Deep underground in the French Alps, researchers are working to detect dark matter, a substance believed to make up much of the universe but which cannot be directly observed. Its existence is inferred through its gravitational effects, such as holding galaxy clusters together and influencing the motion of stars.
A recent study published in Physical Review Letters describes efforts by the DAMIC-M (DArk Matter In CCDs at Modane) international collaboration to search for signs of dark matter using advanced detection methods. The team includes University of Chicago Professor Paolo Privitera, who serves as spokesperson for the project.
One theory is that dark matter consists of unknown particles that interact very weakly with ordinary matter but still exert gravitational force. For decades, experiments have searched for WIMPs (Weakly Interacting Massive Particles), thought to be heavier than protons. “But WIMPs have not been found so far, despite extremely sensitive searches by enormous detectors weighing a ton, including my colleague Luca Grandi’s work with XENONnT,” said Privitera.
Particle accelerator experiments like ATLAS at CERN’s Large Hadron Collider have also failed to detect WIMPs. As a result, scientists are now looking for lighter dark matter particles, which require highly sensitive instruments because their signals are faint and difficult to observe.
The DAMIC-M experiment operates 5,000 feet below ground at the Laboratoire Souterrain de Modane in France. While it did not detect dark matter during its initial run, it was able to rule out several particle candidates known as “hidden sector” dark matter.
Dark matter detectors work on the principle that these particles might occasionally collide with an atom’s nucleus in the detector material. Such collisions could produce signals like emitted light or movement within the atomic lattice. Lighter particles are harder to spot because they transfer less momentum during collisions compared to heavier ones.
“A heavy particle hitting a nucleus is like a bowling ball hitting another bowling ball—it will impart a sizeable momentum,” said Privitera. “A light particle hitting a nucleus would be like a ping-pong ball hitting a bowling ball. It would not move it at all.” However, hidden-sector dark matter could interact with electrons instead of nuclei: “Now it is like a ping-pong ball hitting another ping-pong ball,” Privitera explained.
To capture these rare events, DAMIC-M uses specially designed charge-coupled devices (CCDs) capable of detecting individual electrons generated by potential interactions with hidden-sector dark matter. These CCDs are thicker than standard models and feature skipper readout technology that allows precise electron counting.
Background noise from thermal fluctuations and external radiation can obscure possible signals from dark matter interactions. To reduce this interference, DAMIC-M operates its CCDs at -220 degrees Fahrenheit and shields them with layers of lead—some sourced from ancient shipwrecks due to their low radioactivity—and rock overhead.
For this study, researchers built a prototype called the Low Background Chamber containing two CCD modules weighing 26 grams total and collected data over two and a half months. They identified 144 clusters containing two electrons each and one cluster with four electrons—numbers consistent with expected background levels rather than evidence of dark matter interactions.
“Thus, we have not yet discovered dark matter,” said Privitera. He noted that their results were “orders of magnitude more sensitive than any other experiment, a notable achievement when considering they were obtained with a prototype detector and a small mass.”
The absence of detected signals informs theories about how dark matter formed after the Big Bang—either through “freeze-out,” where ordinary and dark matter were once in equilibrium before separating as the universe cooled; or “freeze-in,” where they never reached equilibrium due to weak interaction rates.
“These theoretical predictions are now probed for the first time by the DAMIC-M null result,” said Privitera. In particular scenarios such as freeze-out, certain hidden-sector candidates can now be excluded based on lack of signal: “Because the team did not detect signals, the experiment completely excludes several hidden-sector candidates—they do not exist.” For freeze-in scenarios though, some possibilities remain open since an absence of signal does not definitively rule out those forms of dark matter.
“The fact that we have not found dark matter in our data excludes that hidden-sector particles constitute the entirety of dark matter in the universe,” said Privitera; however if such particles exist they may only account for part of all dark matter present today.
Following promising results from their prototype detector, researchers plan for full-scale data collection beginning in 2026 using an expanded version of DAMIC-M featuring improved shielding and materials designed to further minimize background interference. According to Privitera: “Our target is still hidden-sector dark matter… but also light WIMPs and other candidates… We expect that DAMIC-M will be the leading experiment in the search for these low-mass dark matter particles for several years to come.”
Other University of Chicago co-authors on this study include research associate professor Radomír Šmída; KICP Fellow Brandon Roach; postdoctoral scholar Julian Cuevas-Zepeda; graduate students Ruixi Lou, Sravan Munagavalasa, Joseph Noonan, Sugata Paul and Rachana Yajur.
Funding sources include European Research Council; National Science Foundation; The Kavli Foundation; Ministry of Science and Innovation (Spain); Swiss National Science Foundation; Centre National de la Recherche Scientifique (CNRS).
Citation: "Probing Benchmark Models of Hidden-Sector Dark Matter with DAMIC-M." K. Aggarwal et al., Phys. Rev. Lett. 135, 071002.