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 鈥� but whatever the substance is, it鈥檚 invisible. Dark matter is, effectively, a placeholder: A well-documented hole in our understanding of the universe.
Researchers have 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鈥檚 prototype have already ruled out one of the leading theories of how dark matter originated.
The August 13 in Physical Review Letters.
鈥淒AMIC-M may be our best shot to answer the dark matter question in the coming years,鈥� said , a 天美影视传媒 associate professor of physics and detector lead for the DAMIC-M (DArk Matter In CCDs at Modane) international collaboration, which conducted this study.
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.
鈥淲e know how much dark matter there is in the universe, but we don鈥檛 know whether it鈥檚 made of many light particles, or fewer, heavier ones,鈥� Chavarria said. 鈥淭he game is to rule out all possible hypotheses until we find something.鈥�
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 鈥渉idden-sector鈥� 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 鈥� though much more sensitive 鈥� 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 , 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.
鈥淲e’re looking for very rare signals in the detector 鈥� maybe on the order of one signal in a year,鈥� Chavarria said. 鈥淵ou need to remove all types of interference from other forms of radiation.鈥�
As advanced as the instrument is, it鈥檚 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 鈥減hotographs,鈥� 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 鈥渘ull鈥� result by the DAMIC-M prototype almost entirely rules out one of the scenarios 鈥� 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.
鈥淚f DAMIC-M doesn鈥檛 see anything, I don鈥檛 think you鈥檒l hear about hidden-sector models of dark matter anymore.鈥�
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 鈥� they鈥檙e the target of . In other words, maybe dark matter is another particle 鈥� 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.
鈥淲e’ve been working on this since I arrived at the UW in 2018,鈥� Chavarria said. 鈥淭he 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鈥檙e pretty confident the full-scale detector is going to work. I鈥檓 very excited. This was the dream.鈥�
Co-authors include , a former UW postdoctoral researcher who is now a postdoctoral fellow at Johns Hopkins University; , who completed this research as a UW graduate student; , a former UW postdoctoral researcher who is now a postdoctoral fellow at the Instituto de F铆sica de Cantabria in Spain and , a UW graduate student. A full list of co-authors is included with the .
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).
For more information, contact Chavarria at chavarri@uw.edu.
This story was adapted from a by the University of Chicago.