Author: Emily Conover / Source: Science News
A scientific tug-of-war is underway over the size of the proton. Scientists can’t agree on how big the subatomic particle is, but a new measurement has just issued a forceful yank in favor of a smaller proton.
By studying how electrons scatter off of protons, scientists with the PRad experiment at Jefferson Laboratory in Newport News, Va., sized up the proton’s radius at a measly 0.83 femtometers, or millionths of a billionth of a meter. That’s about 5 percent smaller than the currently accepted radius, about 0.88 femtometers.
The new figure adds to a muddle of measurements, each of which seems to fall into one of two camps — favoring either the standard radius or one a tad smaller. With the new result from PRad, “if anything, the proton radius puzzle has become even more puzzling,” says physicist Nilanga Liyanage of the University of Virginia in Charlottesville. He presented the result on October 23 at a joint meeting of the American Physical Society Division of Nuclear Physics and the Physical Society of Japan, held in Waikoloa, Hawaii.
The experiment, in which electrons scatter off of the protons contained in hydrogen gas, improves upon previous electron-proton scattering experiments by catching electrons that scatter away at glancing angles, as small as 0.6 degrees. Such electrons help suss out the protons’ size by probing the outermost edges of the protons.
PRad’s small radius is in…
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