Few things have the same air of enigma surrounding them as dark matter. Even the name suggests something hidden in the universe’s shadows and exudes secrecy. A group of scientists working together under the name COHERENT made an effort to draw dark matter out of the darkness of the cosmos and into a somewhat less spectacular location: a narrow, well-lit basement hallway. But that wasn’t just any basement. The team usually focuses on neutrinos, subatomic particles that are found in a region of Oak Ridge National Laboratory known as Neutrino Alley. On a more practical level, they are created as a byproduct of proton interactions in particle accelerators or when stars die and explode as supernovas.
Unsurprisingly, Neutrino Alley is situated just below Oak Ridge’s Spallation Neutron Source, one of the world’s most potent particle accelerators (SNS). There are several detectors located at Neutrino Alley that are made to watch neutrinos as they travel through and clash with them. Yet, SNS’s operations produce more by-products than just neutrinos. Particle accelerators also create dark matter when protons collide, which is distinct from anti-matter, which is a favorite of movie villains. The COHERENT team set out to observe dark matter in Neutrino Alley by utilizing both the power of SNS and the sensitivity of their neutrino detectors, which they did as a result of years of theoretical calculation.
And we missed it,” Scholberg adds. That would have been more exciting if we had seen it, but missing it is still a significant event. She notes that since their neutrino detectors missed dark matter, this has allowed scientists to significantly improve their theoretical predictions of what dark matter would look like. “We were hunting for that specific fingerprint because we know exactly how the detector would react to dark matter if dark matter shared similar features.” The resonant behavior of the atomic nuclei in the neutrino detector when struck by a neutrino, or in this case, by a dark matter particle, is the alleged fingerprint in question.
Pershey compared it to shooting projectiles at a bowling ball on a sheet of ice. In his comparison, the neutrino detector, which in this experiment was a 14.6 kilogram cesium iodide crystal, is represented by the atoms as the bowling balls. You can learn a lot about the projectile as well as the force with which it was fired by observing how much the bowling ball recoils after making contact. Every piece of information is helpful when it comes to dark matter. Nobody truly understands what it is. About a century ago, scientists came to the conclusion that the cosmos could not function as it did if it only contained matter that can be seen.
Jason Newby, group head for neutrino research at Oak Ridge National Laboratory and a co-author of the paper, said, “We’re swimming in a sea of dark matter.” According to physicists, dark matter accounts for 85% of the universe’s mass. To explain how the cosmos behaves, it must be subject to gravity, yet it doesn’t interact with any kind of light or electromagnetic wave, making it appear dark. We discovered it by observing the rotation of massive galaxies around one another and noticing that they revolve far more quickly than they should, which suggests that they contain more mass than first appears, according to Pershey. So now that we are aware that there is additional material available, all we need to do is understand where to seek.
“Even though we’re in the land of basically no results,” asserted Newby, “it’s incredibly crucial that anywhere you can look, you look. So you can rule out a whole lot of options and focus on a new area with strategy instead of just utilizing a ‘spaghetti on the wall’ approach.” We’re expanding the possibilities for dark matter models, which Scholberg called “extremely potent.” She points out that the accomplishment doesn’t end there because the experiment also gave the researchers the opportunity to expand the global hunt for dark matter in a novel method. According to Pershey, the standard method for finding dark matter particles is to dig a hole, construct a highly sensitive detector, and then wait for the particles to simply pass through.
The issue? It’s possible that dark matter particles are moving through the air relatively lazily. They could not have enough energy to reach the detector in order to leave behind a detectable fingerprint if they are likewise very light. This problem is dealt with by the COHERENT team experimental setup. “You produce such particles at substantially higher energies when you enter a particle accelerator,” Pershey added. And that provides them a lot more force with which to strike nuclei and produce the dark matter signal.
What happens next, then? Not quite starting from scratch, but getting close. The much bigger and more powerful detector that will soon be installed at Neutrino Alley will considerably increase the likelihood of detecting one of these nefarious particles when combined with COHERENT’s improved search parameters. Pershey stated, “We’re on the verge of where the dark matter should be.”