The magnet detectives

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.