The preliminary result from the most recent PRad experiment to determine the proton charge radius suggests it is about five percent smaller than previously thought from electron scattering experiments. Researchers are finishing the remaining study on systematic uncertainties and a final result is expected soon. If confirmed, a smaller proton charge radius could have enormous implications.
The preliminary result from the most recent PRad experiment in the United States, co-led by Haiyan Gao, vice chancellor for academic affairs at Duke Kunshan University, suggests that the size of the proton, specifically its charge radius, is about five percent smaller than previously thought from electron scattering experiments.
By studying how electrons scatter off of protons, scientists with the PRad experiment at the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory) in Virginia, United States, measured the proton’s charge radius as being 0.83 femtometers, or millionths of a billionth of a meter.
Other experiments have arrived at two different, incompatible values for the radius. This measurement is consistent with the smaller of these values, and is about 5 percent smaller than the currently accepted radius of about 0.879 femtometers from electron scattering experiments.
The proton’s size is an important property of nature. And the discrepancies in the measurements are discouraging scientists from performing other experiments, such as high-precision testing the theory of quantum electrodynamics, which describes how light and charged particles interact.
“The result is still preliminary,”said Gao, also Henry Newson Professor of Physics at Duke University. “The researchers are now working very hard to finish the remaining study on systematic uncertainties in order to have a final result soon.”
Gao became interested in the proton charge radius in 1999. However, she didn’t start to focus on this issue again until 2010, when a measurement of the energy levels of the so-called munoic hydrogen atom found a significantly lower proton radius, about 0.8409 femtometers.
The PRad experiment, in which electrons were scattered off protons contained in hydrogen gas, improved upon previous electron-proton scattering experiments by catching electrons that scatter away at glancing angles, as small as 0.7 degrees, which help to determine the protons’ size by probing the outermost edges of the protons.
“Assuming the muonic result is correct, then one has to do better experiments both from electron-proton scattering and also normal hydrogen spectroscopy measurements,” said Gao.
“For electron-proton scattering, the charge radius is determined by the slope of the proton electric form factor at a momentum transfer value of zero, which means one has to do a scattering experiment all the way down to a scattering angle near zero, which is not possible,” added Gao. “All previous experiments used magnetic spectrometers, so they cannot get close to zero degree. Our experiment got to as low as 0.7 degree using a non magnetic calorimeter, which is the smallest scattering angle ever achieved.”
Gao said: “at the moment, our preliminary result suggests that we are in better agreement with a smaller proton charge radius.”
The results of a hydrogen spectroscopy measurement experiment published in 2017 in Science, found a small proton charge radius. The same goes for another study reported in July at the International Conference on Atomic Physics in Barcelona. But a large proton radius was found in a study published in May in Physical Review Letters.
Nilanga Liyanage, a physicist at the University of Virginia in Charlottesville and a senior collaborator from the PRad collaboration, presented the PRad result on behalf of the collaboration at a joint meeting of the American Physical Society Division of Nuclear Physics and the Physics Society of Japan in Waikoloa, Hawaii in October 2018, said that with the new result from PRad, “if anything, the proton radius puzzle has become even more puzzling”.
Some think that the discrepancy in the measurements might reveal the existence of new particles or other secrets of physics. Gao thinks if the discrepancies exist and are not due to discrepancies in the measuring techniques, it might mean there are some differences between the electronic system and muonic system, which we are not familiar with and possibly due to new physics.
“In the standard model, one would not expect such a difference because the electron and muon are in the same lepton family,” she said.
“But before we speculate new physics, it is important we do new experiments such as PRad and normal hydrogen spectroscopy experiments,” added Gao. “Future experiments could help resolve the disagreement.”
As one of the subatomic particles— atoms are made of protons, neutrons and electrons (except hydrogen, which lacks a neutron) — the size of the proton should be the same no matter how one measures it. And a precise knowledge of its charge radius is critically important for the understanding of the underlying quark-gluon structure of the nucleon in the theory of strong interactions – Quantum Chromodynamics (QCD).