Patrick Green wins 2023 URA Doctoral Thesis Award

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.

The U.S. Department of Energy’s Fermi National Accelerator Laboratory is going live on June 15, to discuss all things neutrinos with our partners at CERN in Geneva, Switzerland and the Sanford Underground Research Facility in Lead, South Dakota. This Thursday at 11:00 am CDT, join the interactive livestream of, “Particle pursuit, a journey of the Deep Underground Neutrino Experiment.” View the teaser to preview this exciting event here.

The DUNE experiment is hosted by Fermilab, and more than 1000 scientists and engineers from 35 countries spanning five continents – Africa, Asia, Europe, North America and South America – are working on the development, design and construction of the DUNE detectors. The experiment seeks to understand the nature of neutrinos – almost massless particles that could help answer fundamental questions such as why the Universe has much more matter than antimatter.

DUNE will be built at two locations: Fermilab, near Chicago, and SURF, in Lead, South Dakota. The Fermilab particle accelerator complex will provide the world’s most intense beam of high-energy neutrinos and send it 800 miles through Earth to huge neutrino detectors almost a mile underground at SURF. ProtoDUNE, the largest liquid-argon neutrino detector in the world, recorded its first particle tracks from both cosmic rays and a beam created at CERN’s accelerator complex in 2018. ProtoDUNE is all set to begin its second run this year.

The CERN Neutrino Platform has provided a large-scale demonstration of the future DUNE detectors with the construction and operation of two prototypes known as ProtoDUNE. ProtoDUNE, the largest liquid-argon neutrino detector in the world, recorded its first particle tracks from both cosmic rays and a beam created at CERN’s accelerator complex in 2018. ProtoDUNE is all set to begin its second run this year.

To celebrate this, CERN, Fermilab and SURF are joining forces to bring viewers to all three laboratory locations to learn more about neutrinos and the international DUNE project.

CERN will go live from the Neutrino Platform, Fermilab will broadcast from the control room for its neutrino experiments joined by the SURF in Lead, South Dakota streaming live from its Ross Hoistroom.

Join Fermilab on YouTube on June 15, 11 a.m. CDT for a gameshow-style livestream to learn about all things neutrinos and the preparations for DUNE. The live will also be broadcasted simultaneously on CERN (at 6 p.m. CEST (GMT +2) and SURF (10 a.m. MDT) social media channels.