Newswise — New research from Adelaide University has revealed that geological processes dating back billions of years are critical to locating the rare earth elements needed for modern technologies and the global clean energy transition.

Published today in Science Advances, the study shows a strong global link between ancient subduction zones – where tectonic plates collide – and the formation of rare earth element (REE) deposits and carbonatites, a type of hot molten rock called magma, known to host these valuable resources.

Rare earth elements are essential components in technologies such as electric vehicles, wind turbines, smartphones, and defence systems. However, locating economically viable deposits remains a major global challenge.

Led by Professor Carl Spandler from the School of Physics, Chemistry and Earth Sciences, the research team reconstructed Earth’s geological history over the past two billion years using advanced plate tectonic modelling.

They identified regions of the Earth’s mantle that had been fertilised by subduction processes, where material from one tectonic plate is forced beneath another, releasing fluids and elements into the overlying mantle.

The Adelaide University researchers found that these fertilised mantle regions now underlie approximately 67% of carbonatites and 72% of REE deposits formed over the past 1.8 billion years. For older deposits, that figure rises to 92%.

Prof Spandler said the findings provide compelling evidence that ancient subduction zones play a fundamental role in creating the conditions needed for rare earth deposits to form.

“This research shows that the ingredients for these critical mineral deposits were put in place many million to even billions of years ago,” Prof Spandler said. “By identifying where these ancient processes occurred, we can significantly narrow down the search areas for future discoveries.”

The study also challenges previous theories that linked these deposits primarily to mantle plumes –columns of hot material rising from deep within the Earth.

Instead, the research highlights a two-stage process: an initial fertilisation of the mantle during subduction, followed – sometimes hundreds of millions or even billions of years later – by a separate event that triggers melting and magma formation.

“This time lag is one of the most surprising aspects of our findings,” Prof Spandler said. “It shows that the Earth’s mantle can store these enriched zones for incredibly long periods before the right conditions arise to form mineral deposits.”

The research team mapped these regions across the globe, finding they cover around 35% of the Earth’s continental crust. Importantly, areas where multiple subduction events overlapped were found to host particularly high concentrations of REE deposits.

Co-author Dr Andrew Merdith said the work has significant implications for mineral exploration.

“By focusing on these ancient tectonic zones, exploration companies and governments can take a more targeted and efficient approach to finding new deposits,” Dr Merdith said. “This is especially important as demand for rare earth elements continues to grow.”

The findings also provide new insights into Earth’s geological evolution, including how continents have been shaped over billions of years and how deep Earth processes influence surface resources.

Beyond resource exploration, the study highlights the long-term storage of carbon and water in the Earth’s mantle, with implications for understanding past climate and volcanic activity.

The research was conducted in collaboration with the ARC Centre in Critical Resources for the Future.

‘Linking carbonatites, rare earth ores, and subduction-fertilized mantle lithosphere’ is published in Science Advances. DOI: 10.1126/sciadv.aeb2942

BYLINE: Greg Cunningham

Newswise — Researchers at the Department of Energy’s Oak Ridge National Laboratory have uncovered a path to design superionic polymer electrolytes for solid-state batteries and other energy applications that could help ensure a future of abundant and reliable energy for the United States. The scientists demonstrated that by carefully controlling the chemical composition of a lithium salt-based polymer, they could create a material that enables superfast transport of ions in batteries and many other energy storage and conversion technologies.

“Researchers around the world are focusing on unlocking the potential of polymer electrolytes because they have a lot of advantages over the conventional liquid electrolytes,” said Catalin Gainaru, an R&D staff scientist of ORNL’s Chemical Sciences Division. “Achieving fast ion transport has always been a major challenge of polymer electrolytes, but our recent research demonstrates that this may no longer be the case.”

Batteries are made up of two electrodes — a cathode and an anode — separated by an electrolyte material. As a battery charges or discharges, ions need to have a high mobility within the electrolyte as they move back and forth between electrodes. Traditional batteries use liquid or gel electrolytes, but the demand for safer and more efficient power storage has spurred interest in solid-state batteries in which the electrolyte is solid, yielding a battery that is faster charging, safer, more compact and durable. 

The challenge of ion transport in solid-state batteries

Many solid-state concepts use ceramic electrolytes that transport ions so effectively that they are known as superionic ceramics. Unfortunately, these ceramics are prone to break due to brittleness. They are also difficult to roll into thin films and don’t adhere well to the electrodes in a battery. The ORNL researchers demonstrated how a polymeric material can achieve a similar superionic state, in which ions can move up to 10 billion times faster than their surroundings, without the shortcomings of liquids and ceramics. 

Polymers are materials formed by long molecular chains made up of small, repeating building blocks. Well-known examples include a variety of plastics, which are usually made up of repeating units containing carbon and other atoms. The ORNL polymer electrolyte contains polar segments that favor the inclusion of lithium salts and strongly enhance the mobility of ions. 

The research, which was published in Materials Today, was performed as part of the DOE Energy Frontier Research Center (EFRC) known as the Fast and Cooperative Ion Transport in Polymer-Based Materials (FaCT) Center. 

“The goal of the FaCT EFRC is to fully understand how to design novel polymers that change the paradigm of ion transport,” said Tomonori Saito, an ORNL distinguished researcher in ORNL’s Chemical Sciences Division. “We developed a very special polymer in which the segments self-organize to provide a high mobility path for the ions to move through.”

A molecular design strategy enables superionic behavior

The key development was the careful tuning of the structure of the polymer by the addition of precise amounts of molecular groups known as zwitterions. These special functional groups carry both positive and negative charges, which increases local polarity but results in a zero charge for the entire macromolecule. By using careful chemical processes, researchers were able to tailor the number of zwitterionic groups attached to the polymer backbone allowing the ions to assemble into pockets. 

In these pockets, ions interact much like conversationalists at a dinner party. At first, small pockets of diffuse conversations form, isolated throughout the material. Add more pockets, though, and the discussions eventually lose individuality and evolve into a pleasant and cohesive hum. That’s when the ions start to flow like good conversation. But add too many zwitterions, and the cohesive hum devolves into a cacophony and ion transport slows back down. 

Researchers found that the sweet spot was achieved by functionalizing around 80 percent of the units of the polymer electrolyte with zwitterionic groups. At this point, the pockets connect into channel-like structures that allow ions to hop back and forth in an orderly fashion with minimal resistance.

The research team plans to build on this promising early-stage research with additional investigations into the fundamental mechanisms that enable the superionic nature of the polymer. Modeling and simulations using ORNL supercomputing resources as well as robotic autonomous chemistry coupled with AI will help understand what drives this fascinating performance, and neutron scattering studies are planned at the Spallation Neutron Source, a DOE Office of Science user facility at ORNL, to observe the interactions at the molecular level. 

While solid-state batteries are a clear application for the new electrolyte, many energy technologies also rely on effective ion transport. Flow batteries, fuel cells, grid-level energy storage and many other applications could benefit from these newly developed polymers. 

“It’s hard to predict all the technologies that could leverage this discovery,” Saito said. “Anything that needs an impermeable barrier layer, but let ions move freely across it, is a potential application.”

The research was funded by the DOE’s Office of Basic Energy Sciences as part of the FaCT EFRC.

UT-Battelle manages ORNL for the Department of Energy’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. 

BYLINE: Dawn Levy

Newswise — Researchers at the Department of Energy’s Oak Ridge National Laboratory developed a method to convert a commonly discarded hydrocarbon polymer into gasoline- and diesel-like fuels. The team has applied for a patent for the discovery, which treats polyethylene — the stuff of white cutting boards and shopping bags — with aluminum chloride-containing molten salts that serve as both solvent and catalyst. The results were published in the Journal of the American Chemical Society.

The scientists closely monitored the chemical reaction that turns the polymer into petrol to learn the secrets of its success. Soft X-ray spectroscopy and nuclear magnetic resonance showed that charged aluminum atoms each bind to three other atoms to create strongly acidic catalytic sites that break long polymer chains into shorter ones. Isotopic labeling and neutron scattering revealed how simpler polymer chains form gasoline-like fuels and more complex chains form diesel-like fuels.

If scaled beyond the laboratory, the process could strengthen U.S. energy security and industrial competitiveness.

“We developed an efficient and selective polyethylene-to-gasoline conversion,” said Liqi Qiu, a postdoctoral researcher at the University of Tennessee, Knoxville, who performed most of the study’s experiments in the ORNL laboratory of Sheng Dai, of ORNL and UTK. Dai, an ORNL Corporate Fellow and section head for separations and polymer chemistry, is a co-corresponding author of the paper.

The experiments produced a gasoline yield of about 60 percent under mild conditions.

“We converted polymer waste to value-added fuels by using commercially available inorganic salts as the reaction media to provide the catalytic sites,” said Zhenzhen Yang, an ORNL staff scientist who was also a co-corresponding author of the paper. “Unlike traditional techniques for converting polymer to fuel, the new process did not require noble-metal catalysts, organic solvents or external hydrogen. This is the first time molten salts were used as media to produce high-value-added chemicals from waste without any catalytic initiator or solvent and at temperature below 200 degrees Celsius.”

That temperature is comparable to a conventional kitchen oven. Previously, converting polyethylene to gasoline required temperatures of 450 to 500 degrees Celsius through pyrolysis, a heat-driven process that breaks long polymer chains into smaller hydrocarbons.

ORNL has pioneered molten salt research since the 1960s, when its Molten Salt Reactor Experiment showed that molten salt mixtures could serve as both fuel and coolant in a nuclear reactor. 

Dai proposed using molten salts to turn polymer waste into fuel. Molten salts are inorganic compounds that remain stable under harsh reaction conditions. 

“The ORNL system solves two fundamental issues,” Dai said. “One, for a stable system, the process can be radically easier to scale up. Two, the previous system needed an initiator to kick off catalytic reactions. However, the ORNL system does not need one.”

ORNL’s Tomonori Saito managed the project and contributed polymer expertise. “In this case we tackled polyethylene, a widely available commodity polymer, using molten salt,” he said. “We’re trying to understand fundamental science that will lead to discoveries and new economic opportunities.”

Achieving that understanding required multidisciplinary expertise and advanced instruments.

At ORNL, to identify hydrocarbon products formed from reactions with various polymer chains, Luke Daemen employed neutron scattering, and Felipe Polo-Garzon used gas chromatography-mass spectrometry.

When the polymer interacted with an aluminum site, it created a positively charged ion of carbon. Qiu, Yang and Dai labeled that carbon ion with deuterium, an isotope of hydrogen, to track its behavior during the reactions. They also used neutrons at ORNL’s Spallation Neutron Source to track hydrogen.

“The polymer contains a lot of hydrogen,” Dai said. “Neutrons are ideal at discerning light elements including hydrogen and its isotopes, such as deuterium.”

To probe structural changes to aluminum sites during the reaction, Yang used the Advanced Light Source at Lawrence Berkeley National Laboratory. Working with Min-Jae Kim and Jinhua Guo there, she used soft X-rays to examine how aluminum sites interacted with the polymer at atomic and electronic levels. Soft X-rays are ideal for imaging lightweight elements like aluminum.

“The aluminum edge shifted to the low-electron-density edge, which means some electron-rich intermediates formed,” Yang said. “We compared the findings with other techniques and confirmed an aromatic ring intermediate can coordinate with aluminum and cause a binding-energy change.”

That change indicated that the aluminum sites were catalytically active.

Back at ORNL, Bobby Sumpter of the Center for Nanophase Materials Sciences conducted simulations to examine the reaction’s energy dynamics, such as formation and transfer of stable carbon ions to hydrocarbons.

At UTK, Michael Koehler used in situ X-ray diffraction to monitor phase changes in the reaction mixture, and Carlos Alberto Steren used nuclear magnetic resonance to examine aluminum sites.

ORNL’s Tao Wang lent expertise in molten salts. ORNL’s Logan Kearney provided high-density polymers and expert suggestions for their valorization paths.

Although the aluminum-site system is catalytically active and inexpensive, it is hygroscopic, meaning it absorbs water and loses stability. Next, the team hopes to explore ways to confine molten salts, maybe with halogens or carbons, to improve separation and processing.

The findings expand options for producing transportation and industrial fuels. “Polymer source material is abundantly available from consumer waste, and our catalyst system, aluminum molten salts, is very cheap,” Qiu said. “This advance may be promising for industry.”

The DOE Office of Science (Materials Sciences and Engineering Division) primarily supported the research as well as the gas chromatography-mass spectrometry work (Chemical Sciences, Geosciences and Biosciences Division, Catalysis Science program). The research employed DOE Office of Science user facilities at ORNL (the Spallation Neutron Source for neutron scattering at the VISION beamline and the Center for Nanophase Materials Sciences for quantum chemistry calculations) and Lawrence Berkeley National Laboratory (the Advanced Light Source for soft X-ray spectra).

UT-Battelle manages ORNL for DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. — Dawn Levy

Newswise — When astronomers point their telescopes to the heavens, they tend to look toward the light. They may search for the pin pricks of shining stars, the billowing of cold gas clouds, the faint heat of a star nursery or the rhythmic blaze of a pulsar. These faint signals of light beckon in a universe that is mostly dark.

In fact, the visible bits only account for 5% of the universe, while the dark side makes up the other 95%.

Now, an ongoing experiment at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility is aiming a spotlight into this so-called hidden dark sector. It aims to uncover hints of particles that interact with the dark by catching their light. 

“You underestimate the power of the dark side.”

Most of our universe is currently thought to be composed of dark matter and dark energy. While the dark sector isn’t visible to light microscopes, its existence is inferred by how it affects the stars and galaxies we study.

Currently, it’s thought that fully 27% of our universe is dark matter. This is matter that, as far as we know, isn’t made of the ordinary particles found in the Standard Model, the theoretical framework that nuclear physicists use to describe the subatomic particles that build our visible universe, such as protons and neutrons.

According to Liping Gan, the Jefferson Lab Eta Factory (JEF) experiment aims to capture hints of particles that connect the light and dark sides. Gan is a professor at the University of North Carolina, Wilmington, and a spokesperson for the JEF experiment. She says one of the goals of the experiment is to produce and capture hints of dark matter particles.

“One of the first ones is to search for dark gauge bosons. And that will give us some clue about the properties of the dark sector,” she said. 

If they do exist, dark gauge bosons would be rare examples of particles that can interact with the ordinary particles described in the Standard Model, as well as the mysterious dark matter particles.

To search for these bosons, ironically, the physicists will be looking for faint signals of light. 

“There’s no such thing as ‘the unknown’, only things temporarily hidden, temporarily not understood.”

The experiment will be carried out with the Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility accessed by more than 1,700 nuclear physicists worldwide to study the nature of matter.

The original GlueX Forward Calorimeter, as installed in Jefferson Lab’s Experimental Hall D. (Aileen Devlin / Jefferson Lab)

In the experiment, the CEBAF accelerator will generate a beam of energetic electrons and direct them into a diamond crystal. There, the electrons stimulate the diamond to release highly energetic light particles, or photons. These tiny but mighty particles of light then barrel into a target made of hydrogen. 

Hydrogen atoms are the simplest element to explore: a single proton makes up the atom’s nucleus, while the motion of a single electron forms the atom’s outer perimeter. The interactions of those photons with hydrogen’s protons spawn additional particles that will then enter detectors, where many of them can be measured.

JEF experimenters are most interested in producing the eta (η) meson. Like the proton, the η meson is made of a combination of smaller particles called quarks and gluons. But unlike the proton, the η isn’t stable. Instead, it will fall apart, or decay, into other particles in about half of an attosecond (5×10-19 seconds).

“Then our goal becomes looking for a decay. There are very many different decay channels we are going to measure, because a different decay channel tells us a different part of the physics,” explained Gan. 

According to Simon Taylor, a Jefferson Lab staff scientist and spokesperson for the JEF experiment, the experimenters are looking for rare decays.

“And a particular rare decay that we’ve been focusing on is a four-photon final state,” he said. 

“Only 18 out of 10,000 will decay to this. Most of the etas will decay into two gammas. So that’s why we call it rare reaction or rare decays, it happens only for a small fraction of the experiment,” Gan said.

The most interesting decay channels in this experiment are when η may produce a dark gauge boson. Dark gauge bosons are thought to be both exceedingly rare and short-lived. They, too, will decay away into other particles in the tiniest of fractions of a second. 

“These other particles include photons of light. We don’t know if these dark gauge bosons exist or will be produced in these decays, but this decay mode will allow us to search for them,” said Taylor.

The researchers will sift through the data they collect to see if they can discover unexpected peaks in their data, where there are more four-photon groupings than they expect. This will trigger intensive analysis to determine if each of these “bumps” is a tell-tale sign of the η meson decay.

Justin Stevens is the Wilson & Martha Claiborne Stephens associate professor of physics at William & Mary and spokesperson for phase two of the Gluonic Excitations Experiment (GlueX-II). The JEF and GlueX-II experiments are running concurrently using the same apparatus. He explained that one of the biggest challenges the two experiments face is finding those tell-tale bumps of interest inside the ginormous amount of data that will be generated. 

For context, 50 million light photons slam into the hydrogen target every second during the run. Each one has the potential to generate a few to dozens of particles that will stream into the detectors, all at once.

“We are getting something like 5 to 10 times higher luminosity in total compared to the original phase of GlueX,” Stevens said.

Taylor said that there are also other issues that will make finding a rare bump a challenge.

“It can get confused with other reactions,” Taylor explained. “Photons are detected by reconstructing the electromagnetic showers they produce in a calorimeter.  Some tiny fraction of the time, one or more of the photons disappear down the beam line or in a hole in the detector. Or two very close showers of particles may merge in the calorimeter together.”

To ensure the researchers would be able to capture these rare reactions, they needed to optimize one of their main detectors. An upgrade would ensure the detector can measure the photons it catches to such accuracy, the physicists could be sure they are capturing signals from a potential dark gauge boson and not from the remnants of incoming photons or from other reactions.

“You can’t stop the signal, Mal. Everything goes somewhere.”

For this experiment, the JEF collaboration upgraded the existing GlueX Forward Calorimeter (FCAL) to improve its resolution and hardiness. 

A view of the FCAL system during the upgrade. (Aileen Devlin / Jefferson Lab)

FCAL originally consisted of 2,800 lead glass scintillating modules connected to photomultiplier tubes. Each module has a 4 cm square face to collect the incoming particles produced in the experiment. These modules were designed to capture incoming particles and convert them into showers of light as the particles travel down their 45 cm length. This light can then be measured by the photomultiplier tubes and converted into signals that are recorded as data.

The upgrade removed 440 of the lead glass modules at the center of the detector and replaced them with 1,596 smaller lead tungstate (PbWO4) crystals. The new crystals feature only a 2 cm square face and extend 20 cm in length. They form a central square inside the new Eta Calorimeter (ECAL) that is surrounded by 2,360 of the original lead glass modules. 

A lead glass scintillating module (left) with a lead tungstate (PbWO4) crystal (right) are shown here inside Experimental Hall D. (Aileen Devlin / Jefferson Lab)

Zisis Papandreou is a JEF experiment spokesperson and a professor and head of the physics department at the University of Regina in Saskatchewan Canada. He compared the upgraded detector to a digital camera.

“Essentially, we’re reconstructing something akin to an image. If you have a larger number of smaller pixels, you can improve the resolution in the image,” he said.

The new lead tungstate crystals are also radiation-hard, which means they can better withstand the onslaught of particles generated in experiments than the original lead glass modules. Centering the new crystal modules closest to the target allows them to take the brunt of the onslaught, protecting the lead glass modules from the highest-energy particles and resulting in a longer overall detector lifetime.

Funded by the lab, the Department of Energy and the National Science Foundation, the upgrade of FCAL to ECAL cost about $5 million. Several work groups at the lab also contributed to the upgrade, including the Radiation Detector & Imaging group, the Detector Support group, the Radiation Control Department, and the Fast Electronics group. 

After four years of R&D, the full-scale upgrade took about three years to complete, beginning in 2022 and wrapping up in early 2025. According to Alexander Somov, a staff scientist at Jefferson Lab and a spokesperson for the JEF experiment who led the ECAL construction, the detector has already proven its versatility.

“It is the largest lead-tungstate crystal calorimeter in the U.S., and it’s been successfully commissioned and integrated,” said Somov. “It took a few weeks to commission, and all modules are operational. It was working fine for the first run of approximately five months.”

ECAL was successfully commissioned in April 2025, with data acquired for JEF and GlueX-II in late spring and summer 2025. So far, the detector has collected about 75 days of data as counted by the Program Advisory Committee for the two experiments.

“And the first results we’re getting show that the performance of ECAL is in good agreement with our basic expectations. But there’s more analysis to come,” said Somov.

University of North Carolina Wilmington (UNCW) Physics Professor Liping Gan, George Washington University undergraduate student Olivia Nippe-Jeakins, UNCW undergraduate students Shane Whaley and Ben Simpson, Jefferson Lab Hall D Staff Scientist Alexander “Sasha” Somov, and UNCW postdoc Laveen Puthiya Veetil pose for a photo inside Jefferson Lab’s Experimental Hall D. (Aileen Devlin | Jefferson Lab)

The upgraded detector has also served as a training ground for undergraduate physics majors interested in pursuing a career in nuclear physics. In all, 28 undergraduate students from the JEF experiment’s 11 collaborating universities participated in ECAL fabrication, installation and commissioning. The project also involved three graduate students, six postdoctoral nuclear physicists and one visiting scientist. 

“These are valuable experiences to undergraduates and then later on, they go to graduate school and some of them already were talking about going to a university that has GlueX collaborators. So, the hardware work is an absolutely amazing experience for students,” said Papandreou.

“You built a time machine? Out of a DeLorean?”

The second and final run for these experiments is currently scheduled to resume this summer. In it, the collaborators also aim to collect data that may reveal new clues to our understanding of other phenomena, such as why matter beat out antimatter in the first second of the early universe.

George Washington University undergraduate Quinn Stefan, left, and graduate student Phoebe Sharp, right, monitor experiment progress inside the Experimental Hall D counting house at Jefferson Lab in Newport News, Va., on June 17, 2025. (Aileen Devlin | Jefferson Lab)

“For example, right now we see large asymmetry between matter and antimatter in the universe, and one of the goals is to search for maybe there’s some new symmetry violation beyond the Standard Model. The third goal is to understand why quantum chromodynamics (QCD) prevents quarks from being isolated via tests of fundamental symmetries at low energy,” Somov said.

Alongside these searches, the GlueX-II experiment collaborators are looking to expand their data on particles that include charm quarks, such as the J/ψ (J/psi) meson. While the more familiar protons and neutrons are made of up and down quarks, other subatomic particles include other quark flavors, such as the charm quark. These data offer a rare opportunity to look at in depth at how such particles are made.

“So that means we can do things like study J/ψ production with a lot higher statistics and look for the charm production mechanism with a lot more precision than we had before,” said Stevens.

As the collaborators prepare for their final data run, they have also already begun preparing for the next opportunity. The proposed GlueX-III run aims to increase the intensity of the photon beam. For GlueX, it achieved about 10 million photons per second, and GlueX-II has an intensity of about 50 million photons per second. The goal for GlueX-III is to slowly crank that up to around one hundred million photons per second.

“Once you understand the detector, you just drive up the rate,” Stevens said. “So, we’re going to keep pushing up the intensity as high as we can, because we just get more data. You can look at more rare decays, more rare processes.”

In the meantime, the collaborators have their hands full preparing for their next run this summer and are looking forward to what their data reveal about the light and dark sides of our universe.

Further Reading
GWU Research Magazine: The Particle Whisperers
UNCW: Undergraduates Step Into Physics Research
Technical Paper: Eta Decays with Emphasis on Rare Neutral Modes: The JLab Eta Factory (JEF) Experiment
Technical Paper: Update to the JEF proposal (PR12-14-004)
Technical Paper: Light monitoring system for the lead tungstate calorimeter in Hall D at Jefferson Lab

Newswise — The U.S. Department of Energy’s (DOE) Argonne National Laboratory made a major contribution to a high-profile experiment seeking to discover new physics. Hosted at DOE’s Fermi National Accelerator Laboratory (Fermilab), the Mu2e (muon-to-electron conversion) experiment aims to observe an extremely rare process in particle physics. The experiment is a multiyear collaboration among more than 30 institutions and 200 scientists from around the world.

An Argonne team of high energy physics scientists designed and — most recently — commissioned a crucial Mu2e subsystem called the Cosmic Ray Veto (CRV) detector. This subsystem filters out the biggest source of background noise in Mu2e. Background noise refers to signals that mimic the rare process that Mu2e seeks to detect. The commissioning tests demonstrated that the CRV detector is working properly and collecting background data.

The CRV’s development and deployment was a collaborative effort among Argonne, Fermilab, Kansas State University, University of South Alabama, University of Virginia, Northern Illinois University and University of Michigan.

“The CRV detector will enable Mu2e to more reliably and accurately detect an event expected to be vanishingly rare according to current particle physics theory,” said Yuri Oksuzian, an Argonne physicist who has played a key role in Mu2e and the CRV’s development. ​“The CRV is essential because it screens out background noise that could mimic this event. Observing even a few cases of the event would be compelling evidence of new physics.”

Searching for a muon-to-electron conversion

Since the 1970s, the dominant theory in particle physics has been the Standard Model. Widely considered a robust theory, the Standard Model describes the interactions among the fundamental particles and forces in the universe. But it leaves many big questions unanswered. For example, it cannot explain gravity or dark matter, a mysterious type of matter that cannot be observed directly.

As a result, particle physicists are searching for new theories, particles and forces beyond the Standard Model. The aim is to provide a more comprehensive understanding of the universe.

Mu2e seeks to observe a muon changing to an electron without any other particles being produced. A muon is a fundamental particle that is a heavier version of an electron. The Standard Model expects this transition to be so rare that observing it at Mu2e would be a major discovery and strong evidence of new physics.

“If Mu2e detects a muon-to-electron conversion, it would indicate that there’s a new particle or force involved in this process,” said Oksuzian. ​“The discovery would fundamentally change our understanding of how the universe works.”

Screening out cosmic-ray muons

The Mu2e apparatus directs a high-intensity beam of muons to a thin aluminum foil target. Detectors probe the target for conversion events, indicated by the presence of electrons with a precise amount of energy and momentum. Remarkably, Mu2e is expected to be 10,000 times more sensitive to conversions than previous similar experiments.

The apparatus has critical subsystems that measure background — in other words, signals that look like conversions but aren’t. A major background source is cosmic rays. These are high-energy particles from space that collide with atoms in the Earth’s atmosphere. The collisions produce showers of particles, including muons, that reach the ground.

Muons can penetrate the Mu2e apparatus and knock electrons from the aluminum foil. These free electrons can potentially have the exact energy and momentum of an electron from a conversion event. If that happens, Mu2e’s detectors will mistakenly register them as the real signal.

The CRV detector, engineered by Argonne, detects background events caused by cosmic-ray muons. It is essentially a giant cage that covers key parts of the Mu2e apparatus. It consists of 83 modules, which together weigh about 60 tons. The modules are made of thousands of plastic strips that produce light photons when muons pass through them.

Special fibers inside the strips carry the photons to sensors called silicon photomultipliers. The sensors measure the photons, indicating the exact time of the muon’s passage. If the CRV detects a cosmic muon just before an electron appears in the Mu2e apparatus, that electron is rejected from the data.

“We had to carefully design the CRV structure so that there are no gaps between the modules,” said Oksuzian. ​“The objective was to ensure that the detector would not miss any cosmic muons.”

Without the CRV, cosmic-ray muons would produce thousands of ​“fake” conversion events over Mu2e’s three-to-five-year run time. Because muon-to-electron conversions are so rare, even a small number of fake events would compromise Mu2e’s accuracy. As a result, the CRV must detect and reject 99.99% of cosmic-ray muons passing through. Argonne recently evaluated the CRV’s performance over a two-year period and found that it can meet this strict design requirement.

Commissioning the CRV at Mu2e

Recently, the Argonne team transported the CRV components to the Mu2e building at Fermilab in Batavia, Illinois, and commissioned them to detect cosmic-ray muons. The successful test enabled the CRV to fulfill a key DOE technical milestone and performance objective, helping to advance Mu2e’s commissioning. Other Mu2e subsystems will be commissioned and tested over the next year.

The experiment is expected to begin in 2027. Argonne scientists will have important roles in Mu2e operations, including the CRV, data acquisition system and analysis of datasets.

Besides Oksuzian, Argonne’s CRV team also includes Simon Corrodi, Sam Grant, Peter Winter and Lei Xia.

DOE’s Office of Science is a key supporter of Mu2e and Argonne’s CRV research and oversees Mu2e’s implementation.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology by conducting leading-edge basic and applied research in virtually every scientific discipline. Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://​ener​gy​.gov/​s​c​ience.