Oak Ridge National Laboratory neutrino detectors were used to investigate dark matter in a light basement hallway. The crew operates in Neutrino Alley beneath the Spallation Neutron Source, a particle accelerator. The COHERENT team saw dark matter, which is thought to make up 85% of the Universe, after years of theoretical computation. The experiment expanded the global hunt for dark matter, and the team plans to get a larger and more sensitive detector to catch additional dark matter particles.

Dark matter is mysterious. The term implies a cosmic secret.

COHERENT, a team of scientists led by Kate Scholberg, Arts & Sciences Distinguished Professor of Physics, Phillip Barbeau, associate professor of Physics, and postdoctoral scholar Daniel Pershey, attempted to bring dark matter out of the shadows of the Universe and into a brightly lit, narrow basement hallway.

A unique basement. The Oak Ridge National Laboratory team studies neutrinos in Neutrino Alley. Supernovas and particle accelerator proton collisions create them.

Neutrino Alley sits beneath Oak Ridge’s Spallation Neutron Source, the world’s most powerful particle accelerator (SNS). Neutrino Alley has detectors to watch neutrinos passing through and colliding.

SNS produces more than neutrinos. As particle accelerators smash protons, they also create dark matter. The COHERENT team used SNS and its neutrino detectors to see dark matter in Neutrino Alley after years of theoretical computation.

“And we didn’t see it,” Scholberg admits. Of course, seeing it would have been more exciting, but not seeing it is a significant deal.”

She says their neutrino detectors didn’t discover dark matter, so they could update their theoretical models.

“We knew the detector would respond to dark matter with certain properties, so we were seeking that precise fingerprint.”

The fingerprint is the way atom nuclei in the neutrino detector recoil when struck by a neutrino or dark matter particle.

Pershey described it like firing missiles at a bowling ball on ice. The atoms in the neutrino detector, a 14.6 kilogram cesium iodide crystal, are his bowling balls. How much the bowling ball recoils tells you about the projectile and its force.

Any dark matter knowledge is useful. It’s unknown. About 100 years ago, scientists understood that the Universe could not act the way it does if all it contained was the things we can see.

“We’re floating in a sea of dark matter,” said Jason Newby, Oak Ridge National Lab neutrino research group head and study co-author. Physicists agree that dark matter accounts up 85% of the Universe’s mass. To explain the Universe’s behavior, it must be gravity-bound yet black.

COHERENT’s experimental setup handles this.

Pershey noted that accelerators create particles at greater energy. They have greater power to knock into nuclei and produce the dark matter signal.

What now? Not nearly a rewrite. Neutrino Alley is getting a larger, more sensitive detector, which, together with COHERENT’s revised search criteria, will increase the odds of catching one of these diabolical particles.

“We’re at the doorway of where dark matter should be,” Pershey added.

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