title: Three-Loop Slope of the Dirac Form Factor and the 1S Lamb Shift in Hydrogen.
abstract: The last unknown contribution to hydrogen energy levels at order mα^{7}, due to the slope of the Dirac form factor at three loops, is evaluated in a closed analytical form. The resulting shift of the hydrogen nS energy level is found to be 3.016/n^{3} kHz. Using the QED calculations of the 1S Lamb shift, we extract a precise value of the proton charge radius r_{p}=0.883±0.014 fm.
authors.affiliation: Stanford Linear Accelerator Center, Stanford University, Stanford, California 94309.
authors.forename: K
authors.initials: K
authors.lastname: Melnikov
authors.forename: T
authors.initials: T
authors.lastname: van Ritbergen