New measurements of the proton’s radius

Electron measurements
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Scientists used the first new method in half a century for measuring the size of the proton via electron scattering, producing a new value for the proton’s radius.

The PRad collaboration has conducted an experiment at the Department of Energy’s Thomas Jefferson National Accelerator Facility. The experiment has produced a new value for the proton’s radius.

The result of the study has resulted in the most precise measurement from electron-scattering. The new value for the proton radius that was obtained is 0.831 fm, which is smaller that the previous value of 0.88 fm and agrees with recent muonic atomic spectroscopy results.

“We are happy that years of hard work of our collaboration is coming to an end with a good result that will help critically toward solution of the so-called proton radius puzzle,” says Ashot Gasparian, a professor at North Carolina A&T State University and the experiment’s spokesperson.

In 2012, a collaboration of scientists led by Gasparian came together at Jefferson Lab in order to revamp electron-scattering methods in hopes to produce a novel and more precise measurement of the proton’s charge radius. The PRad experiment was given priority scheduling as one of the first experiments to take data and complete it run following an upgrade of the Continuous Electron Beam Accelerator Facility, a DOE User Facility for nuclear physics research.

“When we started this experiment, people were searching for answers. But to make another electron-proton scattering experiment, many skeptics didn’t believe that we could do anything new,” says Gasparian. “If you want to come up with something new, you have to come up with some new tools, some new method. And we did that — we did an experiment which is completely different from other electron-scattering experiments.”

The scientific collaboration instituted three new techniques to improve the precision of the new measurement. The first being the implementation of a new type of windowless target system, funded by a National Science Foundation Major Research Instrumentation grant.

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