The proton radius revisited
The nucleus of all atoms consists of protons and neutrons, and the simplest of all atoms, hydrogen, has just one proton. The radius of the proton is very small, about 1 fm (1 fm is 10−15 m), smaller than the radius of a hydrogen atom by a factor of 60,000. As a proton is such a fundamental particle, much effort is devoted to measuring its size. Since 2010, proton size has been puzzling theorists and experimentalists alike. Measuring transition frequencies in an exotic form of hydrogen, where instead of an electron a muon—an elementary particle 200 times heavier than the electron—is orbiting the proton, a 4% smaller proton size was found (1). The near-6σ discrepancy with both regular hydrogen spectroscopy and results from electron-proton scattering was coined the “proton-size puzzle” and finding a solution initiated intense scientific debate, so far without a definite outcome (2). On page 79 of this issue, Beyeret al.(3) present a measurement of the 2S-4P transition frequency in regular hydrogen, one of the lines of the Balmer series. The value of the proton size they deduce from their spectra agrees with the value from muonic hydrogen spectroscopy and disagrees with most previous measurements in regular hydrogen—and there were many. They also find a value for one of the most accurately determined constants of nature, the Rydberg constant, which disagrees with the literature value by more than three standard deviations.