Puzzling problem: Karsten Schuhmann (left) and Aldo Antognini of the Paul Scherrer Institute inspect the laser system that was used for the laser spectroscopy measurements on muonic deuterium.

After years of confusion, a new study confirms the proton is tinier than once thought. That enables a test of the standard model of particle physics.

Physicists get closer to solving the proton radius puzzle with unique new measurement of the charge radius of the proton Textbook pictures make atomic nuclei seem tidy, but they are actually fuzzy ...