If equal amounts of matter and antimatter had formed in the Big Bang more than 13 billion years ago, they would have annihilated one other upon meeting — and today’s universe would be full of energy but no matter to form stars, planets and life.

Yet, matter exists now. That fact suggests something is wrong with the Standard Model of Physics — which as written states that there is symmetry between subatomic particles and their so-called “antiparticles.” In a study March 26 in , collaborators of the , an experiment led by the Department of Energy’s Oak Ridge National Laboratory and including researchers at the 91̽��, have shown they can shield a sensitive, scalable 44-kilogram germanium detector array from background radioactivity.

This accomplishment, which involves a collaboration among 129 researchers from 27 institutions and 6 nations, is critical to developing a much larger future experiment to study the nature of neutrinos. 91̽��researchers in the collaboration are based in the and the .

“The excess of matter over antimatter is one of the most compelling mysteries in science,” said of ORNL and the University of North Carolina, Chapel Hill, who leads the MAJORANA DEMONSTRATOR. “Our experiment seeks to observe a phenomenon called ‘neutrinoless double-beta decay’ in atomic nuclei.”

Observing neutrinoless double-beta decay would show that neutrinos are their own antiparticles, according to Wilkerson. If so, physicists would have to rewrite the Standard Model.

“Observing neutrinoless double-beta decay would be a major step forward in understanding the predominance of matter in the universe,” said , a 91̽��professor of physics and co-spokesperson for the MAJORANA Collaboration. “It is one of the most compelling questions in theoretical physics and impacts fundamental questions about where we come from and why we exist.”

Neutrinoless double-beta decay has never been observed, though have sought it. One of the keys to detecting this long-theorized form of atomic nuclear decay lies in minimizing background effects that could be mistaken for the real phenomenon.

That was the key accomplishment of the MAJORANA DEMONSTRATOR. This experiment was completed in South Dakota in September 2016 at the . Setting the experiment under nearly a mile of rock was the first of many steps collaborators took to reduce interference from background effects. Other steps included a cryostat made of the world’s purest copper and a complex six-layer shield to eliminate interference from cosmic rays, radon, dust, fingerprints and naturally occurring radioactive isotopes.

There are many ways for an atomic nucleus to fall apart. In two-neutrino double-beta decay — a process that has been observed — two neutrons decay simultaneously to produce two protons, two electrons and two antineutrinos. But the MAJORANA Collaboration seeks evidence for a decay process in which no neutrinos are emitted.

Observing neutrinoless double-beta decay would contradict a principle that was written into the Standard Model: The conservation of the number of leptons. Leptons are subatomic particles such as electrons and neutrinos. Many theorists believe that lepton number is actually not conserved, and that the neutrino and the antineutrino are really the same particle spinning in different ways. Italian physicist introduced this concept in 1937, predicting the existence of particles that are their own antiparticles.

The MAJORANA DEMONSTRATOR uses germanium crystals as both the source of double-beta decay and the means to detect it. The scientists distinguish between two-neutrino and neutrinoless decay modes by their energy signatures.

“It’s a common misconception that our experiments detect neutrinos,” said Detwiler, who is also a co-author on the paper. “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.”

The MAJORANA DEMONSTRATOR has shown that the neutrinoless double-beta decay half-life of germanium-76 is at least 10²⁵ years — 15 orders of magnitude longer than the age of the universe. That’s a long time to wait.

“We get around the impossibility of watching one nucleus for a long time by instead watching on the order of 10²⁶ nuclei for a shorter amount of time,” said co-spokesperson Vincente Guiseppe of the University of South Carolina.

Chances of spotting a neutrinoless double-beta decay in germanium-76 are no more than 1 for every 100,000 two-neutrino double-beta decays, Guiseppe said. But using detectors containing large amounts of germanium atoms increases the probability of spotting the rare decays.

Between June 2015 and March 2017, the scientists observed no events with the energy profile of neutrinoless decay, an absence that had been expected given the small number of germanium nuclei in the detector. But they were encouraged to see many events with the energy profile of two-neutrino decays, verifying the detector could spot the decay process that has been observed.

The MAJORANA Collaboration’s results coincide with new results from , a parallel experiment in Italy.

“The MAJORANA DEMONSTRATOR and GERDA together have the lowest background of any neutrinoless double-beta decay experiment,” said ORNL’s .

The DEMONSTRATOR was designed to demonstrate that backgrounds can be low enough to justify building a larger detector. The MAJORANA DEMONSTRATOR will continue to take data for two or three years. Meanwhile, a potential merger with GERDA is in the works to develop a one-ton detector called .

“This merger leverages public investments in the MAJORANA DEMONSTRATOR and GERDA by combining the best technologies of each,” said LEGEND co-spokesperson Steve Elliott of Los Alamos National Laboratory, who was a long-time spokesperson for MAJORANA until 2017.

Scientists hope to start on the first stage of LEGEND by 2021.

Other 91̽��co-authors on the paper are 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, , Nicholas Ruof and A.G. Schubert. The research was funded by the U.S. Department of Energy Office of Science and the National Science Foundation.

###

Adapted from a by ORNL.

For more information, contact Detwiler at 206-543-4054 or jasondet@uw.edu.

The prize recipients, Takaaki Kajita and Arthur McDonald, respectively led the in Japan and the in Canada. As the Emerald City awoke to the news, the two teams of 91̽�� researchers who were members of these multinational, decades-long scientific groups began to celebrate.

“It feels great,” said 91̽��physics professor . “We’re glad the recognition came for the hard work everyone has done.”

Meanwhile, physics professor has led the 91̽��team working with the Sudbury Neutrino Observatory, or SNO, which includes over thirty professors, students and other researchers from the 91̽��on experiments that were in 2006.The endeavors honored by the 2015 Nobel Prize in Physics were large-scale, multiyear experiments to measure the fundamental properties of neutrinos. Wilkes and the late 91̽��physics professor began the UW’s involvement with the experiments at Super-Kamiokande, or Super-K, when the facility was still under construction in the early 1990s. A team of about a dozen 91̽��postdoctoral researchers, graduate students and engineers later joined them on the Super-K Collaboration.

“Before these experiments, people used to think neutrinos had no mass, and there were many other unanswered questions about their behavior,” said 91̽��physics professor , who worked on the SNO collaboration and still conducts neutrino research.

As early risers for the Oct. 6 announcement learned, neutrinos are the second most common type of particle in our universe — after light. Yet neutrinos are difficult to detect and measure with precision. They are ubiquitous and easily form when other types of particles collide or undergo radioactive decay. But they are tiny, electrically neutral — neutrino means “little neutral one” — and can easily pass through light years of matter without interacting with it. Neutrinos also come in three “flavors,” but the differences between flavors have proven difficult to tease out. At times neutrinos, especially those generated by the sun, seemed to disappear as they traveled to Earth.

In the 1990s, two international groups of researchers constructed elaborate facilities to learn some basic facts about neutrino behavior. Both teams wanted to detect neutrinos that form far above our heads — Super-K focused on neutrinos generated in Earth’s atmosphere while SNO sought out neutrinos from the sun. But to do this, they had to conduct their experiments in underground mines.

“Cosmic rays sometimes hit the upper atmosphere, creating a shower of particles that you and I don’t notice, but they completely block out any signal from neutrinos on the surface,” said Detwiler. “So you have to take your neutrino detectors far underground.”

Both Super-K and SNO used massive tanks of ultrapure liquids to detect neutrinos. In the SNO facility, in a spherical tank made of clear acrylic served as the detection medium.

“When a neutrino comes in and interacts with the water, it makes little flashes of light,” said Detwiler. “Outside this huge spherical tank, they mounted almost 10,000 light detectors to measure these flashes.”

Super-K used a tank of purified water to detect neutrinos. Both facilities required years of construction and calibration before experiments could begin. 91̽��teams pored over and analyzed data along with their collaborators at other institutions. But within just a few years of each other, both international teams announced major discoveries.

The Super-K collaboration discovered that atmospheric neutrinos can “oscillate” between at least two flavors. In addition, months before Young’s death in 1998, the group that neutrinos do indeed have mass. Three years later, SNO confirmed neutrino oscillation among all three flavors, which explained why solar neutrinos seemed to vanish by the time they reach Earth. On Oct. 6, the Nobel committee specifically cited these groundbreaking discoveries during the prize announcement.

91̽��researchers are not yet finished with the subject.

“There are still many questions about neutrinos,” said Wilkes. “For example, we don’t know their mass — just that they have mass, it’s very small and we don’t know why.”

Neutrinos may also be a route toward detecting new types of physical interactions, added Detwiler.

Unanswered questions aside, the 91̽��Department of Physics plans to celebrate the SNO and Super-K experiments. On Monday Oct. 12 at 4:00 p.m., the department will hold a colloquium in the in room A102 to revisit the contributions of 91̽��researchers.

###

For more information, contact Wilkes at wilkes@u.washington.edu or 206-543-4232 and Robertson at rghr@uw.edu or 206-616-2745.