New dark matter detectors look for ‘wimpier’ particles

The hunt to confirm the existence of mysterious dark matter just got a powerful weapon, one that has been deployed deep beneath the French Alps.  

The highly sensitive detector—developed by an international collaboration including Johns Hopkins University researchers—is making it possible for the first time in decades to expand the search for the elusive particles believed to make up roughly 85% of the universe but that have never been directly observed in a lab.

The advance could either generate the first direct evidence of dark matter or rule out broad classes of theories that have yet to be tested, the researchers said.

“Dark matter is one of the most important ingredients that shape our universe and also one of the greatest cosmological mysteries,” said co-author Danielle Norcini, an experimental particle physicist and an assistant professor of physics and astronomy at Johns Hopkins University. “Our prevailing theories about the nature of dark matter aren’t yielding results, even after decades of investigation. We need to broaden our search, and now we can.”

The new devices, which rely on technology similar to the light-sensitive microchips in camera phones, are designed to detect dark matter particles that are far lighter than what traditional detectors have unsuccessfully looked for over the past several decades.  

The findings are published in Physical Review Letters.

Dark matter detectors are designed to capture the tiny jolts of energy that might result from dark matter colliding with atoms. But traditional detectors have long relied on the use of heavier atoms, such as xenon and argon, in their search. If a dark matter particle were to encounter an atom’s nucleus and they were roughly the same size, the collision could cause the nucleus to recoil—like one billiard ball bumping into another. Detectors record the energy from the recoil as a potential dark matter signal. 

For the past four decades, most dark matter research focused on finding these nucleus-sized particles, called weakly interacting massive particles (WIMPs). However, if nucleus-sized dark matter particles existed, the collisions would likely have been observed by now, the researchers said.

 If the particles turn out to be lighter than predicted, they would have gone undetected because they wouldn’t have the heft necessary to nudge an entire nucleus. Instead of two billiard balls, it would be more like a ping pong ball striking a bowling ball. The new technology allows researchers to look for those lighter, weaker particles—making them, in Norcini’s words, “WIMPier than the WIMPs.”

These new devices, called silicon skipper CCDs, can detect signals from single electrons, the much smaller particles bound to and orbiting an atom. This unprecedented achievement allows scientists to look for dark matter similar in size to an electron rather than a nucleus. 

Because the new detector is so sensitive, experiments must be conducted in carefully shielded environments. Researchers operate the devices in the Laboratoire Souterrain de Modane, about 2 kilometers underground in the French Alps, using the environment and materials to reduce interfering signals: The bedrock blocks cosmic rays, while layers of ancient, low-radioactivity lead and special lab-grown copper surround the experiment to minimize background radiation. 

“Trying to lock in on dark matter’s signal is like trying to hear somebody whisper in a stadium full of people. That’s how small the signal is,” Norcini said. “While we haven’t discovered dark matter yet, our results show that our detector works as designed, and we are starting to map out this unexplored region.” 

Next, the team plans to scale up from eight skipper CCDs in the proof-of-concept prototype to a full array of 208 sensors. The larger area will boost the chances of capturing an interaction. The fully constructed experiment, called DAMIC-M, will be the most sensitive detector in the world searching for this “WIMPier” type of dark matter.