New Evaluation of Proton Structure Corrections to Hydrogen Hyperfine Splitting

We consider the proton structure corrections to hydrogen ground-state hyperfine structure, focusing on a state-of-the-art evaluation of the inelastic nucleon corrections--the polarizability corrections--using analytic fits to the most recent data. We find a value for the fractional correction Delta_{pol} of 1.3 \pm 0.3 ppm. This is 1--2 ppm smaller than the value of Delta_{pol}$ one would deduce using hyperfine splitting data and elastic proton structure corrections obtained from modern form factor fits. In addition, we discuss the derivations of the relevant formulas, paying attention to lepton mass effects and to questions surrounding the use of unsubtracted dispersion relations.