When scientists observe the cosmos, they see stars whizzing around their galaxies faster than the laws of physics should allow and clusters of galaxies attracting each other too strongly. They theorize that something must be producing more gravity than all the visible matter in existence could explain - but whatever the substance is, it's invisible. Dark matter is, effectively, a placeholder: A well-documented hole in our understanding of the universe.
Researchers have floated numerous theories to explain what dark matter might be, but to date, no experiment has turned up compelling evidence to support any of them. An international team of physicists is now working on a new kind of dark matter detector with the goal of capturing the first direct observation of the puzzling material. Results from the detector's prototype have already ruled out one of the leading theories of how dark matter originated.
The new research was published August 13 in Physical Review Letters.
"DAMIC-M may be our best shot to answer the dark matter question in the coming years," said Alvaro Chavarria, a University of Washington associate professor of physics and detector lead for the DAMIC-M (DArk Matter In CCDs at Modane) international collaboration, which conducted this study.
A DAMIC-M detector module with silicon CCDs. The module is enclosed in a high-purity copper frame for installation in the detector prototype.DAMIC-M Collaboration
Most physicists think that dark matter is made of particles, just like all other matter in the universe. For reasons unknown, this class of particles does not interact much with conventional matter or with photons of light. But it could interact just enough to be observed by a highly sensitive instrument as the dark matter particles zip through the Earth.
"We know how much dark matter there is in the universe, but we don't know whether it's made of many light particles, or fewer, heavier ones," Chavarria said. "The game is to rule out all possible hypotheses until we find something."
For years, the leading candidate for dark matter was a heavy theoretical particle known cheekily as the WIMP, or Weakly Interacting Massive Particle. But experiments have not revealed a single WIMP, so many researchers have pivoted their search to lighter candidates called "hidden-sector" particles. Lighter particles would be that much harder to measure, so to meet the challenge, Chavarria and the DAMIC-M team developed a new class of detector.
The new device works a bit like a digital camera, which uses a silicon sensor called a CCD made of millions of pixels. The sensor detects photons and turns them into an image. The dark matter detector is made of similar - though much more sensitive - CCDs that can pick up tiny and rare particle interactions.
Chavarria and his team assembled and tested the CCD modules in their UW clean room lab. They then sent the device straight to the Laboratoire Souterrain de Modane, a facility located beneath 5,000 feet of rock in the French Alps. There, it was encased in lead to protect it from radioactive elements in the surrounding rock, and brought online. All of this was done to conduct the experiment with pristine machinery.
"We're looking for very rare signals in the detector - maybe on the order of one signal in a year," Chavarria said. "You need to remove all types of interference from other forms of radiation."
Researchers install the copper box containing the detector modules. Surrounding the box is shielding made of lead from ancient Rome - the team chose lead so old that any radioactive contaminants within it would have already decayed.DAMIC-M Collaboration
As advanced as the instrument is, it's just a prototype. The DAMIC-M team is building a much larger, more sensitive detector right now; they plan to bring it online early next year. Still, the prototype has proven useful. For two and half months, it captured several thousand "photographs," which the team scoured for evidence of dark matter collisions. It found none.
But in the game of dark matter detection, the absence of a finding is a finding in itself.
Historically, scientists have weighed two possible scenarios for how hidden-sector particles could have formed early in the life of the universe. Each scenario makes a different prediction for how the particles might turn up today. If the hidden-sector theory is correct, one of those two scenarios should be accurate. The "null" result by the DAMIC-M prototype almost entirely rules out one of the scenarios - and the full-scale detector is sensitive enough to finish the job. Either the new detector will discover dark matter, Chavarria said, or it will be time to test new theories.
"If DAMIC-M doesn't see anything, I don't think you'll hear about hidden-sector models of dark matter anymore."
The DAMIC team closes up the prototype detector after installing the CCDs.DAMIC-M Collaboration
Other possibilities exist. Perhaps hidden-sector particles exist, but only account for a small amount of all the dark matter in the universe. Perhaps tiny particles called axions are in the mix too - they're the target of another detector housed at the UW. In other words, maybe dark matter is another particle - or more than one.
But with DAMIC-M, researchers can narrow down the number of existing theories to those worth investigating, all while building the technology necessary to do so.
"We've been working on this since I arrived at the UW in 2018," Chavarria said. "The module development alone took almost five years of work here on campus. And now, thanks to the amazing result we got from the prototype, we're pretty confident the full-scale detector is going to work. I'm very excited. This was the dream."
Co-authors include Heng Lin, a former UW postdoctoral researcher who is now a postdoctoral fellow at Johns Hopkins University; Kellie McGuire, who completed this research as a UW graduate student; Michelangelo Traina, a former UW postdoctoral researcher who is now a postdoctoral fellow at the Instituto de Física de Cantabria in Spain and Kush Aggarwal, a UW graduate student. A full list of co-authors is included with the paper.
This research was funded by the European Research Council; National Science Foundation; The Kavli Foundation; The Ministry of Science and Innovation, Spain; Swiss National Science Foundation; and Centre National de la Recherche Scientifique (CNRS).
