title: Constraints on proton structure from precision atomic-physics measurements.
abstract: Ground-state hyperfine splittings in hydrogen and muonium are very well measured. Their difference, after correcting for magnetic moment and reduced mass effects, is due solely to proton structure-the large QED contributions for a pointlike nucleus essentially cancel. The rescaled hyperfine difference depends on the Zemach radius, a fundamental measure of the proton, computed as an integral over a product of electric and magnetic proton form factors. The determination of the Zemach radius, (1.019+/-0.016) fm, from atomic physics tightly constrains fits to accelerator measurements of proton form factors. Conversely, we can use muonium data to extract an experimental value for QED corrections to hydrogenic hyperfine data. There is a significant discrepancy between measurement and theory, in the same direction as a corresponding discrepancy in positronium.
authors.affiliation: Stanford Linear Accelerator Center, Stanford University, Stanford, California 94309, USA.
authors.forename: Stanley J
authors.initials: SJ
authors.lastname: Brodsky
authors.forename: Carl E
authors.initials: CE
authors.lastname: Carlson
authors.forename: John R
authors.initials: JR
authors.lastname: Hiller
authors.forename: Dae Sung
authors.initials: DS
authors.lastname: Hwang