The proton was discovered just over a hundred years ago and has been intensely studied ever since. Yet, there’s still more to learn about this important building block of the visible universe. Now, work toward a better understanding of the proton carried out at the Department of Energy’s Thomas Jefferson National Accelerator Facility has earned Weizhi Xiong the 2020 Jefferson Science Associates (JSA) Thesis Prize.
A proton is a stable particle with a positive electric charge. Its charge radius is related to the charge distribution of its constituent objects. The goal of the PRad experiment is to help advance our understanding of the proton with a precision measurement of the proton’s charge radius.
Xiong’s outstanding doctoral dissertation describing PRad and its results not only helped him earn a Ph.D. in physics last year from Duke University, it also won him the thesis prize.
“It’s certainly a great honor,” said Xiong, 32, who journeyed from China as an 18-year- old to begin studying physics at the University of South Carolina. “I remember 15 years ago, when I first came to the U.S., I couldn’t even speak English very well. I couldn’t even say, ‘Sorry, I dialed the wrong number.’ It almost feels like yesterday, and now I’m here receiving an award at this scale — it’s definitely quite a milestone for me.”
Xiong’s dissertation, “A High Precision Measurement of the Proton Charge Radius,” examines one of the key aspects of this essential particle that continues to intrigue physicists. While the charge radius is one key aspect, others include its spin and mass.
There are two well-established methods to measure a proton’s charge radius: electron scattering and hydrogen spectroscopy.
In electron scattering, a beam of electrons hits a gas or liquid hydrogen target. (A hydrogen atom is a proton orbited by an electron.) The proton target causes the electron beam to deflect or scatter. Then, by measuring the angular distribution of the scattered electrons, you can measure the size of the protons, in what’s called electron-proton elastic scattering.
For a long time, the two methods came up with very similar results. Then, a decade ago, a new method called muonic hydrogen spectroscopy swapped out the orbiting electron with its heavier cousin, called the muon, resulting in a new and much more precise measurement that surprised physicists.
Since then, they’ve been trying to resolve what they call the “proton charge radius puzzle.”
To aid in that effort, Jefferson Lab used its unique and world-class Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Nuclear Physics User Facility, to conduct a different kind of electron scattering experiment.
One of the ways that PRad changed things up is by using a “windowless” target — basically, a container for the hydrogen target with holes for the electron beam to pass through. This reduced the amount of “noise” or extraneous data that such experiments can collect when a particle beam hits both sides of a container as it enters and exits. Xiong said it’s the first time the lab used a windowless target.
In addition, PRad measured not only electron-proton scattering, but electron-electron scattering. By comparing those two results, Xiong said, they were able to get a very precise charge radius measurement.
“We were really just trying to measure this quantity using a completely different approach compared to other electron scattering experiments,” Xiong said. “We didn’t know whether it was going to agree with the old result or the new muonic result.”
It turns out that PRad agreed more closely with the new muonic measurement, he said.
Still, the puzzle persists, particularly between electron scattering results: The slight difference in the results was enough for the lab to plan to run another experiment to try to hone the measurement even more.
The quest continues because an ultra-precise measurement of the charge radius is essential for a better understanding of quantum chromodynamics, or the theory that describes the strong interaction between quarks and gluons that make up protons and other particles.
Paying it forward
As accomplished as Xiong has become in his physics education, it didn’t start out that way. He says he was an average student in high school with little interest in the subject.
What inspired him to study harder was a certain girl in his physics class who also struggled and so would seek help from other students.
“But she never asked me, because I also was terrible,” Xiong said. “I thought, ‘Well, if I’m good at this, she will ask me.’ That’s how I got into it.”
He never got the girl, but physics eventually hooked him, and he threw himself into it.
“You have a clear goal,” Xiong said of its appeal. “You’re curious about the nature and the beauty in physics, and so you try to understand it.”
At the University of South Carolina, professors mentored him and assigned him projects that also grabbed his interest. That convinced him to go for his doctorate. At Duke, his advisor and other gifted peers inspired him even more. He credits all of them for helping him earn the 2020 JSA Thesis Prize.
The prize is given every year to the top Ph.D. thesis on Jefferson Lab-related research. Judges consider four criteria: the quality of the writing, the student’s contribution to the research, the work’s impact on the field of physics, and how the work contributes to science at Jefferson Lab or other experiments. Winners get a $2,500 cash award and a commemorative plaque.
“Graduate students play a central and important role in advancing the science program of Jefferson Lab, and the Jefferson Lab User Organization, with the support of JSA, is ecstatic to be able to recognize their efforts with the awarding of this prize,” said JLUO Chair Edward Brash, professor of physics at Chritsopher Newport University. “As usual, the theses submitted were of very high quality and this made our decision challenging. I want to thank the selection committee, lead by Carlos Munoz, for its careful examination of the submissions. In the end, we are delighted to recognize the outstanding work that Weizhi has produced.”
The prize is funded by the JSA Initiatives Fund program, which provides support for programs, initiatives and activities that further the scientific outreach, and promote the science, education and technology missions of Jefferson Lab and benefit the laboratory’s user community. The prize was established in 1999 by the Southeastern University Research Association (SURA), one of the joint owners of JSA.
Toward the Future
Doctorate in hand, Xiong says he intends to secure a position teaching physics at the high school level. He’s looking to return to his former high school in the large port city of Guangzhou near Hong Kong.
In a high school setting, said Xiong, “I really get to teach and influence kids. I think that maybe has a bigger impact — not just on their physics knowledge, but on other things relating to their life. What they want to do in the future. So it’s more than just teaching physics.
“I hope that I will have a lot more things to relate to them, because I sort of started as an average, or even below-average, kid in high school in terms of academics. So I know a lot about struggle. I probably know also how to make them interested in things like research and academia.”
Jefferson Science Associates, LLC, a joint venture of the Southeastern Universities Research Association, Inc. and PAE, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy's Office of Science.
DOE’s 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, visit https://energy.gov/science.