Phoebe Hamilton’s (M.S. ’11, Ph.D. ’13, physics) research career at the University of Maryland could have ended when she earned her Ph.D. Instead, it marked the start of an exciting challenge—for Hamilton and her fellow high-energy physics researchers in UMD’s Department of Physics.
Phoebe Hamilton, Elizabeth Kowalczyk and Othello Gomes check a photodetector.In fall 2012, Distinguished University Professor and Gus T. Zorn Professor Hassan Jawahery eyed a new opportunity after his research group wrapped up with BaBar, a collider experiment in California.
“BaBar had finished collecting new data and we were looking for the next gig for the group,” Hamilton recalled. “Hassan was my Ph.D. advisor and we talked about how exciting it would be to move to the Large Hadron Collider beauty—LHCb—experiment.”
Just two months before Hamilton defended her dissertation, Jawahery’s group learned that they had been formally accepted into the LHCb experiment, which is named after its primary research subject: a particle called the beauty quark, also known as a bottom quark or b quark. By studying bottom quarks produced by proton-proton collisions at the Large Hadron Collider located near Geneva, Switzerland, researchers hope to come closer to understanding why there is so much matter but so little antimatter in the universe.
Excited by the opportunity to discover new physics at the world’s most powerful particle accelerator, Hamilton stayed at UMD. As a postdoctoral researcher from 2012 to 2020 and a faculty specialist from 2020 to 2023, she developed tools that enabled the LHCb to take measurements previously thought “impossible” by some scientists.
Now, as an assistant professor of physics, her contributions continue to level up the LHCb’s abilities, improving its chances of making groundbreaking findings.
“Getting to stay a postdoc as long as I did at Maryland was a real blessing,” Hamilton said. “I wasn’t sure I’d actually have the chance to become an assistant professor, but I'm very happy to get to do it. Maryland is such a home and such a family to me.”
Raising the BaBar
Hamilton’s interest in physics began in high school and she nurtured it with books about string theory by physicist Brian Greene. After graduating, she enrolled at Youngstown State University to pursue computer science, another one of her interests, but switched to physics after realizing that it inspired and challenged her more than any other subject.
“I like knowing how things work,” said Hamilton, who also enjoys learning new musical instruments for similar reasons. “The fact that physics is orderly and follows these predictable rules has always been fascinating to me.”
Hamilton quickly took to particle physics, and after earning her bachelor’s degree in 2007, she decided to pursue particle theory research in UMD’s graduate program. She chose UMD because of its wide range of research possibilities, which allowed her to try out different specializations before committing.
“I thought I knew what I wanted to do, but there was doubt,” she said. “With UMD, I thought to myself, ‘This is where I’m going to be able to thrive no matter what I do.’”
Hamilton soon discovered she enjoyed the experimental side of particle physics much more than theory. So when Associate Professor Doug Roberts put up flyers seeking student researchers for the BaBar experiment, Hamilton jumped at the chance.
BaBar was Hamilton’s introduction to experimental studies of CP violation, which occurs when two conservation laws of particle physics—charge conjugation and parity—are broken. By measuring CP violation at experiments like BaBar, researchers can begin to understand the differences between matter and antimatter.
“I fell in love with it very quickly,” Hamilton said of BaBar. “It was a fantastic machine and a fantastic experiment.”
Hamilton’s research contributed to the first measurement of how Bs mesons, a family of subatomic particles called mesons that contain a bottom quark and a strange antiquark, are produced at different collision energies. Ultimately, the BaBar experiment shed light on how antimatter is produced and set the stage for Hamilton’s participation in an even bigger—but messier—collider.
“The beautiful thing at BaBar was that you would get two hadrons containing bottom quarks and nothing else, so it was very clean and very easy to measure what was going on,” Hamilton said. “Here [at the LHCb], colliding protons is like colliding handfuls of rock salt. You get 100 reconstructed particles in every event and you have to sort through it.”
Achieving the ‘impossible’
For the last 12 years, Hamilton has been working to make those messy collisions a little easier to interpret. UMD’s contribution to the LHCb experiment falls within the realm of lepton flavor universality: a physics principle stating that the only difference between different “flavors,” or types, of leptons—including electrons, muons and tau leptons—is their mass.
The LHCb is a good fit for this type of research because it analyzes a large number of particles containing b quarks, which transform, or decay, into leptons. In the beginning, though, some scientists thought that lepton flavor universality couldn’t be done at the LHCb because either one or three neutrinos escape undetected during collisions, making it difficult to determine all of the energies and momenta needed to distinguish muons from tau leptons.
“Because of the messy nature of these proton-proton collisions, the consensus was that this was too hard for LHCb to do,” Hamilton said. “But Jawahery and I worked together on a technique to make some wild approximations and figure out a way to do it anyway.”
And they did figure out a way. Developed from 2013 to 2015 in collaboration with LHCb researcher Greg Ciezarek, their method of analyzing decays led to measurements of lepton flavor universality between muons and tau leptons that were previously thought impossible.
“It was interesting to go from ‘This is probably another dead-end’ to ‘Oh, this might actually be worth something’ to ‘This is actually the star of the experiment right now,’” Hamilton said. “This is still an active area of research for us. We extended and superseded the 2015 measurement in 2023 and are working on the next generation of this in the data from the second run of the LHC.”
Cracking the K-pi puzzle
Over the years, Hamilton has also played a key role in making the LHCb’s equipment more durable and better at discerning different particles. She helped develop electronics for the Upstream Tracker sub-detector for the experiment’s first upgrade from 2022 to 2023 and is now testing new photodetectors in her lab. These new detectors would measure the light produced in upgraded modules for the LHCb’s calorimeter, which stops particles as they pass through and enables researchers to measure the energy deposited.
This planned upgrade to the calorimeter aims to make energy measurements more precise, which can ultimately help researchers determine which particles were produced in a collision event.
“One of the big motivations for upgrading the calorimeter is making some of the granularity smaller so that you can tell different particles apart,” Hamilton explained. “Along with the ability to precisely measure the time different particles arrive, it should in principle be able to cope with five times the collision rate.”
Whether Hamilton is toiling in the lab or analyzing data from the LHCb, she continues to find inspiration in physics’ most puzzling questions. She recently submitted a research proposal to dive deeper into matter-antimatter asymmetries and continues to work on developing new and improved techniques for her research.
From 2014 to 2015, she and her colleagues at UMD developed a way to study b-hadron decays with only one reconstructed trajectory, meaning that certain key information is missing. She believes this technique can now be applied to a persistent challenge in physics called the K-pi puzzle.
“The K-pi puzzle is the possibility that the Standard Model fails to explain the pattern of matter-antimatter asymmetry in b-hadron decays to two pseudo-stable mesons, pions or kaons—or one of each. The Standard Model predicts specific patterns to their CP asymmetries, which we can use to check the Standard Model’s validity, but theorists need measurements of them all,” Hamilton explained. “Some of these involve two trajectories to reconstruct and identify the b-hadron but many do not, and these tend to be the less understood ones.”
Going forward, Hamilton hopes to make more “impossible measurements”—and perhaps challenge or reshape the Standard Model of physics in the process.
“We have an opportunity to contribute to understanding this puzzle in some of the areas that are fuzziest right now,” Hamilton said, “and I think there's exciting things to be tried there.”
Written by Emily Nunez