Physicists Probe Proton Periphery

Guy Ron is a teaching assistant and Ph.D. student at Tel Aviv University and a Jefferson Lab user.

Sixty years after Hudeki Yukawa won the Nobel Prize for his prediction of mesons, nuclear physicists are looking for them once again.

Yukawa won the 1949 Nobel Prize for his prediction of a new class of particle. While calculating the properties of the force that binds protons and neutrons in the nucleus of the atom, he predicted the mass of a particle that was needed to carry the binding force between protons and neutrons.

He also predicted that the particle, the meson, would be found in cosmic rays, particles that continually bombard the Earth. Mesons and other particles are produced when cosmic rays strike atoms in the Earth's atmosphere, and some of these particles travel to the surface. Within three years of Yukawa's prediction, mesons were spotted.

Now scientists are looking for mesons again. This time, they are searching inside nucleons (protons and neutrons), where Yukawa first predicted their presence. According to Guy Ron, a teaching assistant and Ph.D. student at Tel Aviv University, the scientists are looking for a particular meson: the pion.

Ron says that researchers want to resolve whether our present picture of the proton in the nucleus is accurate. This picture describes the proton as a particle made up of an inner core of its own building blocks, quarks, surrounded by a cloud of pions that interact with other protons and neutrons. Researchers think they've seen evidence of pions in previous experiments.

"All of the previous experiments have hints of interesting results, but the precision was such that you couldn't really say a lot about it. You could say that there was something non-trivial, but it was kind of hard to define. So we went ahead and proposed to do a high-precision survey of this," he says.

Jefferson Lab Hall A researchers probed the structure of the proton using relatively low-energy electrons from Jefferson Lab's CEBAF accelerator. "This is like looking at the proton as a whole rather than probing the individual quarks inside. So it's like looking at the way these quarks interact in a global fashion."

Ron and his colleagues surmise that the new, high-precision data will help to better resolve the picture of the proton. "The most probable explanation to any deviations in our new data is that you're actually seeing the effects of the virtual pion cloud that is around the proton," he says.

The experiment ran last spring. Xiaohui Zhan, a Ph.D. student at the Massachusetts Institute of Technology, is currently analyzing the data taken in this experiment for her Ph.D. thesis. Look for preliminary results in the coming year.