Nuclear Recoil Effect in the Lamb Shift of Light Hydrogenlike Atoms

We report high-precision calculations of the nuclear recoil effect to the Lamb shift of hydrogenlike atoms to the first order in the electron-nucleus mass ratio and to all orders in the nuclear binding strength parameter Zα. The results are in excellent agreement with the known terms of the Zα expansion and allow an accurate identification of the nonperturbative higher-order remainder. For hydrogen, the higher-order remainder was found to be much larger than anticipated. This result resolves the long-standing disagreement between the numerical all-order and analytical Zα-expansion approaches to the recoil effect and completely removes the second-largest theoretical uncertainty in the hydrogen Lamb shift of the 1S and 2S states.