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Cryostat
Researchers work on the delicate wiring of a cryostat, which chills the germanium detectors at the heart of the Majorana Demonstrator experiment. (Sanford Underground Research Facility Photo / Matthew Kapust)

An experiment conducted deep underground in an old South Dakota gold mine has given scientists hope that a future detector could help solve one of physics’ biggest puzzles: why the universe exists at all.

Put another way, the puzzle has to do with the fact that the universe is dominated by matter.

That may seem self-evident, but it’s not what’s predicted by Standard Model of particle physics as currently understood. Instead, current theory suggests that the big bang should have given rise to equal parts of matter and antimatter, which would annihilate each other within an instant.

Scientists suspect that there must have been something about the big bang that gave matter an edge more than 13 billion years ago. So far, the mechanism hasn’t been identified — but one leading theory proposes that the properties of neutrinos have something to do with it.

The problem is, neutrinos interact so weakly with other particles that it’s hard to detect what they’re doing. The experiment conducted in the nearly mile-deep Sanford Underground Research Facility in South Dakota was aimed at figuring out whether a detector could be shielded well enough from background radiation to spot the effect that scientists are looking for.

In a study published today in Physical Review Letters, an international research team reports that it’s possible. The finding thus strengthens the case for building a bigger detector to delve into the mystery.

“The excess of matter over antimatter is one of the most compelling mysteries in science,” John Wilkerson, a physicist at the Oak Ridge National Laboratory in Tennessee as well as at the University of North Carolina in Chapel Hill, explained in a news release.

“Our experiment seeks to observe a phenomenon called ‘neutrinoless double-beta decay’ in atomic nuclei,” said Wilkerson, who leads the six-nation Majorana Collaboration. “The observation would demonstrate that neutrinos are their own antiparticles and have profound implications for our understanding of the universe.”

Detecting neutrinoless double-beta decay was rated as a top priority for the nuclear physics community in a 2015 report prepared by the U.S. Nuclear Science Advisory Committee for the U.S. Department of Energy and the National Science Foundation.

The concept relates to a mode of nuclear decay in which two neutrons are typically converted into two protons, emitting two electrons and two antineutrinos in the process. That’s what’s known as two-neutrino double-beta decay.

The Majorana Collaboration is looking for a different kind of decay mode, involving the emission of two electrons, a neutrino and an antineutrino. The neutrino and its antimatter counterpart would cancel out each other in a flash — which would be the signature of neutrinoless decay.

Finding evidence of neutrinoless decay would demonstrate that the rules of the Standard Model needn’t always apply when it comes to the balance of matter and antimatter particles.

The collaboration’s supercooled experimental apparatus, known as the Majorana Demonstrator, monitored the energy that was released as crystals of radioactive germanium-76 underwent nuclear decay to turn into selenium-76. (Majorana refers to Italian physicist Ettore Majorana, who came up with the theory behind neutrinoless decay in the 1930s.)

“It’s a common misconception that our experiments detect neutrinos,” said University of Washington physicist Jason Detwiler, who is a co-spokesperson for the collaboration. “It’s almost comical to say it, but we are searching for the absence of neutrinos. In the neutrinoless decay, the released energy is always a particular value. In the two-neutrino version, the released energy varies but is always smaller than for neutrinoless double-beta decay.”

Between June 2015 and March 2017, the researchers saw many events with the energy profile of two-neutrino decay. They didn’t see any events with the profile of neutrinoless decay, but that wasn’t surprising. From the start, the researchers assumed that the rule-breaking effect would be exceedingly rare — at least 100,000 times rarer than two-neutrino decay.

The bottom line is that the case hasn’t yet been cracked, but researchers should be able to see the signature of neutrinoless decay if they can build a bigger detector. Such a detector, known as LEGEND (Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay), is in the planning stage.

“Observing neutrinoless double-beta decay would be a major step forward in understanding the predominance of matter in the universe,” Detwiler said. “It is one of the most compelling questions in theoretical physics, and impacts fundamental questions about where we come from and why we exist.”

The study published in Physical Review Letters is titled “Search for Neutrinoless Double-Beta Decay in 76Ge with the Majorana Demonstrator.” In addition to Detwiler, the UW co-authors include Sebastian Alvis, Micah Buuck, Clara Cuesta, Peter Doe, J.A. Dunmore, Z. Fu, Julieta Gruszko, Ian Guinn, R.A. Johnson, A. Knecht, J. Leon, M.G. Marino, Michael Miller, Walter Pettus, Hamish Robertson, Nicholas Ruof and A.G. Schubert.

The Majorana Collaboration also has researchers from the Pacific Northwest National Laboratory: C.E. Aalseth is the principal author, and other co-authors affiliated with PNNL include E. Aguayo, I.J. Arnquist, R.L. Brodzinski, J.E. Fast, E. Fuller, E.W. Hoppe, T.W. Hossbach, J.D. Kephart, R.T. Kouzes, B.D. LaFerriere, J.H. Merriman, H.S. Miley, J.L. Orrell, N.R. Overman and J.H. Reeves.

For more about the quest to detect neutrinoless decay and its implications, check out “The Hunt for No Neutrinos” on the APS Physics website.

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