Twelve faculty members at the 91±ŹÁÏ have been elected to the Washington State Academy of Sciences. They are among 36 scientists and educators from across the state July 17 as new members. Election recognizes the new member’s “outstanding record of scientific and technical achievement and willingness to assist the Academy in providing the best available scientific information and technical understanding to inform complex policy decisions in Washington.”Ìę

The 91±ŹÁÏ faculty members were selected by current WSAS members or by their election to national science academies. Eleven were voted on by current WSAS members:Ìę

, professor, Bill & Melinda Gates Chair, and director of the Paul G. ÌęAllen School for Computer Science & Engineering, for “contributions in data management for data science, big data systems, cloud computing and image/video analytics and leadership in data science education.”Ìę

professor of civil & environmental engineering and of industrial & systems engineering, for “pioneering work in human mobility analysis and infrastructure resilience, which have transformed transportation systems in terms of both demand and supply, and shaped the future directions of transportation systems research on community-based solutions and disaster resilience.”Ìę

Lloyd and Kay Chapman Endowed Chair for Oral Health and associate dean for research in the 91±ŹÁÏ School of Dentistry, and professor in the Department of Health Systems & Population Health, for “leadership in understanding and addressing children’s oral health inequities through community-based socio-behavioral interventions and evidence-based policies.”Ìę

professor of biology, for “internationally recognized leadership in the biology of sleep, including groundbreaking research on molecular and genetic aspects of the brain, human behavioral studies on learning under varied sleep schedules, and contributions that have shaped policy on school schedules and standard time.”Ìę

, the Virginia and Prentice Bloedel professor of physics and of electrical & computer engineering, for “foundational contributions to fundamental and applied research on the optical and spin properties of quantum point defects in crystals and for service and leadership in the quantum community.” Ìę

, professor and chair of human centered design and engineering, for “award-winning leadership in HCI computing, whose research has advanced health and education technology, influenced policy, and shaped the HCI field of through impactful scholarship, interdisciplinary collaboration and inclusive, real-world technology design.”Ìę

, professor and associate dean for research in the 91±ŹÁÏ School of Social Work, for “contributions to understanding psychosocial and physiological factors that moderate the effectiveness of their interventions and ultimately improve the health of children with abdominal pain disorders.”Ìę

, professor of medicine in the 91±ŹÁÏ School of Medicine and of pharmacy, “for leadership in health economics and cancer research, including work on financial toxicity, cost- effectiveness, and healthcare policy that has influenced national discussions, improved cancer care access, and shaped policies for equitable and sustainable healthcare.” Ramsey is also Director of the Cancer Outcomes Research Program at Fred Hutch.Ìę

, professor of bioengineering and Vice Dean of Research and Graduate Education in the 91±ŹÁÏ School of Medicine, for “national leadership in biomedical research, research policy, and graduate education, including pioneering novel drug delivery approaches for regenerative medicine applications in the nervous system and other tissues such as bone, cartilage, tendon and skin.”Ìę

, Rabinowitz Endowed Professor of Earth and space sciences, for “revolutionizing our understanding of climate change in Antarctica through pioneering ice core extractions under hazardous Antarctic conditions and their subsequent analyses over two decades, and for applying that expertise to advance climate research in Washington State.”Ìę

, professor of pharmaceutics, for “pioneering contributions to pharmaceutical and translational sciences, including groundbreaking research on drug transporters, PBPK modeling and maternal-fetal pharmacology that have helped shaped drug safety policies.”Ìę

The Academy also welcomed new members who were selected by virtue of their election to the National Academies of Science, Engineering or Medicine. Among them is , the Arthur B. McDonald professor of physics and director of the Center for Experimental Nuclear Physics and Astrophysics. Hertzog was elected to the National Academy of Sciences last year. Ìę

Physicists use a theory called to describe how the universe works at its most fundamental level. Scientists from around the world have tested this theory in order to determine if there are other forces or particles we have not yet discovered.

One test involves trying to precisely measure the of a subatomic particle called a muon (pronounced “mew-on”). Muons are similar to electrons, but about 200 times more massive. Precisely measuring the muon’s magnetic moment, or the magnetic anomaly, will help scientists understand whether it is interacting solely with known particles and forces, or if unknown particles or forces exist.

Scientists working on the (pronounced “mew-on gee-minus-two”), released two earlier datasets that attempted to precisely measure the muon’s magnetic moment. These datasets had some discrepancy between the measured values and calculated values.

On June 3, the team released the third and final measurement of the muon magnetic anomaly. This result agrees with the published results from 2021 and 2023 but with a much better precision of 127 parts per billion, surpassing the original experimental design goal of 140 parts per billion. This new measurement is of the magnetic anomaly. These findings have been submitted to the journal Physical Review Letters.

“Now we’ve published the final dataset and again we obtained a consistent result, but with even smaller uncertainties,” said , a 91±ŹÁÏ professor of physics and director of the 91±ŹÁÏ Center for Experimental Nuclear Physics and Astrophysics. “This is a legacy experiment with extraordinary precision on a very fundamental quantity. It probes the Standard Model in a broad and impactful manner. If these newest results are true, they would suggest no new physics, but it’s ‘stay tuned’ for now because it will take more time for researchers in the theory community to come to their own precise prediction.”

The Muon g-2 experiment looks at what’s known as “the wobble” of the muon. Like electrons, muons have a quantum mechanical property called “spin” that can be interpreted as a tiny internal magnet. In the presence of an external magnetic field, the internal magnet will wobble — or precess — like the axis of a spinning top.

The precession speed depends on the muon’s magnetic moment, typically represented by the letter g. At the simplest level, theory predicts that g should equal 2. Any difference of g from 2 — or “g minus 2” — could be attributed to the muon’s interactions with other particles pulling at the muon’s precession.

The Standard Model predicts how g should change based on the electromagnetic, weak nuclear and strong nuclear forces, as well as particles such as photons, electrons, quarks, gluons, neutrinos, W and Z bosons, and the Higgs boson. But if the measured value of g is different from the Standard Model’s value, it could suggest the possible existence of as-yet-undiscovered particles or forces that could contribute to the value of g-2 — and this could open the window to exploring new physics.

When measurements taken at Brookhaven National Laboratory in the late 1990s and early 2000s showed a possible discrepancy with the theoretical calculation at that time, physicists decided to upgrade the Muon g-2 experiment to make a more precise measurement.

For this experiment, the Muon g-2 collaboration repeatedly sent a beam of muons into a 50-foot-diameter superconducting magnetic storage ring, where muons circulated about 1,000 times at nearly the speed of light. Detectors lining the inside of the ring — including 91±ŹÁÏ-designed and built NMR probes to measure the magnetic field and calorimeters to reconstruct the decay positrons — helped them determine how rapidly the muons were precessing.

This experiment, which included improved techniques, instrumentation and simulations, collected data for six years before shutting down the muon beam on July 9, 2023 with a dataset more than 21 times the size of the Brookhaven’s original dataset.

This final measurement is based on the analysis of the last three years of data, taken between 2021 and 2023, combined with the previously published datasets. This more than tripled the size of the dataset used for their second result in 2023, and it enabled the collaboration to finally achieve the precision goal proposed in 2012.

The latest experimental value of the magnetic moment of the muon from the Fermilab experiment is:

(g-2)/2 = 0.001165920705 +/- 0.000000000114(stat.) +/- 0.000000000091(syst.)

The first number is the calculation, and the second and third are statistical and systematic uncertainties, respectively.

This dataset also represents an analysis of the experiment’s best-quality data. Toward the end of their second data-taking run, the Muon g-2 collaboration finished tweaks and enhancements to the experiment that improved the quality of the muon beam and reduced uncertainties.

The Muon g-2 experiment is based at the , a Department of Energy facility near Chicago. At last count, the team includes 179 scientists at 37 institutions in seven countries.

Researchers with the 91±ŹÁÏ have been part of the Muon g-2 team from the beginning, designing and constructing detectors as well as leading efforts to analyze the massive amounts of data collected. In addition to Hertzog, other 91±ŹÁÏ scientists involved in the team’s latest efforts includes , research professor of physics; Erik Swanson, a research engineer with CENPA; and current and former postdoctoral researchers Jarek Kaspar, Zach Hodge, Svende Braun, Christine Claessens, Brynn MacCoy and Joshua LaBounty. Hertzog noted that seven former 91±ŹÁÏ doctoral students — Rachel Osofsky, Matthias Smith, Nathan Froemming, Aaron Fienberg, Hannah Binney, Brynn MacCoy and Joshua LaBounty — based their dissertations on this experiment.

“Our 91±ŹÁÏ group even spawned three new g-2 groups as postdocs who went on to be professors Ìębuilt their own groups,” Hertzog said.

This result will remain the world’s most precise measurement of the muon magnetic anomaly for many years to come. Despite recent challenges with the theoretical predictions that reduce evidence of new physics from Muon g-2, this result provides a stringent benchmark for proposed extensions of the Standard Model of particle physics.

“This is a very exciting moment because we not only achieved our goals but exceeded them, which is not very easy for these precision measurements,” said Peter Winter, who was a postdoctoral researcher in Hertzog’s group and is now a physicist at Argonne National Laboratory and the co-spokesperson for the Muon g-2 collaboration. “With the support of the funding agencies and the host lab, Fermilab, it has been very successful overall, as we reached or surpassed pretty much all the items that we were aiming for.”

While the experiment’s main analysis has come to an end, there is more to be mined from the six years of Muon g-2 data. In the future, the collaboration will produce measurements of a property of the muon called the electric dipole moment as well as tests of a fundamental property of physical laws known as charge, parity and time-reversal symmetry.

“Of course, it’s sad to end such an endeavor because it’s been a large part of many of our collaborators’ lives,” said Winter, who has been part of the collaboration since 2011. “But we also want to move to the next physics that’s out there, to do our best to advance the field in other areas. I think it will be a textbook experiment that will be a long-lasting reference for many future decades to come.

For more information, contact Hertzog at hertzog@uw.edu.

Adapted from by Fermilab.

A particle physics experiment decades in the making — the — looks increasingly like it might set up a showdown over whether there are fundamental particles or forces in the universe that are unaccounted for in the current , the comprehensive theory that physicists use to describe how the universe works at its most fundamental level.

On Aug. 10, the international team of scientists behind Muon g-2 — pronounced “g minus 2” — the world’s most precise measurement yet of the anomalous magnetic moment of the muon. Muons are subatomic particles similar to electrons, but about 200 times more massive. Calculating the muon’s magnetic moment at a high precision will indicate whether it is interacting solely with the particles and forces known today, or if unknown particles or forces are out there.

“The result we released today confirms earlier findings, but at a much higher level of precision,” said , a 91±ŹÁÏ professor of physics and director of the 91±ŹÁÏÌę. “Further on, we still have 75% of the data to analyze and already we have exceeded our uncertainty goals. The experiment has been extremely successful”.

The Muon g-2 experiment is based at the , a Department of Energy facility near Chicago. At last count, the team includes more than 180 scientists at 33 institutions in seven countries.

Researchers with the 91±ŹÁÏÌęÌęhave been part of the Muon g-2 team from the beginning, designing and constructing detectors as well as leading efforts to analyze the massive amounts of data collected. In addition to Hertzog, other 91±ŹÁÏ scientists involved in the team’s latest efforts includeÌę, research professor of physics; Erik Swanson, a research engineer with CENPA; and current and former postdoctoral researchers Svende Braun, Christine Claessens, Jarek Kaspar and Zach Hodge. Hertzog noted that seven 91±ŹÁÏ doctoral students, including recent graduates Brynn MacCoy and Hannah Binney, based their dissertations on this experiment. An eighth 91±ŹÁÏ doctoral degree from muon endeavors is forthcoming from Joshua Labounty.

“Working on Muon g-2 has been incredibly exciting,” said Labounty. “As we continue to push toward a higher-precision measurement, we’ve encountered new puzzles, which required a great deal of out-of-the-box thinking. Even processing the petabytes of data necessary for this result has been a challenge.”

The collaboration’s new measurement for the muon’s anomalous magnetic moment is twice as precise as a previous measurement released by the team in 2021. Their findings have been submitted to the journal Physical Review Letters.

By comparing theories built using the Standard Model to experimental results, physicists have been trying to discern whether the theory is complete — that is, whether all particles and forces are known — or if there is physics “beyond the Standard Model.” Muons have been playing an increasing role in the gentle tug-of-war between theorists and experimentalists.

Like their less massive cousin, the electron, muons have a tiny internal magnet that, in a magnetic field, precesses, or wobbles, like the axis of a spinning top. The precession speed depends on the muon’s magnetic moment, typically represented by the letter g; at the simplest level, theory predicts that g should equal 2. Any difference of g from 2 — or “g minus 2” — could be attributed to the muon’s interactions with particles blinking in and out of existence in a quantum foam that surrounds it.

The Standard Model predicts how the quantum foam should change g based on the electromagnetic, weak nuclear and strong nuclear forces, as well as photons, electrons, quarks, gluons, neutrinos, W and Z bosons, and the Higgs boson. But physicists are excited about the possible existence of as-yet-undiscovered particles or forces that could contribute to the value of g-2 — and would open the window to exploring new physics.

The new result, based on the first three years of data collected at the team’s experimental set-up at Fermilab, is:
g-2 = 0.00233184110 +/- 0.00000000043 (stat.) +/- 0.00000000019 (syst.)

The first number is the calculation, and the second and third are statistical and systemic uncertainties, respectively.

To make the measurement, the Muon g-2 collaboration repeatedly sent a beam of muons into a 50-foot-diameter superconducting magnetic storage ring, where muons circulated about 1,000 times at nearly the speed of light. Detectors lining the inside of the ring — including 91±ŹÁÏ-designed and built NMR probes to measure the magnetic field and calorimeters to reconstruct the decay positrons — helped them determine how rapidly the muons were precessing.

The Fermilab experiment reused a storage ring originally built for a predecessor experiment at Brookhaven National Laboratory that concluded in 2001. Officials carefully transported the storage ring 3,200 miles from Long Island to Fermilab. The Muon g-2 experiment, which included improved techniques, instrumentation and simulations, collected data for six years before shutting down the muon beam on July 9, 2023 with a dataset more than 21 times the size of the Brookhaven’s.

This new measurement of g-2, which comes from analyzing the first three years of data, corresponds to a precision of 0.20 parts per million.

In 2020, the Muon g-2 Theory Initiative, a related group, announced its best Standard Model prediction for muon g-2 based on data available at the time. A newer experimental measurement of the data, as well as a calculation based on a different theoretical approach, are in tension with the 2020 calculation. The initiative aims to have a new, improved prediction available in the next couple of years that considers both theoretical approaches.

That should come right around the time that the Muon g-2 collaboration anticipates releasing its final, most precise measurement of the muon magnetic moment — setting up an ultimate showdown between Standard Model theory and experiment.

Until then, physicists have a new and improved measurement of muon g-2 that is a significant step toward the endeavor’s final physics goal.

For more information, contact Hertzog at hertzog@uw.edu.

Adapted from a by Fermilab.

An international research team, including scientists from the 91±ŹÁÏ, has established a new upper limit on the mass of the neutrino, the lightest known subatomic particle.

In a published Feb. 14 in Nature Physics, the collaboration — known as the or KATRIN — reports that the neutrino’s mass is below 0.8 electron volts, or 0.8 eV/c². Honing in on the elusive value of the neutrino’s mass will solve a major outstanding mystery in particle physics and equip scientists with a more complete view of the fundamental forces and particles that shape ourselves, our planet and the cosmos.

KATRIN, based in Germany at the Karlsruhe Institute of Technology, has been hunting for the neutrino’s mass since the experiment began collecting data in 2018. The team’s first reported measurement in 2019 cut the upper limit for this value almost in half, from 2 eV/c² to about 1.1 eV/c². With the new findings reported this month, the upper limit drops below 1 eV/c² for the first time.

The of particle physics once predicted that neutrinos shouldn’t have a mass. But experiments in the early 2000s at the Super-Kamiokande and the Sudbury Neutrino Observatory detectors demonstrated that they actually do have a small mass, a discovery in 2015 with the Nobel Prize in Physics.

Though that mass is very small, it has had a major impact because neutrinos are so numerous, according to co-author , a KATRIN team member and research professor of physics at the 91±ŹÁÏ.

“There are almost as many neutrinos in the universe as there are photons,” said Doe. “So, although the neutrino mass is tiny, their abundance results in them playing an important role in the evolution of the large-scale structures of the universe, such as the distribution of galaxies. Determining the neutrino mass would also enable further refinement of the standard models of and of . For these reasons, the measurement of the mass scale of the neutrino is of great importance to both particle physics and cosmology.”

To measure neutrino mass, KATRIN makes use of the beta decay of tritium, an unstable isotope of hydrogen. The team takes precision measurements of the energy spectrum of electrons released by the decay process. The neutrino mass is revealed in a minute distortion within that spectrum. But collecting data about these small particles is a big undertaking: The experiment utilizes the worldÂŽs most intense tritium source as well as a giant spectrometer to measure the energy of decay electrons with extremely high precision.

“KATRIN is an experiment with the highest technological requirements and is now running like perfect clockwork,” said co-author and KATRIN co-spokesperson of the KIT.

The 91±ŹÁÏ is a founding member of the KATRIN collaboration, which was formed in 2001. Under the direction of co-author , a 91±ŹÁÏ professor emeritus of physics, the 91±ŹÁÏ was the lead U.S. institution for designing and acquiring KATRIN’s electron detection system. Led by co-author , a 91±ŹÁÏ research associate professor of physics, 91±ŹÁÏ efforts now focus on developing data analysis tools for KATRIN experiments, as well as understanding systematic errors in the detector system.

Data taken by the experiment in 2019 and 2021 allowed KATRIN scientists to narrow the upper limit on the neutrino mass by more than a factor of two. The KATRIN experiment will continue to collect data until 2024, with the goal of reaching a sensitivity 4 times greater than what the collaboration has achieved to date.

Previous, indirect experiments by other groups suggest that the lower limit for the neutrino’s mass at 0.02 eV/c².Ìę But the technique employed by KATRIN cannot practically determine a mass below 0.2 eV/c². A new endeavor, , plans to reach an upper limit sensitivity of 0.04 eV/c², according to Doe. Project 8 will measure the neutrino’s mass by making use of an atomic tritium source — rather than molecular tritium — and will track the electron energy using a novel detection technique that was recently demonstrated at the 91±ŹÁÏ.

Menglei Sun, a former postdoctoral researcher in the 91±ŹÁÏ Center for Experimental Nuclear Physics and Astrophysics, is also a co-author on the paper. KATRIN efforts in the U.S. are funded by the U.S. Department of Energy’s Office of Nuclear Physics.

For more information, contact Doe at pdoe@uw.edu.

Adapted from a by the Massachusetts Institute of Technology.

The first results from the at the U.S. Department of Energy’s have revealed that fundamental particles called muons behave in a way that is not predicted by scientists’ best theory to date, the of particle physics. This landmark result, , confirms a discrepancy that has been gnawing at researchers for decades.

The strong evidence that muons deviate from the Standard Model calculation might hint at exciting new physics. The muons in this experiment act as a window into the subatomic world and could be interacting with yet-undiscovered particles or forces.

“This experiment is a bit like a detective story,” said team member , a 91±ŹÁÏ professor of physics and a founding spokesperson of the experiment. “We have analyzed data from the Muon g-2’s inaugural run at Fermilab, and discovered that the Standard Model alone cannot explain what we’ve found. Something else, perhaps beyond the Standard Model, may be required.”

The Muon g-2 experiment is an international collaboration between Fermilab in Illinois and more than 200 scientists from 35 institutions in seven countries. 91±ŹÁÏ scientists have been an integral part of the team through the Ìę— constructing sensitive instruments and sensors for the experiment, and leading data analysis endeavors. In addition to Hertzog, current 91±ŹÁÏ faculty and lead scientists involved include , research professor of physics; Erik Swanson, a research engineer with the 91±ŹÁÏ’s , or CENPA; Jarek Kaspar, a research scientist; and , a professor of physics.

“The 91±ŹÁÏ custom-built instrumentation would not have been possible without the extraordinary dedication and expertise of our CENPA technical staff, who work closely with our postdocs and graduate students,” said Hertzog.

A muon is about 200 times as massive as its cousin, the electron. They occur naturally when cosmic rays strike Earth’s atmosphere. Particle accelerators at Fermilab can produce them in large numbers. Like electrons, muons act as if they have a tiny internal magnet. In a strong magnetic field, the direction of the muon’s magnet precesses, or “wobbles,” much like the axis of a spinning top. The strength of the internal magnet determines the rate that the muon precesses in an external magnetic field and is described by a number known as the g-factor. This number can be calculated with ultra-high precision.

As the muons circulate in the Muon g-2 magnet, they also interact with a “quantum foam” of subatomic particles popping in and out of existence. Interactions with these short-lived particles affect the value of the g-factor, causing the muons’ precession to speed up or slow down slightly. The Standard Model predicts with high precision what the value of this so-called “anomalous magnetic moment” should be. But if the quantum foam contains additional forces or particles not accounted for by the Standard Model, that would tweak the muon g-factor further.

Hertzog, then at the University of Illinois, was one of the lead scientists on the predecessor experiment at Brookhaven National Laboratory. That endeavor concluded in 2001 and offered hints that the muon’s behavior disagreed with the Standard Model. The new measurement from the Muon g-2 experiment at Fermilab strongly agrees with the value found at Brookhaven and diverges from theory with the most precise measurement to date.

The accepted theoretical values for the muon are:

• g-factor: 2.00233183620(86)
• anomalous magnetic moment: 0.00116591810(43)

The new experimental world-average results announced by the Muon g-2 collaboration today are:

• g-factor: 2.00233184122(82)
• anomalous magnetic moment: 0.00116592061(41)

The combined results from Fermilab and Brookhaven show a difference with theoretical predictions at a significance of 4.2 sigma, a little shy of the 5 sigma — or 5 standard deviations — that scientists prefer as a claim of discovery. But it is still compelling evidence of new physics. The chance that the results are a statistical fluctuation is about 1 in 40,000.

“This result from the first run of the Fermilab Muon g-2 experiment is arguably the most highly anticipated result in particle physics over the last years,” said Martin Hoferichter, an assistant professor at the University of Bern and member of the theory collaboration that predicted the Standard Model value. “After almost a decade, it is great to see this huge effort finally coming to fruition.”

The Fermilab experiment, which is ongoing, reuses the main component from the Brookhaven experiment, a 50-foot-diameter superconducting magnetic storage ring. In 2013, it was transported 3,200 miles by land and sea from Long Island to the Chicago suburbs, where scientists could take advantage of Fermilab’s particle accelerator and produce the most intense beam of muons in the United States. Over the next four years, researchers assembled the experiment; tuned and calibrated an incredibly ; developed new techniques, instrumentation, and simulations; and thoroughly tested the entire system.

The Muon g-2 experiment sends a beam of muons into the storage ring, where they circulate thousands of times at nearly the speed of light. Detectors lining the ring allow scientists to determine how fast the muons are “wobbling.”

Many of the sensors and detectors at Fermilab were constructed at the 91±ŹÁÏ, such as instruments to measure the muon beam as it enters the storage ring and to the telltale particles that arise when muons decay. Dozens of scientists — including faculty, postdoctoral researchers, technicians, graduate students and undergraduate students — have worked to assemble these sensitive instruments at the 91±ŹÁÏ and then install and monitor them at Fermilab.

91±ŹÁÏ scientists have also been involved in theoretical work around the Muon g-2 collaboration.

“The prospects of the new result triggered a coordinated theory effort to provide our experimental colleagues with a robust, consensus Standard-Model prediction,” said Hoferichter, who was a 91±ŹÁÏ research assistant professor from 2015 to 2019. “Future runs will motivate further improvements, to allow for a conclusive statement if physics beyond the Standard Model is lurking in the anomalous magnetic moment of the muon.”

In its first year of operation, in 2018, the Fermilab experiment collected more data than all prior muon g-factor experiments combined. The Muon g-2 collaboration has now finished analyzing the motion of more than 8 billion muons from that first run. The 91±ŹÁÏ team was central to this effort, leading to four doctoral theses to date.

Data analysis on the second and third runs of the experiment is under way; the fourth run is ongoing, and a fifth run is planned. Combining the results from all five runs will give scientists an even more precise measurement of the muon’s “wobble,” revealing with greater certainty whether new physics is hiding within the quantum foam.

“So far we have analyzed less than 6% of the data that the experiment will eventually collect,” said Fermilab scientist Chris Polly, who is a co-spokesperson for the current experiment and was a lead University of Illinois graduate student under Hertzog during the Brookhaven experiment. “Although these first results are telling us that there is an intriguing difference with the Standard Model, we will learn much more in the next couple of years.”

“With these exciting results our team, in particular our students, is enthusiastic to push hard on the remaining data analysis and future data-taking in order to realize our ultimate precision goal,” said Kammel.

Hertzog will present the results at a 91±ŹÁÏ Department of Physics on April 12.

Adapted from a by Fermilab.

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 91±ŹÁÏ, 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. 91±ŹÁÏ 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 91±ŹÁÏ 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 91±ŹÁÏ 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.

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Adapted from a by ORNL.

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