Brian Clark Hunts for the Most Elusive and Energetic Neutrinos

Brian Clark, a new assistant professor of physics at the University of Maryland, arrived with a mission: to detect the first ultra-high-energy neutrinos.

If spotted, these tiny, almost weightless particles that pass through matter with barely any interactions could offer a new lens through which to view the universe. The highest-energy neutrinos have about 100 times more energy than any of the particles speeding through the Large Hadron Collider, the world’s highest-energy particle smasher running underneath the France-Switzerland border. And scientists think that the energetic neutrinos are created by astrophysical events that might not leave any other detectable trace. 

“I think the general theme of the work I do,” Clark said, “is that anytime you have a chance to look at the universe in a new way, you tend to learn something. So, I'm pro finding new ways to look at the universe.”

Brian Clark

Clark brings to UMD a wealth of experience, a reputation of excellence in mentoring students and a history of leadership within scientific collaborations. His passion for looking at the universe in novel ways and for community building dates back to his undergraduate years studying physics at Washington University in St. Louis. 

During his freshman and sophomore years, Clark had a few false starts. He wasn’t a fan of quantum mechanics for its reliance on mathematical tricks and worked in a couple of condensed matter physics labs that left him unexcited. Then, he stumbled into the astrophysics research group of Physics Professor Henric Krawczynski, who was working on detecting the polarization of celestial X-rays—Clark’s first exposure to a new way of looking at the universe. 

“What really caught me with that project was how well astrophysics as a concept integrated the physics,” Clark recalled. “It doesn't utilize just one branch of physics—you have to think really cross-disciplinarily. You need to know statistical mechanics and electromagnetism and a little bit of dynamics to put all of these stories together. And I enjoyed thinking broadly in that sense.” 

In Krawczynski’s group, Clark contributed to the development of a new method for analyzing data from a type of telescope that didn’t yet exist. This type of telescope, known as an X-ray polarimeter, would look at the polarization—or orientation—of X-ray light. While X-rays coming from remote reaches of the universe have been detected before, pinning down their polarization had always been difficult. Nevertheless, polarization was expected to carry extra information about the source of the X-rays, potentially elucidating things like the way matter collapsed into a black hole. Recently, an X-ray polarimeter called IXPE launched into orbit, and the work Clark contributed to has emerged as the main data analysis method for scientists using the telescope. 

“It's been fun to watch the citation count tick up,” Clark said. 

In his undergraduate years, Clark also participated in many community-oriented activities. He was a residential advisor at his dorm and volunteered as both an orientation leader and a tour guide. During his senior year, Clark was torn between his passion for physics and his enthusiasm for community building. He applied to physics and higher education administration graduate programs. However, he couldn’t imagine leaving science altogether. By the end of his senior year, he firmly settled on physics. 

Clark enrolled at Ohio State University and joined the group of Physics Professor Amy Connolly to work on what would become the subject of his career—high-energy neutrinos. 

“He walked in my office looking for a spot and he was just glowing with enthusiasm, like he always is,” Connolly recalled. 

With Connolly’s support, Clark landed a National Science Foundation Graduate Research Fellowship to support his studies. 

“Neutrinos are really young as a discipline, especially high-energy neutrinos,” he said. “They're a pretty new way of looking at the night sky. And that was really intriguing to me. It’s a really nascent field that's just finding its feet.”

Neutrinos are a peculiar type of particle. Of all the particles that have mass, they are the lightest. They have no electrical charge and only interact weakly via gravity and the nuclear forces responsible for radioactivity. Because of this, they largely pass through matter, including humans, unimpeded, which has earned them immortality in a poem by John Updike. It’s estimated that 100 trillion neutrinos pass through you every second.

The reluctance of neutrinos to interact with anything else meant that they remained undetected until 1956. But once they were discovered, it didn’t take long for scientists to conceive of using neutrinos as a sort of telescope to look at the universe. Scientists first spotted neutrinos coming from the sun in 1968, but it took until 2013 for anyone to find neutrinos originating from outside the solar system. These were detected by an experiment called IceCube, located at the South Pole in Antarctica. 

The basic idea of the IceCube experiment is simple: Neutrinos almost never interact, but one way to catch them is to provide a really big target and wait around for signs that a neutrino smashed into it. Luckily, the big target already exists—ice around the South Pole is dense enough to provide a decent chance of interaction, and there’s lots of it.  So, scientists buried detectors deep into Antarctic ice sheets. If, by chance, a neutrino comes close enough to one of the atomic nuclei inside the ice molecules, it will cause something akin to an explosion, sending out a shower of particles and radiation, from flashes of optical light to radio waves. 

The IceCube detectors capture the light, which indicates what direction a neutrino came from and how much energy it had. This provides a new way to look at the universe, distinct from optical imaging with tools like the James Webb Space Telescope or gamma and X-ray detectors. Neutrinos carry information about powerful cosmic events, like exploding stars and black holes, that are hard to see by other means. 

IceCube’s method for detecting astronomical neutrinos works well for neutrinos that carry up to about a trillion times the energy of a typical electron meandering around a hydrogen atom. But for the ultra-high energies that Connolly’s group was interested in—another million times more energetic—it was destined to fall short. 

Ultra-high-energy neutrinos are likely produced by cosmic ray bursts that are otherwise too difficult to see because their heavier components get bent by magnetic fields and everything but neutrinos gets bogged down in the celestial sludge left over from the early universe known as the cosmic microwave background. These neutrinos are extremely rare, and the volume of ice filled with IceCube detectors is too small to stand a reasonable chance of catching one. 

To increase the volume of detection, scientists turned to radio-wave detectors. The radio signal produced by neutrino explosions in ice is stronger for the highest-energy neutrinos, and it’s also able to travel much farther through ice without getting absorbed. So, fewer radio detectors can be placed larger distances apart to capture signals from a greater volume. In hopes of catching some ultra-high-energy neutrinos, scientists started building the Askaryan Radio Array (ARA), an array of radio detectors near the IceCube experiment, to detect neutrino events anywhere within 10 cubic kilometers of ice (compared to 1 cubic kilometer for IceCube). 

As a graduate student, Clark got his hands dirty with every aspect of the ARA experiment. He built two of the five radio detectors that were deployed and refurbished two more. He then went down to Antarctica, where he played a role in installing the detectors into the ice. 

“Antarctica was a really wild place,” Clark said. “It's constantly trying to kill you. But it's also a really beautiful place with a really unique community. It was great.”

After returning from Antarctica, Clark led the most comprehensive data analysis of ARA’s high-energy neutrino search to date. So far, they haven’t caught any of the elusive particles. But for this type of detector, they set the tightest limits on how many ultra-high-energy neutrinos could be hitting Earth. They developed efficient data analysis techniques that could be applied to a variety of detectors, operating over many years. 

“The detectors always look and behave a little bit differently, but you have to rely on all of them working together in order to do the science,” Clark said. “We demonstrated that you can do this kind of analysis at scale—that you can take data from detectors that look a little bit different, behave a little different and have been running for a long time.”

In addition to his hardware contributions, Clark also helped develop software that models neutrino interactions to aid in designing the next generation of detectors. In particular, he focused on how the signal from a neutrino explosion propagates through the ice. This is no easy task—the ice changes density from bottom to top, and the model needs to simulate millions of optical or radio signals traveling through it. Clark and his collaborators found and implemented an efficient way to model the curved trajectories the signals take, enabling the simulation to run in a reasonable timeframe. 

Clark’s mastery of all aspects of the ARA experiment, as well as his approachable nature, made him a valuable resource for junior members of the project and other collaborators. 

“One thing that’s really characteristic of Brian is what a fantastic mentor he is,” Connolly said. “He's always there to help, almost to a fault. I would always tell him, ‘Brian, you can point people to other places to get information.’ But everyone always wants to talk to Brian.”

After earning his Ph.D., Clark was awarded the prestigious NSF Astronomy and Astrophysics postdoctoral fellowship. He joined the physics and astronomy department of Michigan State University and brought the high-energy neutrino program there for the first time. 

“He was largely independent,” said Tyce DeYoung, one of Clark’s co-advisers at Michigan State and an associate professor of physics and astronomy. “He was sort of breaking new ground for the group here at Michigan State.

While there, Clark extended the search for ultra-high energy neutrinos to optical signals by finding new ways of analyzing data from IceCube. He also played key roles in planning the next generation of IceCube and a new neutrino-detection experiment being built in Greenland called the Radio Neutrino Observatory in Greenland (RNO-G), which has become part of his research focus at UMD. 

Throughout his graduate and postdoctoral career, Clark carried on his passion for community leadership and mentorship. As a graduate student, he helped design and facilitate Ohio State’s ASPIRE program, a hands-on physics research workshop for high school girls. He also brought physics demos to elementary schools and served for 18 months on his department’s Climate and Diversity Committee. As a postdoc, he was an early career scientist representative to the IceCube governance board and served on the IceCube collaboration’s working group on diversity and inclusion.

DeYoung appreciated Clark’s involvement with community building. 

“It was a great pleasure to work with him and to see him broaden his horizons while he was here, and to start to think about not just the scientific aspects of how you build a detector and how you analyze data, but also expanding to thinking about how you organize people to build these big projects and to operate these big projects,” DeYoung said. “How do you communicate with the scientific community and foster the sort of scientific community that we want?”

Now, Clark is looking forward to bringing his passions for the high-energy neutrino search and community building to UMD. 

“UMD is a really exciting nexus,” Clark said. “It's a really unique blend of experimental approaches, that makes it a unique place.”


Written by Dina Genkina

Sasha Philippov Investigates Plasma Around Black Holes and Neutron Stars

Curiosity about the cosmos has attracted many young astrophysicists to their careers, but that was not the case for Alexander “Sasha” Philippov. While growing up in Russia, he was a natural at science and even competed in the national Physics Olympiad for high school students. However, his interests at the time were more down-to-earth.Sasha PhilippovSasha Philippov

“I was not into astrophysics whatsoever,” said Philippov, who joined the University of Maryland’s Department of Physics as an assistant professor in May 2022. “When I went to college, I was hoping to do theoretical particle physics, but my advisor and mentor, Professor Vasily Beskin, showed me that you can do very interesting, detailed modeling in astrophysics. It looked like real physics to me—not just looking into a telescope and watching the stars.”
Philippov has been starstruck ever since, using a combination of theory and computer modeling to tackle big questions in high-energy astrophysics and publish over 60 papers. 

After completing his undergraduate studies at the Moscow Institute of Physics and Technology, he earned a Ph.D. in astrophysical sciences from Princeton University in 2017. It was during his graduate studies that he started specializing in the physics of plasmas—a state of matter comprising hot, ionized gas that surrounds “some of the most mysterious and exotic objects in the universe,” including black holes and neutron stars, Philippov said. 

He was then named a NASA Einstein and Theoretical Astrophysics Center Fellow and completed his postdoc at UC Berkeley, where he co-developed computer code to model the behavior of charged particles around rotating black holes.

“This sort of thing had never been modeled before, because you need to take into account the effects of general relativity, how the electromagnetic fields are modified by the spinning black hole and how they react to the dynamics of charged particles,” Philippov explained. “Ours was the first code that could do it.” 

His code enabled a flurry of studies—including five publications in the journal Physical Review Letters—which offered new insights into the extraction of energy from rotating black holes, the production of electron-positron plasmas and electromagnetic flares near the event horizons of black holes, and the accretion of collisionless plasma.

After his postdoc, Philippov worked as an associate research scientist at the Simons Foundation’s Flatiron Institute in New York City, where he continued to study the emission mechanisms of pulsars—magnetized neutron stars that rapidly rotate. During this time, he constructed the first models capable of explaining the mysterious coherent emission of pulsars, which were found to produce relativistic lightning strong enough to shoot beams of radio waves out into space. Although this cosmic phenomenon was discovered more than 50 years ago, its cause was not fully understood until Philippov’s study.

Recently, he was named deputy director of a multi-institutional Simons Foundation project called the Simons Collaboration on Extreme Electrodynamics of Compact Sources (SCEECS). The team will model electrodynamic processes related to neutron stars and black holes, which have magnetic fields that are 10 billion times stronger than those associated with the Large Hadron Collider in Switzerland.

The group will use kinetic plasma simulations and large-scale fluid-type modeling to study the underlying emission mechanisms of magnetars—neutron stars with the strongest magnetic fields—and spinning black holes. Advances in high-performance computing and telescope technologies help make this work possible.

“It’s a big investment for research that is completely theoretical,” Philippov said of the grant. “We will be looking at a collection of both completely new and very old problems—the latter having been revitalized by the power of computer simulations and new observations. It’s a great time to do this.”

Philippov also recently received a $600,000 grant from the National Science Foundation to support the modeling of phenomena in extreme plasmas around black holes and neutron stars. In addition, he continues to study the radiation of neutron stars (including enigmatic fast radio bursts), flares from accreting supermassive black holes and merging neutron stars, and particle acceleration in jets produced by black holes. 

As a Fellow in the Joint Space-Science Institute (JSI), a collaboration between the NASA’s Goddard Space Flight Center and UMD’s astronomy and physics departments, Philippov is helping to organize a workshop in October 2023 that will share the latest expertise on “winds throughout the universe,” including everything from the sun’s powerful wind to the gusts produced by merging neutron stars.

In his day-to-day work, Philippov works closely with students and postdoctoral fellows and said he “tremendously enjoys mentoring and interacting with early career researchers.” 

Though he was not instantly smitten with astrophysics as a high school student, he is happy he pursued this career path. For him, having the chance to ponder big questions in astrophysics—and better yet, to answer them—is what continues to drive his research.

“The time that you get to study something completely new is why we are in this business,” Philippov said. “The joy of getting to finally understand something after a long time is remarkable.”


Written by Emily Nunez.

In Memoriam

It is with much sadness that the Department of Physics announces the passing of several members of our community.

  • Donald Robert Benton (Ph.D., '85) died on May 5, 2023.
  • Larry Lambert Burton (Ph.D., '77) died on March 23, 2023.
  • Richard Durkin (M.S., '72) died on April 22, 2023. 
  • Lewis Fulcher, a former postdoctoral associate, died on May 9, 2023.
  • Professor Emeritus Charles William Misner died on July 24, 2023. A memorial service is planned for November 10 & 11More
  • Sherman Poultney, a faculty member who worked on the Lunar Laser Ranging Reflector for Apollo 11, died on February 9, 2023.
  • Bruce Rowley, a talented machinist in the Department of Physics from 2004 until his retirement in 2022,, died on July 19.
  • Arthur W. Ruff (Ph.D., '64), died on April 24, 2023.
  • Philip DiLavore III, a who served as an Assistant Professsor before moving to Indiana State University in 1971, on died July 16, 2023.
  • Jonathan San Miguel (B.S., '17), died on April 16, 2023, in Stanford, CA.

Faculty, Staff, Student and Alumni Awards & Notes

We proudly recognize members of our community who recently garnered major honors, began new positions and more.

Faculty and Staff 
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Buonanno Elected to Italian National Academy of Sciences

Alessandra Buonanno has been elected a member of the Accademia Nazionale dei Lincei, the Italian National Academy of Sciences

Buonanno is the director of the Astrophysical and Cosmological Relativity Department at the Max Planck Institute for Gravitational Physics  (Albert Einstein Institute) in Potsdam and a Research Professor at the University of Maryland.

Buonanno's research has spanned several topics in gravitational wave theory, data analysis and cosmology. She is a Principal Investigator of the LIGO Scientific Collaboration, and her waveform modeling of cosmological events has been crucial in the experiment’s many successes. Her work has merited election to the U.S. National Academy of Sciences and Leopoldina, the German National Academy of Sciences.

In 2018, Buonanno received the Leibniz Prize, Germany's prestigious research award. Other accolades include the Galileo Galilei Medal of the National Institute for Nuclear Physics (INFN), the Tomalla Prize, the Dirac Medal (with Thibault Damour, Frans Pretorius, and Saul Teukolsky) and the Balzan Prize (with Damour).

Alessandra Buonanno © A. Klaer Alessandra Buonanno © A. Klaer

She is a Fellow of the American Physical Society and the International Society of General Relativity and Gravitation and a recipient of the Alfred P. Sloan Foundation Fellowship and the Richard A. Ferrell Distinguished Faculty Fellowship.

Buonanno, Charlie Misner, Peter Shawhan and others detailed UMD's contributions to gravitational studies in a 2016 forum, A Celebration of Gravitational Waves