Scientists measure precise proton radius to help resolve decade-old puzzle
Distinguished research professor Eric Hessels in his physics lab at York University. Credit: York University

York University researchers have made a precise measurement of the size of the proton—a crucial step towards solving a mystery that has preoccupied scientists around the world for the past decade.

Scientists thought they knew the size of the proton, but that changed in 2010 when a team of physicists measured the proton-radius value to be four percent smaller than expected, which confused the scientific community. Since then, the world’s physicists have been scrambling to resolve the proton-radius puzzle—the inconsistency between these two proton-radius values. This puzzle is an important unsolved problem in fundamental physics today.

Now, a study to be published in the journal Science finds a new measurement for the size of the proton at 0.833 femtometres, which is just under one trillionth of a millimetre. This measurement is approximately five percent smaller than the previously-accepted radius value from before 2010.

The study, led by researchers in York University’s Faculty of Science, presents a new electron-based measurement of how far the proton’s extends, and it confirms the 2010 finding that the proton is smaller than previously believed.

“The level of precision required to determine the proton size made this the most difficult measurement our laboratory has ever attempted,” said Distinguished Research Professor Eric Hessels, Department of Physics

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