In Neutrino Alley's basement, I get to see dark matter

In Neutrino Alley's basement, I get to see dark matter ...

This image is a dark matter particle. Oak Ridge National Laboratory scientists attempted to observe these obscure particles using their neutrino detectors in Neutrino Alley.

Dark matter has an aura of mystery similar to few things. The name itself elicits secrecy, suggesting something hidden in the dark.

COHERENT is a collaborative effort of scientists led by Kate Scholberg, an Arts & Sciences Distinguished Professor of Physics, Phillip Barbeau, an associate professor of Physics, and postdoctoral scholar Daniel Pershey. Dark matter was brought out of the Universe's shadows and into a somewhat less glamorous destination: a brightly lit, narrow corridor in a basement.

The lab is not just a basement, either. Neutrino Alley, in Oak Ridge National Laboratory's Neutrino Alley, is a research lab dedicated to subatomic particles called neutrinos. They are formed when stars die and become supernovas, or, at a deeper level, as a result of proton collisions in particle accelerators.

NASA/CXO/Fabian/Marsolais et al. X-ray: NASA/AUI/NSF Optical: NASA, SDSS

The COHERENT team sought to utilize SNS's power and sensitivity to observe dark matter in Neutrino Alley.

Scholberg adds, "It's unimaginable." "It's certainly possible that we'd have seen it sooner, but not seeing it is a big deal."

The fact that their neutrino detectors failed to detect dark matter allows them to greatly refine their theoretical understandings of what dark matter may be.

"If dark matter had certain characteristics, we knew exactly how the detector would respond to dark matter," said the researcher. We were looking for that specific fingerprint.

Grayson Rich, her co-author, and Philip Barbeau are all working on this project. Credit: Long Li/Duke University

When struck by a neutrino or in this case, by a dark matter particle, the nuclei of the neutrino detector recoil.

Pershey explained that throwing projectiles at a bowling ball on ice is like throwing a ball at a bowling ball. The bowling balls are the atoms contained in the neutrino detector, which in this case was a 14.6 kg cesium iodide crystal.

Any information is good information when it comes to dark matter. No one really knows what it is. Almost 100 years ago, physicists realized that the Universe would not function as it did if all it contained was the information we can see.

"We're dangling in a sea of dark matter," said Jason Newby, the study's group leader at Oak Ridge National Lab. It's estimated that dark matter accounts for about 85% of the Universe's mass, but it does not interact with any sort of light or electromagnetic radiation, thus becoming dark.

At Neutrino Alley, Jason Newby and co-author Yuri Efremenko hold a remarkable 14.6 kg cesium iodide neutrino detector used to search for dark matter. Credit: Genevieve Martin/Oak Ridge National Laboratory, US Department of Energy

“We discovered this by looking at large galaxies rotating around each other and finding that they rotate faster than they ought to, implying that they have more mass than they appear to have,” said Pershey.

"Even if you're in the realm of mostly no results," said Newby, "it's really important that wherever you can look, you look, and then you can rule out a whole number of possibilities and focus on a new area with strategy rather than just using a'spaghetti on the wall' approach."

Daniel Pershey. Credit: Duke University

"We're broadening our understanding of what dark matter models are capable of, and that's very powerful," Scholberg said.

She adds that the achievement does not stop there: the experiment allowed the group to search for dark matter in a different way.

"The typical detection technique is to go underground, construct a very sophisticated detector, and wait for these dark matter particles to just pass through," according to Pershey.

What is the problem? Dark matter particles may be traveling quite leisurely through the air. If they also happen to be very light, they may not be able to detect a fingerprint.

This issue is addressed by the COHERENT team's experimental setup.

Pershey said: "You produce those particles at significantly higher energies when you go to an accelerator." "And that gives them a lot more oomph to knock into nuclei and make the dark matter signal appear."

So, what do we do now? Neutrino Alley is currently working on a larger and more sensitive detector that, combined with COHERENT's more sophisticated search parameters, will significantly increase the probability of catching one of these nasty things.

Pershey said "we're at the point where the dark matter should be."

D. Akimov and his colleagues conduct a FIRST PROBE of Sub-GeV Dark Matter Beyond the Cosmological Expectation with the COHERENT CsI Detector at the SNS, 3 February 2023, in Physical Review Letters. DOI: 10.1103/PhysRevLett.130.051803

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