Vincente Guiseppe, co-spokesperson of the Majorana Collaboration and a research staff member at ORNL, in front of the Majorana Demonstrator shield on the 4850 Level of SURF. Credit: Nick Hubbard/Sanford Underground Research Facility
For nearly six years, the Majorana Demonstrator quietly listened to the universe. Nearly a mile underground at the Sanford Underground Research Facility, or SURF, in Lead, South Dakota, the experiment collected data that could answer one of the most perplexing questions in physics: Why is the universe filled with something instead of nothing?
With the publication of the experiment’s final results, humanity is one step closer to finding that answer.
On Feb. 10, the Majorana Collaboration published its final results in Physical Review Letters. The experiment ran from 2015 through 2021 and was managed by Oak Ridge National Laboratory for the Department of Energy Office of Nuclear Physics with support from the National Science Foundation.
The final results prove that the techniques used by the collaboration could be deployed on a much larger scale to search for the rare, never-before-seen decay that could help explain the existence of matter in our universe.
“The purpose of the Majorana Demonstrator was to prove that our detector design and technology were advanced enough to justify the creation of a ton-scale experiment,” said Vincente Guiseppe, co-spokesperson of the Majorana Collaboration and a research staff member at ORNL. “This paper—the culmination of six years of data and the final word on the Majorana Demonstrator—proves that we have achieved what we set out to do.”
As focus shifts to the next-generation experiment, the collaboration reflects on success that was far from guaranteed a decade ago.
The question
The Majorana Demonstrator is linked to one of the biggest unanswered questions in particle physics: Why does matter exist in the universe, when everything we know about physics says it shouldn’t?
According to theory, the Big Bang should have created equal parts matter and antimatter—substances that annihilate upon meeting, leaving nothing in their wake but pure energy and, theoretically, an empty universe. Despite this prediction, we find ourselves in a universe replete with matter. Some unknown, hidden rule of nature must have tipped the scales to favor matter.
A leading hypothesis predicts that ghostly subatomic particles called neutrinos once had super-heavy partners whose decays in the early universe gave rise to the imbalance of matter and antimatter we see today. This hypothesis also predicts that a neutrino can act as its own antiparticle (also known as a “Majorana particle”). If this strange trait is observed, it could support the hypothesis and solve this mystery.
“A Majorana particle is one that is indistinguishable from its antimatter partner. This sets it apart from all other particles,” Guiseppe said. “With the Majorana Demonstrator, we are looking for this particle to induce a rare occurrence called neutrinoless double-beta decay.”
Just how rare is this proposed decay? To observe it in just two atoms, you’d have to wait over 1026 years.
“If we watch just one atom, waiting anxiously for it to decay, we would have to watch it for longer than the age of the universe. To win this game, we have to increase the mass we are watching,” Guiseppe said.
Researchers knew they were unlikely to detect neutrinoless double-beta decay with the amount of mass in the Majorana Demonstrator. The collaboration was in a global competition with other experiments to demonstrate its detector design and technology and to secure a bid for the scaled-up, next-generation experiment.
The cavern
In 2010, Cabot-Ann Christofferson, a South Dakota School of Mines and Technology chemist with the Majorana Demonstrator, swept her headlamp across the drift nearly a mile underground, watching dust waft through the beam of light. The ground was slick with mud, and the air was warm and thick with humidity. She wondered, “How will this ever be the laboratory we need?”
Because the signal that the collaboration sought was so faint, its detector had to be built at SURF, where 4,850 feet of rock would shield the detector from unwanted “background noise” such as the cosmic rays pelting the Earth’s surface.
The Majorana Demonstrator’s inner detector consisted of 30 kilograms of an enriched isotope of germanium suspended by strings of ultrapure copper and encased in a supercooled cryostat vessel. This inner detector was concealed behind further layers of shielding, including a layer of 108,000 pounds of lead. This structure would be in a clean room with as few as 100 particles of dust per cubic foot. Because of the Majorana Demonstrator’s sensitivity, even a few particles of dust or a single bead of human sweat would produce enough background radiation to render the detector useless.
Crews at SURF were tasked with transforming the former mine and its century-old infrastructure into the United States’ deepest underground science laboratory, capable of meeting the high standards of modern research.
But SURF was invested in the experiment’s success; teams of scientists, engineers and infrastructure technicians worked closely with the collaboration to create a space tailor-made for the experiment. “Every detail of the facility design reflected the cleanliness protocols and the environment that the experiment required to meet its science goals,” said Jaret Heise, SURF science director.
“The Majorana clean room is the cleanest lab space at SURF,” said Heise, noting that the laboratory’s cleaning protocols were informed by world experts in cleanroom operations. “In this laboratory, you must clean according to a detailed process, rather than any visual cue, because you’re often cleaning dirt that can’t be seen.”
The copper
With cavern construction underway, the collaboration got to work on a painstaking process that would prove to be a hallmark of the experiment’s success: electroforming the world’s purest copper.
“We were in a global competition with other experiments, with no time to waste, so we started electroforming in a temporary cleanroom on the 4850 Level before our permanent cavern was constructed,” Christofferson said.
Electroforming is a fantastically slow process where copper nuggets are first dissolved in a bath of ultrapure sulfuric acid. An electrical current pulls the copper, ion by ion, onto the surface of a metal cylinder. The process slowly forms a layer of exquisitely pure copper at the rate of one millimeter per month, leaving any trace impurities behind.
“The electroforming process creates such pure copper—a percentage with so many trailing nines—the purity couldn’t even be measured at the onset of the experiment in 2011,” Christofferson said. “Today, the technology has advanced, and we can measure the purity to parts per quadrillion.”
Over five years, more than five thousand pounds of copper were grown, then machined into detector components using dedicated tools in an underground machine shop.
The wait
By 2015, the cavern was unrecognizable: The detector was sealed behind a castle-like fortress of lead bricks in a bright, white-walled lab, silently taking in data.
Still, questions lingered. Would these efforts be enough to eliminate background noise? Could this new underground laboratory support world-class science? With the publication of first results in 2017, answers arrived.
While it didn’t detect the particle decay, the Majorana Demonstrator proved that a scaled-up experiment—one that is more than 33 times its size—might be able to do so.
“We created an environment so clean and so pure that we did not see any candidate events in our initial data caused by backgrounds, ” said Guiseppe upon the publication of first results. “We need to continue operating the demonstrator to study its performance and better estimate the backgrounds.”
The impact
After its initial science run, the collaboration pressed on, refining the detector design and analysis techniques.
With this rich set of data, the collaboration multitasked, adding exotic dark matter searches to its repertoire, as well as searches for possible interactions beyond the Standard Model of particle physics. The collaboration published 27 scientific articles, including six in 2022 and three so far in 2023, with additional papers expected.
The Majorana Demonstrator also provided an environment for scores of undergraduates and more than 25 Ph.D. candidates to gain experience in the design, construction and analysis of a world-leading experiment.
“Majorana has served as a training ground for a generation of outstanding young physicists,” said John Wilkerson, principal investigator of the experiment and professor of physics at the University of North Carolina, Chapel Hill. “And I think the experience of our students and postdocs being able to work underground really was fantastic.”
The future
The Majorana Demonstrator completed its science run in 2021. The collaboration’s triumphs, outlined in its recent publication, include a world-leading energy resolution and a confirmation that the half-life of neutrinoless double-beta decay in germanium-76 is greater than 8.3×1025 years—which is more than a million billion times the age of the universe.
The next-generation experiment, named LEGEND (Large Enriched Ge Experiment for Neutrinoless ββ Decay), will ultimately use 1 ton of enriched germanium to conduct the search. By scaling up exponentially, LEGEND may be able to strong-arm the neutrino into showing its hand.
“When we started this project, there were many risks and no guarantee that we could achieve our goals, as we were pushing into unexplored territory,” Wilkerson said. “Today, we’re one step closer to understanding the imbalance in the universe—and why we exist at all.”
Abridged from an article by Erin Lorraine Woodward of Sanford Underground Research Facility. The original version of this article is posted here.
