Might the secret to the missing mass of the universe lie in particles lighter than anything ever trapped before? Under 1.2 miles of rock in the French Alps, researchers have stationed a new generation of dark matter detectors that can listen for the cosmic whispers that come from particles significantly smaller than the long-hunted WIMPs that have evaded scientists for decades.
For almost four decades, searches for dark matter have been focused on Weakly Interacting Massive Particles, employing gigantic tanks of xenon or argon to capture the minute nuclear recoils such particles would impart. The idea was straightforward: a WIMP striking a heavy atomic nucleus would impart sufficient momentum to create a detectable burst of light or ionization. But as Paolo Privitera, DAMIC-M collaboration spokesperson, pointed out, WIMPs have not been found so far, despite extremely sensitive searches by enormous detectors weighing a ton. The failure to detect any signal has driven physicists towards lighter candidates sometimes referred to as “WIMPier than WIMPs” which interact with electrons rather than nuclei.
To find such low-mass particles, one needs a completely different approach. The DAMIC-M experiment uses silicon skipper charge-coupled devices (CCDs), a technology borrowed from astronomical imaging. Each CCD is a finely crafted slab of silicon split into millions of pixels, which can detect the tiny charge remaining after a single electron is released from its atom. Differing from regular CCDs, skipper devices can read out the same pixel multiple times without annihilating the signal, down to essentially zero readout noise and permitting the unambiguous counting of single electrons. This functionality is essential for detecting the small energy depositions usually no more than a few electronvolts such a sub-GeV dark matter particle might leave on an electron.
The current prototype in the Laboratoire Souterrain de Modane is comprised of eight skipper CCDs contained in a cryostat at -220°F to mitigate thermal noise. The site, buried beneath over 5,000 feet of alpine rock, employs the mountain itself as a shield, reducing the cosmic ray flux by more than six orders of magnitude. Surrounding the detector, sheets of ancient lead recovered from Roman wrecks and several-century-old Spanish galleons are supplementary shielding against gamma rays, and high-purity copper screens for radiation from residual radioactivity in the shielding. These materials are selected only after extensive low-background assays to guarantee contamination levels of uranium, thorium, and potassium well below the levels that would simulate a dark matter signal.
The engineering challenge goes beyond shielding. Even in underground deep laboratories, natural radioactivity in detector elements can free electrons or produce Cherenkov photons in nonconductors, giving rise to false signals. To reduce it, DAMIC-M reduces insulating material close to the CCDs and has a modular layout so that several detectors can veto spurious events. As Privitera described it, “Trying to lock in on dark matter’s signal is like trying to hear somebody whisper in a stadium full of people.”
The physics case for such sensitivity is strong. In the “hidden sector” models DAMIC-M targets, dark matter particles are light enough that they cannot push on a nucleus, but can scatter off electrons with measurable impact. These models naturally find their place in cosmological models such as “freeze-in” and “freeze-out,” which explain how dark matter abundance might have been stabilized following the Big Bang. The null result of the first run of the prototype itself 144 two-electron events and one four-electron event, all background-compatible has already ruled out complete classes of hidden-sector particles within the freeze-out model.
Scaling up next. The entire DAMIC-M array, for operation in 2026, will grow to 208 skipper CCDs, raising the target mass from the gram range to the kilogram range and enhancing background rejection using still purer materials and optimized shielding geometries. With a bigger capture area, the experiment will explore interaction probabilities several orders of magnitude below present limits, possibly discovering particles down to the electron mass. If successful, it would be an independent technology to validate any such discovery complementing other sub-GeV searches with superconductors, superfluid helium, and noble liquids.
In the wider community of particle physics, DAMIC-M is an indicator of the direction towards high-precision, diversified instruments that can probe the vast parameter space uncharted by conventional WIMP detectors. By combining semiconductor engineering advances with the radical radiopurity and shielding methods developed in neutrino and rare-event physics, it is extending the boundary of how weak a signal humanity can access from the dark sector of the universe.
