Scientists are about to unleash a powerful new weapon in the hunt for dark matter, the mysterious substance that accounts for around 85% of the “stuff” in the universe. Like a super-weapon developed by a stereotypical supervillain, this new dark matter detector is hidden over a mile deep beneath the French Alps.
This highly sensitive detector, developed by an international team of researchers including scientists from Johns Hopkins University, will expand the search for potential dark matter particles beyond its current parameters. It could thus provide evidence for the existence of a particular dark matter candidate particle, or the detector could help rule out some suspects.
This evidence for or against certain candidates for dark matter could potentially find new particles less massive than many current dark matter candidates. Or, as team member and Johns Hopkins researcher Danielle Norcini puts it, “WIMPier than the WIMPS (Weakly Interacting Massive Particles).”
“Dark matter is one of the most important ingredients that shape our universe and also one of the greatest cosmological mysteries,” Norcini said in a statement. “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.”
New dark matter detector goes underground
Dark matter is such a mystery for scientists because, despite outweighing everyday particles in the universe by a ratio of 5 to 1, we have no idea what dark matter is. We do know what it (probably) isn’t, however.
Dark matter is effectively invisible because it doesn’t directly interact with electromagnetic radiation, or light, or it does, and this interaction is so weak we can’t see it. Dark matter does interact gravitationally, and this has allowed astronomers to discover that entire galaxies like the Milky Way are embedded in vast haloes of dark matter that extend far beyond the reaches of those galaxies’ visible matter.
The particles that comprise atoms, electrons, protons, and neutrons do interact with light, however, so we know that dark matter isn’t the same “stuff” that comprises stars, planets, moons, asteroids, animals and everything else we can see.
This has prompted a search for particles beyond the so-called standard model of particle physics, which was completed when scientists discovered the Higgs Boson at the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, back in 2012.
But, despite scientists using instruments like the LHC to smash together protons and atomic nuclei together at near the speed of light, a potential dark matter particle has thus far failed to manifest in the lab. That is also despite 4 decades of searching.
Traditional dark matter detectors are designed to spot tiny flashes of energy caused when dark matter particles, whatever they may be, collide and interact with particles of ordinary matter. Current detectors use heavy atoms like xenon and argon, which should recoil if their nucleus is struck, much like colliding billiard balls. The energy from this recoil would be recorded and assessed as a potential dark energy signal.
The problem with this is that recoiling, essential for detection, only occurs if the dark matter particle that strikes the atomic nucleus has a similar mass to the struck nucleus.
That means attempts to detect dark matter particles in this way have historically focused on particles with nucleus-sized masses, or WIMPs. However, this team reasons that if WIMPs existed, the 40 or so years of hunting that have been conducted thus far should have turned up a signal.
However, if dark matter particles are of smaller masses, this detection method won’t work. Going back to the billiard ball analogy, imagine replacing the billiard balls with bowling balls and the cue ball with a ping-pong ball. Lighter dark matter particles wouldn’t have the heft to cause a nucleus of xenon or argon to recoil. However, they could cause recoil when striking much less massive particles like electrons. That smaller recoil would result in a smaller flash of energy requiring a more sensitive detector to spot it
To develop a more sensitive dark matter detecting kit, this team turned to silicon skipper charged-couple devices or “CCDs.” These advanced sensors employ silicon to detect much lower-energy events than other CCDs can spot.
The device is capable of detecting signals emitted by single electrons as they orbit a much larger atomic nucleus. This should allow researchers to hunt for dark matter particles similar in size to electrons.
Such sensitivity requires an extremely well-shielded environment to prevent any signal from being washed out by unwanted signals or “noise” from surrounding naturally occurring events. Hence, this team is taking their detector around 1.2 miles (2 kilometers) beneath the French Alps.
In this underground lair, vast amounts of bedrock can block out cosmic rays, charged particles streaming to Earth from space, filtering out signals caused when they strike atoms, while ancient lead and special lab-grown copper reduce background radiation and noise associated with that.
The current detector is a proof-of-concept prototype that features 8 silicon skipper CCDs. The next step is to scale this up to 208 sensors to create a full-sized experiment, which has been dubbed DAMIC-M.
The larger capture area of DAMIC-M will boost the chances of capturing an interaction between electrons and dark matter particles, making this detector the most sensitive in the world to potential “WIMPier” particles.
“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 concluded. “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.”
The team’s research was published on Aug. 13 in the journal Physical Review Letters.
