Contract awarded for acquisition of large cryogenic system for DUNE detectors in South Dakota

A newly awarded multi-year contract for the acquisition of a large cryogenic plant to cool tens of thousands of tons of liquid argon brings the Deep Underground Neutrino Experiment one step closer to realization.

DUNE and its cryogenic plant will be assembled in the Long-Baseline Neutrino Facility, an ambitious project hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory. The experiment will explore the mysterious behavior of elementary particles called neutrinos in more detail than ever before. A neutrino beam powered by Fermilab’s PIP-II accelerator will travel about 1,300 kilometers (about 800 miles) through the earth to the massive, liquid-argon-filled neutrino detectors in the LBNF caverns in South Dakota, located at the Sanford Underground Research Facility.

Providing the equipment for the cooling of the 17,500 metric tons of liquid argon in each of the large cryostats of the far detector modules is the sizeable task of Air Products, an industrial gases company based in Allentown, Pennsylvania. The overall scope of this contract includes the engineering, manufacturing, installing and commissioning of a liquid nitrogen refrigeration system to cool down and keep the argon at minus 186 degrees Celsius, or minus 303 degrees Fahrenheit. Both the nitrogen and argon systems will be closed systems — neither will actively vent into the environment.

Argon is a noble gas about 10 times heavier than its groupmate helium. It becomes a liquid at low temperature. The liquid nitrogen refrigeration system will cool the argon in the DUNE detectors while the argon flows through a separate, closed loop. As the argon slowly boils (a thermodynamic inevitability), the resulting gaseous argon will travel through heat exchangers cooled by liquid nitrogen. Because the liquid nitrogen is colder than the temperature at which argon liquifies, any gaseous argon will condense back into a liquid. This recovered liquid will then go through a purification process before being incorporated back into the liquid argon used in the DUNE detectors and cryostats.

Keeping the liquid nitrogen cold is more complex. During operation, the liquid nitrogen in the refrigeration system warms and boils off as nitrogen gas. Instead of cooling the nitrogen with an even colder material, the nitrogen gas will be recondensed through a series of pressure changes, taking advantage of proprietary turboexpander technology and the Joule-Thomson effect. When a gas is significantly compressed and then forced through a small opening, like a valve, and allowed to expand, the gas cools. If a gas is compressed, cooled, and then expanded in this way multiple times, it will eventually become cold enough to liquify. This method is well known and has been used for more than 100 years. Fortunately, it has become more efficient as technology has advanced.

“You could do the same with the argon,” said David Montanari, the deputy project manager for the Far Detector and Cryogenics Infrastructure Subproject. “The problem is the purity requirements for the argon don’t allow us to,” he said. The argon in the detectors has to reach purities of parts per trillion (impurity concentrations must be less than a trillionth of a percent), which is not possible with the compression-expansion method of gas liquification.

Air Products will be responsible for engineering, manufacturing and installing the entire liquid nitrogen cryogenic system. Integral to this cryogenic system is the incorporation of modular compression technology used to compress the nitrogen gas and turboexpanders used to expand and cool the nitrogen.  The system will also have the added capability of generating its own nitrogen using Air Products’ proprietary membrane technology. The membrane system uses hollow-fiber technology to extract high-purity nitrogen from compressed air.

When the engineering phase is completed, the company will manufacture the system at one of its facilities, then dismantle it so that it can fit down the 5-foot-by-13-foot shaft opening and be taken piecewise a mile underground and into the newly excavated LBNF caverns that will house DUNE.

From an engineering perspective, the liquid nitrogen system is unique, it is one of a kind,” said Montanari. “It’s unique from an industry perspective, too. They have never built such a system underground because nobody builds anything under the ground in the industry; there’s no need to.”

The engineering effort to produce the final system is expected to take about 10 months. After the design is approved, the system will be manufactured and installed, ultimately coming online in the 2026 timeframe. Once operational, the DUNE far detectors are expected to be the largest underground cryogenic system in the world.

Find more information about the cryogenics of the LBNF/DUNE far detectors here.

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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, please visit science.energy.gov.

Predicting the numerical value of the magnetic moment of the muon is one of the most challenging calculations in high-energy physics. Some physicists spend the bulk of their careers improving the calculation to greater precision.

Why do physicists care about the magnetic properties of this particle? Because information from every particle and force is encoded in the numerical value of the muon’s magnetic moment. If we can both measure and predict this number to ultra-high precision, we can test whether the Standard Model of Elementary Particles is complete.

Muons are identical to electrons except they are about 200 times more massive, are not stable, and disintegrate into electrons and neutrinos after a short time. At the simplest level, theory predicts that the muon’s magnetic moment, typically represented by the letter g, should equal 2. Any deviation from 2 can be attributed to quantum contributions from the muon’s interaction with other—known and unknown—particles and forces. Hence scientists are focused on predicting and measuring g-2.

Several measurements of muon g-2 already exist. Scientists working on the Muon g-2 experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory expect to announce later this year the result of the most precise measurement ever made of the muon’s magnetic moment.

Simultaneously, a large number of scientists are working on improving the Standard Model prediction of the value of muon g-2. Several parts feed into this calculation, related to the electromagnetic force, the weak nuclear force and the strong nuclear force.

The contribution from electromagnetic particles like photons and electrons is considered the most precise calculation in the world. The contribution from weakly interacting particles like neutrinos, W and Z bosons, and the Higgs boson is also well known. The most challenging part of the muon g-2 prediction stems from the contribution from strongly interacting particles like quarks and gluons; the equations governing their contribution are very complex.

Even though the contributions from quarks and gluons are so complex, they are calculable, in principle, and several different approaches have been developed. One of these approaches evaluates the contributions by using experimental data related to the strongly interacting nuclear force. When electrons and positrons collide, they annihilate and can produce particles made of quarks and gluons like pions. Measuring how often pions are produced in these collisions is exactly the data needed to predict the strong nuclear contribution to muon g-2.

For several decades, experiments at electron-positron colliders around the world have measured the contributions from quarks and gluons, including experiments in the US, Italy, Russia, China and Japan. Results from all these experiments were compiled by a collaboration of experimental and theoretical physicists known as the Muon g-2 Theory Initiative. In 2020, this group announced the best Standard Model prediction for muon g-2 available at that time. Ten months later, the Muon g-2 collaboration at Fermilab unveiled the result of their first measurement. The comparison of the two indicated a large discrepancy between the experimental result and the Standard Model prediction. In other words, the comparison of the measurement with the Standard Model provided strong evidence that the Standard Model is not complete and muons could be interacting with yet undiscovered particles or forces.

A second approach uses supercomputers to compute the complex equations for the quark and gluon interactions with a numerical approach called lattice gauge theory. While this is a well-tested method to compute the effects of the strong force, computing power has only recently become available to perform the calculations for muon g-2 to the required precision. As a result, lattice calculations published prior to 2021 were not sufficiently precise to test the Standard Model. However, a calculation published by one group of scientists in 2021, the Budapest-Marseille-Wuppertal collaboration, produced a huge surprise. Their prediction using lattice gauge theory was far from the prediction using electron-positron data.

In the last few months, the landscape of predictions for the strong force contribution to muon g-2 has only become more complex. A new round of electron-positron data has come out from the SND and CMD3 collaborations. These are two experiments taking data at the VEPP-2000 electron-positron collider in Novosibirsk, Russia. A result from the SND collaboration agrees with the previous electron-positron data, while a result from the CMD3 collaboration disagrees with the previous data.

What is going on? While there is no simple answer, there are concerted efforts by all the communities involved to better quantify the Standard Model prediction. The lattice gauge theory community is working around the clock towards testing and scrutinizing the BMW collaboration’s prediction in independent lattice calculations with improved precision using different methods. The electron-positron collider community is working to identify possible reasons for the differences between the CMD3 result and all previous measurements. More importantly, the community is in the process of repeating these experimental measurements using much larger data sets. Scientists are also introducing new independent techniques to understand the strong-force contribution, such as a new experiment proposed at CERN called MUonE.

What does this mean for muon g-2? The Fermilab Muon g-2 collaboration will release its next result, based on data taken in 2019 and 2020, later this year. Because of the large amount of additional data that is going into the new analysis, the Muon g-2 collaboration expects its result to be twice as precise as the first result from their experiment. But the current uncertainty in the predicted value makes it hard to use the new result to strengthen our previous indication that the Standard Model is incomplete and there are new particles and forces affecting muon g-2.

What is next? The Fermilab Muon g-2 experiment concluded data taking this spring. It will still take a couple of years to analyze the entire data set, and the experiment expects to release its final result in 2025. At the same time, the Muon g-2 Theory Initiative is working to shore up the predicted value using new data and new lattice calculations that should also be available before 2025. It will be a very exciting showdown. In the meantime, the high-energy physics community is eagerly anticipating the announcement of the world’s best measurement from the Fermilab Muon g-2 experiment later this year.

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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, please visit science.energy.gov.

Particle accelerators and detectors are the workhorses for researchers plumbing the depths of the quantum realm. Every upgrade gives scientists more opportunities to study the building blocks of our universe.

Now, a unique collaboration of researchers, led by research engineer Davide Braga of the U.S. Department of Energy’s Fermi National Accelerator Laboratory, is designing and fabricating three components of a new, superconducting particle detector: chip, circuits and sensor. Each piece of this project is cutting-edge on its own, and together they make something completely new.

The detector will be able to function in the ultracold, strong-magnetic-field, high-radiation environment found at particle accelerator facilities where others cannot. When completed, scientists could use this detector for groundbreaking experiments in fields ranging from nuclear physics to the search for dark matter. The detector’s unique hybrid design enables near-sensor computing modeled after the human brain and will ultimately make the detectors scalable without sacrificing performance.

“At the fundamental level, how this sensor works is completely different from the common detectors we’ve been working with for 100 years in particle physics,” said physicist Whitney Armstrong, a scientist from Argonne National Laboratory collaborating on the project. “The exciting part is thinking up all the new applications that you can do with this new technology.”

In 2021, DOE announced a $54-million call for labs to apply for a three-year grant for fundamental microelectronics co-design research. Fermilab was one of 10 institutions to receive the award to pursue this innovative project.

“DOE is putting a conscious effort in trying to get people with different domain expertise to work together, so that they all influence each other,” said Braga, the principal investigator of the co-design team. “That’s why we brought together this collaboration, which is quite diverse,” he explained.

The design team consists of Fermilab scientists and researchers from seven other institutions, including the Massachusetts Institute of Technology and Argonne. In January 2023, the team met for the first time in person to discuss their progress.

The funded proposal aims to revolutionize cryogenic detectors able to detect single particles or photons. To this end, the team is developing two complementary classes of cryogenic detectors, one based on ultra-low-noise semiconductor sensors, and one based on superconducting nanowire single-photon detectors, or SNSPDs, that operate below minus 268 Celsius. Though “photon” is in the name, Braga explained that these detectors can detect charged particles as well.

“Now, we are trying to incorporate this technology into particle detectors for accelerators and collider experiments,” he said.

Building a better detector

There are advantages to using these superconducting detectors in the high-magnetic-field environment of a particle accelerator that arise from how a signal is generated in the sensor. When a particle — photon or otherwise — hits the superconducting nanowire, it warms the wire enough to break the superconducting state. The nanowire exhibits electrical resistance, and the resulting voltage spike is relayed to a custom microchip for signal processing before being sent to a connected computer.

This process is markedly different from more traditional detectors that rely on ionization to generate a signal: When a charged particle zips through such a detector, it knocks electrons off atoms in its way. This causes an electric signal that is detected, amplified and ultimately sent to a computer.

Because the motion of electrons and ions is affected by the pull of high magnetic fields, ionization detectors, or charge-collection-based detectors, don’t work well inside accelerator tunnels where high-powered magnets shape particle beams. Superconducting detectors, however, may fill that niche.

“One thing that I really want to do is to push the idea of integrating superconducting detectors into the cold mass of superconducting magnets, especially for the Electron-Ion Collider,” said Armstrong. This future collider will smash electrons into ions, like the proton, to probe the inner 3D structure of the particles.

There are no other detectors that can effectively operate in those high magnetic fields and low temperatures, he explained, and this new technology could help researchers tune their accelerators more efficiently and accurately. And, in general, these detectors just work well. “They have excellent position and time resolution, and radiation hardiness,” Armstrong said.

Although typically viewed as disadvantageous, working at cryogenic temperatures may be a boon for new nuclear and particle physics applications. “It’s not necessarily very difficult for accelerators to get to liquid-helium temperatures,” he said.

The microelectronics co-design team’s device has three different components to develop and perfect: a specialized microchip; an interfacing layer of superconducting electronics; and a superconducting nanowire sensor, all three operating at a few degrees above absolute zero. Each component is being tackled by a different group on the team.

Cool microchips

Braga is the leader of the application-specific integrated circuit, or ASIC, development team designing the chips for the final device. Normal chip-integrated circuits, or microchips, operate around room temperature, said Braga, though sometimes they’re heated to 100 degrees C or cooled to minus 40 degrees C for industrial applications. It is a far cry from the researchers’ target operating temperature of minus 269 degrees C, the temperature at which helium becomes a liquid and the working temperature of the rest of the device.

“It’s not straightforward to design and operate a complex circuit at that temperature,” Braga said, “and there are no good models of how the transistors behave at that temperature.”

Yet it is important for the detector’s performance that the microchip operates as close as physically possible to the sensor. As the chip moves further away from the sensor, the device’s performance decreases, and it becomes more difficult to make the detector larger while retaining its desirable qualities.

Braga’s team, the Fermilab microelectronics group, specializes in designing high-performance, custom integrated circuits for extreme environments. It is currently developing a set of transistor models necessary to optimize the power and performance for this system.

“Everything that you do at minus 269 degrees C has to be very low power,” he explained. Having a good model helps minimize the amount of guess-and-check work that goes into developing a reliable and optimized microchip. Making the chips is expensive, Braga said, so “it’s important to have good models so that we’re confident in the performance before we go to manufacture.”

The model used by the team has paid off — the ASIC team is testing several promising prototypes now.

The challenge of cryogenic circuits

A few states away at MIT in Boston, Karl Berggren, professor of electrical engineering, leads a team of graduate students in designing and building the circuitry that will connect the cryoASIC chip designed at Fermilab to the Argonne lab sensor. But it’s not a simple plug-and-play; the MIT group will build the circuitry from the same superconducting nanowire material used on the detector.

“One of the challenges with this technology is that it’s very nascent,” said Reed Foster, a graduate student working on designing the system. “Researchers haven’t wrung out how specific design factors exactly impact the performance of the circuits that we build with these nanowires,” he said.

It means every circuit they build must be carefully tested to ensure it’s functioning as expected.

Foster explained that normally they test circuits by building a smaller circuit nested in the overall design. Self-tests from the smaller circuit let a user know if the whole circuit is functioning properly. “Currently, we’re not able to do that,” Foster said. Instead, they must couple the superconducting cryogenic circuits to room temperature circuits and use the room temperature circuits to self-test.

These two devices are not easy to combine. Although a huge benefit of superconducting circuits is how quickly information travels through them, this process requires very high-speed room temperature circuits, which are costly and can usually only test a few circuits at a time. Their first prototype can test eight circuits simultaneously, Foster said, but he plans to scale that up to 16 or more. While they continue to hammer out how to integrate a self-testing circuit into the final design, Foster’s room temperature setup will continue to ensure that the circuits work properly.

Developing cryogenic sensors

Back in Illinois, at Argonne National Laboratory, Armstrong leads the Argonne team in developing the superconducting nanowire sensor. “We just wrapped up some tests at the Fermilab test beam facility,” he said, “We took the sensors that were fabricated here at Argonne, put them into a cryostat and essentially tested them with the 120 GeV proton beam to see that we can get signal.”

The sensors fabricated by the Argonne researchers are silicon wafers inlaid with a maze of nanowire. Armstrong’s team tested the detection ability of differently sized nanowires, ranging from 100 to 800 nanometers — almost 1,000 times thinner than a strand of hair. Although wider nanowires are easier to fabricate, they don’t necessarily have the same detection capabilities of thinner nanowires.

The team are still analyzing the results from the testing, but so far it looks promising, said Armstrong. “We hope to optimize the sensor design for particle detectors at the Electron-Ion Collider,” he said.

Now they can start focusing their efforts toward scaling up the sensors.

Ultimately, these three components will be combined into a fully functioning device capable of detecting particles at liquid-helium temperatures.

“I think that, in the future, there’s going to be a large number of users of this technology,” said Armstrong.

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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, please visit science.energy.gov.

A mile below the surface in South Dakota, massive subterranean caverns are taking shape. These will one day become part of the the Long-Baseline Neutrino Facility, home to the international Deep Underground Neutrino Experiment. Since 2021, construction crews in South Dakota have extracted about 550,000 tons of rock.

As excavation nears completion, the LBNF/DUNE project team is now preparing for the next phase of construction, which will turn the underground cavities into a laboratory-ready space.

“The excavation is exceeding 70% completion,” said Ron Ray, the U.S. deputy project director of LBNF/DUNE. “As soon as the caverns have been excavated, then we have to turn them into usable laboratory space.”

That’s where the Building and Site Infrastructure subproject, or BSI, comes in. During this phase, workers will outfit the caverns with all the systems — including electricity, plumbing, telecommunications and IT, air conditioning, fire detection and protection, elevators and additional ventilation — needed to run the underground research facility. In March, the U.S. Department of Energy gave the LBNF/DUNE project team the approval to proceed with the BSI subproject.

DUNE, hosted by DOE’s Fermi National Accelerator Laboratory, is an experiment focused on the study of elusive particles known as neutrinos. DUNE scientists hope to solve some the universe’s great mysteries by exploring the behavior of neutrinos: Why is our universe composed of matter? How do black holes and neutron stars form? Can the four forces of nature be combined in a single, unified theory?

LBNF will provide the space, infrastructure and particle beam for DUNE. This includes the underground caverns that will house DUNE’s detectors — a near detector at Fermilab and a far detector 800 miles away at the Sanford Underground Research Facility in South Dakota.

Once the excavation at SURF is complete, the site will be composed of three caverns and a network of tunnels that, together, span the area of approximately eight soccer fields. The north and south caverns, each about the height of a seven-story building, will house the far detector modules, which each will contain 17,500 tons of ultrapure liquid argon — a highly stable element ideal for studying neutrinos — cooled to minus 184 degrees Celsius. The third cavern, known as the central utility cavern, will house the cryogenic support systems, detector electronics and data acquisition equipment.

“The biggest part of our scope is providing the power and the corresponding cooling systems to support the experiment,” said Josh Willhite, BSI subproject manager. “Throughout all of the drifts and caverns, we will install lighting, sprinklers and fire alarm systems, and then the central utility cavern is where we put in an electrical substation and a chilled water system that ultimately removes the heat from underground.” The BSI team will also set up a structure on the surface for receiving deliveries of argon.

The architecture and engineering company Arup USA developed the blueprints for the infrastructure that will be installed during BSI. The contractor Kiewit Alberici Joint Venture will manage the BSI work. KAJV will procure subcontractors from areas including South Dakota and nearby regions over the next six-to-eight months to carry out this next phase of the LBNF/DUNE construction. KAJV will also perform some of the BSI work in-house.

“Most of the recommended subcontractors to date are from South Dakota,” said Scott Lundgren, project manager for KAJV. “We’ve also got quite a few from North Dakota, Minnesota, Wyoming and Denver.”

Excavation and BSI are two of five LBNF/DUNE subprojects. The third, which comes after BSI, is the Far Detector and Cryogenics subproject that includes the installation of the huge neutrino detectors in the caverns. The final two subprojects will take place at Fermilab. One involves building the facilities to support the near detector and beam line, as well as building the beam line itseslf. The other is to build the near detector.

The LBNF caverns provide space for four far detector modules. Members of the DUNE collaboration have successfully tested the technology and assembly process for the first detector module, and mass production of the components of this module is currently underway. Preparations for testing the technology underlying the second detector module are underway at the European research laboratory CERN.

The FDC phase will involve several steps: constructing the cryostats; installing the detectors and cryogenic systems; and filling the detectors with liquid argon. It will involve more than 1,300 collaborators from 204 institutions in 33 countries plus CERN.

“This is a huge international effort,” Ray said. “We have significant contributions from our partners all over the world.”

The BSI phase is expected to start in summer 2024. Workers will begin in the north cavern, with the goal of completing infrastructure installation in that cavern by fall 2024. Once that north cavern is complete, the construction of the first far detector will begin. The FDC team aims to have the first detector module complete and operational before the end of 2028.

“One of the big milestones would be getting started with BSI next summer, which means finishing the excavation and transitioning into the first of the cavern fit-out work,” Lundgren said. “If everything lines up, the first of the cryostat assembly will start late 2024, which inches things ever closer to the ultimate milestone: starting the science.”

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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, please visit science.energy.gov.

While pursuing his doctorate at the University of Manchester, Patrick Green dug into data from the past, looking for new physics with liquid argon detectors, while developing a method to improve computer modeling of results for future detectors.

Now a postdoctoral researcher at the University of Oxford, Green won this year’s Universities Research Association Doctoral Thesis Award for his work involving liquid argon time projection chambers. This annual award recognizes an outstanding doctoral thesis written on research conducted at Fermilab or in collaboration with Fermilab scientists.

“URA is excited to congratulate Patrick Green and his contributions to liquid argon time projection chamber neutrino detectors,” said URA President John Mester. “His thesis will allow higher precision neutrino measurements which will help answer some of the most pressing mysteries presented by the Standard Model.”

During his doctoral research, Green analyzed data from ArgoNeuT, a liquid argon neutrino detector at Fermilab that collected data until 2010.

“This year the committee was happy to review many excellent theses spanning the science at Fermilab,” said Chris Stoughton, chair of the URA Thesis Award Committee.

Green collaborated with theoretical physicists who were looking for insights from the ArgoNeuT data. He developed new techniques to identify the highly energetic muons of interest that would provide clues for a couple of new particles beyond the Standard Model, the current understanding of particle physics. The results of these searches set new constraints on the characteristics of these theoretical particles, Green said.

“This really close collaboration between experiment and theory is really nice, because both sides of it can be done as rigorously as you need,” he said. “You have people on the theory side that truly understand these theories and are involved in creating them, and the experiment side has a deep understanding of the detector.”

On top of the science itself, the age of the ArgoNeuT code presented its own challenges.

“I developed new techniques to select these, including developing a brand-new simulation of these models in ArgoNeuT and MINOS,” Green said. “This involved a lot of archaeology, obviously with very old detectors, digging in some very old code that no one really remembers how it works. It was challenging, but fun.”

Specifically, Green looked for signs of two theoretical particles: heavy neutral leptons, commonly called sterile neutrinos; and heavy quantum chromodynamics, or QCD, axions, a boson that would solve a mystery of the strong force.

Sterile neutrinos are a theorized new type of neutrinos beyond the known three flavors. These types of neutrinos are thought to be heavier than the very low-mass active neutrinos. They are also thought to be right-handed: other types of particles in the standard model have versions with both left-handed and right-handed spin, but all observed neutrinos so far have been left-handed.

On the lower end of the potential masses of sterile neutrinos, they could explain certain anomalies in the observations of previous neutrino experiments like the MiniBooNE experiment. Mid-mass sterile neutrinos could be a candidate for dark matter. And if sterile neutrinos have high masses, they could explain why active neutrinos are so light.

The other subject of Green’s search, QCD axions, are a solution that would explain a discrepancy between theory and observation in the strong force, known as the strong charge parity violation problem. In the weak nuclear force, the laws of physics don’t work exactly the same when particles’ charge and parity, or spatial coordinates, are flipped — in this case, CP symmetry is broken.

On the other hand, there’s no reason why charge and parity should still be conserved in interactions that use the strong nuclear force, the force that holds quarks together to make particles like protons and neutrons. For example, if CP symmetry is violated with the strong force, neutrons should have different charges on opposite sides, but this hasn’t been observed; if it’s there, it’s incredibly, and arbitrarily, small.

One solution is a new particle, the axion, which would minimize the effect of the charge asymmetry of neutrons. QCD axions could also be a candidate for dark matter.

“We don’t really have a real motivation for why CP violation in the strong force is so small compared to theory predictions,” Green said. “The axion is a very elegant model that can explain this.”

Green’s other main project during his doctoral research was developing a more computationally efficient way to model the light produced in particle interactions in liquid argon time projection chambers, or LArTPC.

“Patrick’s thesis covered both the theoretical and experimental aspects of searches for Beyond Standard Model Physics,” Stoughton said. “Specifically, his new method for simulating light production applies to all liquid argon time projection chamber experiments that are a crucial part of Fermilab’s mission.”

In an LArTPC, when a neutrino hits an atom of liquid argon, light is produced in a process called scintillation. This process creates a lot of photons, and tracking each of these photons’ movements through the detector can take up a lot of computing power. A current method to help handle this computing load is a series of lookup tables that model the light in different regions of the detector. But as these detectors get bigger, such as the LArTPC neutrino detectors in the upcoming Deep Underground Neutrino Experiment, this method becomes unwieldy.

Instead, Green and his collaborators developed a new method that predicts the behavior of the light using geometric calculations. The effects of scintillation can then be treated as small corrections to that prediction. This model, which can be scaled up more easily, will be used in the Deep Underground Neutrino Experiment and the Short Baseline Near Detector.

“If you’re using a gigantic lookup table, if you double the size of your detector, you double the size of your lookup table,” Green said. “At some point, you run out of memory and computing resources to handle that large lookup table. Whereas, if you’re just making a geometric calculation each time, with some corrections, you can predict any size of detector.”

Now, Green continues to work with LArTPC detectors, but has shifted his focus to the electron flavor of neutrinos. He is also excited for the new data that will be produced by SBND and DUNE that will let researchers look for hints at something beyond the Standard Model.

“I’d like to thank my supervisor, Andrzej Szelc, for his help throughout all of this, and the many people at Fermilab and various places,” Green said. “All these things are collaborative efforts; many people have contributed to this.”

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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, please visit science.energy.gov.

Accelerator science is simply smashing. And new technologies, like those developed at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, continue to push the frontier of collision experiments. A team led by physicist Jonathan Jarvis in the lab’s Accelerator Research Department has created a toolbox for this next generation of particle accelerators and storage rings. One vital component: technology to make particle beams dense enough to ensure an abundance of particle collisions.

The density of a particle beam is largely tied to how “cold” it is. “Particles in a beam have some momentum going in directions that diverge from the ideal direction,” said Jarvis. It makes the beam “fluffy” he explained, “which makes it less likely that particles will collide when you’re running collision experiments.”

Jarvis received the 2023 Early Career Award from the Universities Research Association in recognition of his work developing a functional optical stochastic cooling system, a new technology able to rapidly cool a diffuse beam into a dense beam. The annual award recognizes a Fermilab researcher at the beginning of their career who’s contributed outstanding work to the community.

“It is important to recognize remarkable scientists like Jonathan Jarvis who co-led experiments, such as the Optical Stochastic Cooling Experiment, which could help potentially increase the beam cooling rates at particle accelerators by up to four orders of magnitude,” said John Mester, president of the University Research Association.

“Jonathan also embodies the Fermilab spirit of fostering the next generation of scientists through his leadership and involvement with early career programs across the lab.”

How optical stochastic cooling works

If you imagine something being cooled, you may envision icy drinks on a hot day or a dip in a cold pond – placing a hot object in close contact with a cold object will eventually equalize their temperatures, cooling the hotter thing. But ice cubes and cold dips don’t cut it in particle accelerators.

Instead, stochastic cooling takes a more direct approach. Rather than cooling the beam by combining it with something colder, researchers use a detector to take a snapshot of the particles within the beam and then nudge each particle’s momentum toward an ideal using magnets, lenses and other optics. “What you want to do is basically remove disorder from the beam,” Jarvis said.

It’s similar to geese slowly converging into a V while migrating. When the flock first scatters into the air, the group may be going in the same overall direction, but each individual bird follows a slightly different trajectory. Some may veer up or down, others left or right, and still others fly slightly faster or slower than their compatriots. But eventually, each individual settles into the ideal formation, and the flock flies south as densely packed as comfortably possible.

Particles, however, cannot correct their own momentum to perfectly match their beam partners. So, the researchers help the particles out. “We send the beam through a device called a pick-up, which measures structural information about the beam,” Jarvis explained, “And then we use that structural information to correct the differences in momentum for each particle.”

The pick-up essentially takes snapshots of narrow slices of the beam, and the precision of those snapshots depends on the wavelength of the light that’s used – the shorter the wavelength, the thinner the slice. Conventional pick-ups use microwaves — the same radiation that’s in a microwave oven — with centimeter-long wavelengths. The individual snapshots contain millions or billions of particles, said Jarvis, which makes it impossible to measure a single particle’s momentum with a single snapshot. “All I can really see is the average of a million particles,” he said.

So Jarvis, and other researchers using stochastic cooling, send the beam repeatedly through the pick-up. With each pass, the average momentum of all the particles in the beam is tweaked toward the ideal value, but individual particles can still veer off course. It takes many thousands of passes to get each particle to cooperate and create a bright, dense beam.

It was this technology that won its inventor, Simon van der Meer, the 1984 Nobel Prize in Physics and enabled the discovery of the W and Z bosons in the CERN Super Proton Synchrotron collider.

Speeding up cooling

In the mid-1990s, researchers proposed a way to make stochastic cooling faster: change the light used to detect the particles.

The time it takes to cool a beam is proportional to the wavelength used in the pick-up because this wavelength determines how many particles are captured in each beam snapshot. Using near-infrared or visible light with wavelengths 1,000 times shorter than microwaves would capture a much narrower snapshot of the beam containing far fewer particles. With fewer particles in each snapshot, they could be nudged more precisely, cooling the beam more efficiently.

“Our big innovation was actually demonstrating optical stochastic cooling experimentally for the first time,” Jarvis said. His team published their results in 2022 in the prestigious journal Nature, nearly 30 years after it was first theorized. Their demonstration carried out at Fermilab’s IOTA storage ring, was a rousing success. “You had this beautiful Nobel Prize-winning technique in stochastic cooling,” Jarvis said, “and we managed to advance its fundamental mechanism by a factor of 1,000.”

Now, the team is working on amplifying the signal collected from the pick-up and sending it back into the beam as an even more powerful correction.

Jarvis explained that this is the perfect example of the kind of experiment his team usually runs. “We’re building the toolkit for the accelerator designers and operators of tomorrow,” he said, adding that they can also expand, or “heat,” the beam with this technique.

“We can envision plug-and-play versions of this technology for certain systems that lets you freely tune the beam structure,” Jarvis said. This can mean anything from creating pulses in the beam to having sections with complicated structure.

“We’re looking to benefit not just high-energy physics, but the whole accelerator research ecosystem,” he said.

It’s the kind of work that URA committee chair Laura Fields of the University of Notre Dame considers both valuable and often overlooked. “Accelerator science is a field that’s extremely critical to almost all of the science that’s done at Fermilab,” she said, “But it’s often not appreciated as much as it should be.”

Not only did Jarvis impress the committee with his scientific contributions, so did his extensive outreach at Fermilab, Fields pointed out. Jarvis leads an undergraduate research group and summer internship program, and is part of multiple working groups, she said.

“The work that Jonathan is doing is going to impact a huge number of scientists in the future at Fermilab and around the world,” Fields said, “We wanted to recognize that.”

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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, please visit science.energy.gov.