dark matter detectors have used giant vats of liquid xenon. In the past few years, however, a new generation of smaller solid detectors have debuted. Known by clever acronyms such as EDELWEISS III, SENSEI and CRESST-III, they are made of materials such as germanium, silicon, and scheelite and are sensitive to dark matter collisions that would create just a single electron.
But all detectors, no matter how well-shielded, experience noise from sources such as background radiation. So over the past year or so, when scientists operating several dark matter detectors began seeing more signals at low energies than expected, they stayed rather quiet about it.
The paper by Kurinsky and his colleagues was the first to point out the remarkable similarity between the low-energy “excesses” seen across disparate dark matter experiments. Several excesses seem to cluster around a value of 10 hertz per kilogram of detector mass. Because the detectors are made of different materials, are located in different places and operate under different conditions, it is difficult to come up with a universal reason for this uncanny harmony—except, that is, for the subtle influence of dark matter. This discussion caught the attention of other physicists, such as Lin, who quickly jumped to work on plasmon calculations. But even she has doubts that what the experiments are currently seeing are the results of dark matter creating plasmons. “I'm not saying it couldn’t be dark matter,” Lin says. “But it doesn’t seem convincing to me so far.”
As more data come in from the latest generation of dark matter detectors, the hypothesis will be put to the test. But whether or not the detectors are currently seeing the mysterious substance may be beside the point. Researchers in the field are now thinking and talking about plasmons and other ways in which low-mass dark matter could behave. An exploration of the precision frontier is underway.
“There are many ways in which we can be wrong,” Krnjaic says. “And they’re all exciting.”