MSU physics faculty’s breakthrough research resolves years-old proton size puzzle


Newswise — STARKVILLE, Miss.—Research by Mississippi State scientists has helped resolve the decades-old “proton radius puzzle,” a question that for years has had physicists experimenting and theorizing to unravel the mystery.

This research, titled the “Proton Radius Experiment” and performed by a national scientific team including three MSU physicists, publishes today in Nature, the international journal of science. Known as PRad, the investigation involved MSU Department of Physics Professors Dipgankar Dutta, corresponding author, and James A. Dunne, as well as Assistant Professor Lamiaa El-Fassi.

Dutta explained their breakthrough confirms one of the most accurately determined, fundamental constants of physics is slightly smaller than previously thought.

“Our results show there is no discrepancy in proton size when measured using ordinary hydrogen atoms or an exotic form of hydrogen atoms,” Dutta said.

The first new method in 50 years for measuring proton size using electron scattering from regular hydrogen atoms, the experiment—conducted at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility—discovered a new value for the proton’s radius.

Known as its charge radius, the size of a proton is a fundamental quantity in physics.

“All visible matter in the universe is built on protons—a cloud of three quarks bound together with strong force energy. The proton, the center of every atom, has been the subject of numerous studies and experiments aimed at revealing its secrets,” according to recently released JLab information.

For decades, the charge radius of the proton had been obtained from high precision measurement of the energy levels of the hydrogen atom or by scattering electrons from hydrogen atoms, which until recently both produced proton charge radius measurements that corroborated.

Dutta said the charge radius of the proton obtained from muonic hydrogen—a lab-created exotic form of hydrogen—was found to be “significantly” smaller than those obtained from regular hydrogen atoms.

“Given the precision of the measurements, the muonic hydrogen and regular hydrogen results being this different, just by chance, was less than about 1 in a 100 billion,” Dutta said. “This was called the ‘proton charge radius puzzle’ and led to a rush of experimental as well as theoretical efforts to understand why the size of the proton appears to be different when measured in regular hydrogen versus muonic hydrogen.”

“Many physicists were excited by the possibility that the ‘puzzle’ was an indication of a possible new fifth force that acted differently on electrons and muons,” Dutta continued. “The results of the PRad experiment effectively seem to resolve the ‘proton radius puzzle,’ and close the door on the possibility that the ‘puzzle’ was an indication of the existence of a new fifth force in nature.”

The PRad collaboration instituted new techniques to improve the precision of the new measurement from prior electron-scattering experiments. The first technique, a new type of windowless target system, was funded by a National Science Foundation Major Instrumentation grant.

The collaborators also were able to close the angle of the electrons bouncing off the hydrogen target to less than one degree. This new technique was achieved by using a large area electron detector, a calorimeter, to precisely measure the energy and position of the electron.

“In electron scattering, in order to extract the radius, we have to have as small a scattering angle as possible,” said Dutta. “To get the proton radius, you need to extrapolate to zero angle, which you cannot access in an experiment. So, the closer to zero you can get, the better.”

“In all previous electron scattering experiments, knowing exactly how many hydrogen atoms were hit by the electron beam was one of the largest sources of uncertainty,” Dutta said. “The innovative design of the calorimeter helped reduce and precisely control the experimental uncertainties.

“Our new method has pushed the frontier of precision that can be achieved in electron scattering experiments and opens the door for further high precision measurements and hopefully exciting new discoveries,” Dutta said.

The experiment is part of a larger collaboration led by MSU and Duke, Idaho State and North Carolina A&T State universities. MSU’s involvement was funded in part by DOE’s Office of Science and the NSF.

Dutta serves as the experiment’s co-spokesperson along with Ashot Gasparian, North Carolina A&T; Haiyan Gao, Duke University; and Mahbub Khandaker, Idaho State University.

Former MSU graduate student Li Ye, a 2018 applied physics doctoral graduate, served the project as a thesis student. In addition to Dunne and El-Fassi, the university’s post-doctoral research associates Krishna Adhikari, Latiful Kabir and Mitra Shabestari contributed, as well as former graduate student Adesh Subedi, a 2014 applied physics doctoral graduate, and former undergraduate presidential scholar Ben Emmich of Olive Branch, a spring 2019 mathematics and physics graduate. Graduate student Pubuduni Ekanayaka participated, and physics graduate students Hem Bhatt, Deepak Bhetuwal, and Abishek Karki currently are active participants in Dutta’s research.

Part of MSU’s College of Arts and Sciences, complete details about the Department of Physics may be found www.physics.msstate.edu.

MSU is Mississippi’s leading university, available online at www.msstate.edu.

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