New experimental constraints on polarizability corrections to hydrogen   hyperfine structure
  We present a state-of-the-art evaluation of the polarizability corrections--the inelastic nucleon corrections--to the hydrogen ground-state hyperfine splitting using analytic fits to the most recent data. We find a value $\Delta_{\rm pol} = 1.3 \pm 0.3$ ppm. This is 1--2 ppm smaller than the value of $\Delta_{\rm pol}$ deduced using hyperfine splitting data and elastic nucleon corrections obtained from modern form factor fits. 
Vahagn Nazaryan, Carl E. Carlson, and Keith A. Griffioen (College of   William and Mary and Hampton University)