Loop-after-loop contribution to the second-order Lamb shift in hydrogenlike low-Z atoms

Abstract

We present a numerical evaluation of the loop-after-loop contribution to the second-order self-energy for the ground state of hydrogenlike atoms with low nuclear charge numbers Z. The calculation is carried out in the Fried-Yennie gauge and without an expansion in Z α. Our calculation confirms the results of Mallampalli and Sapirstein and disagrees with the calculation by Goidenko and coworkers. A discrepancy between different calculations is investigated. An accurate fitting of the numerical results provides a detailed comparison with analytic calculations based on an expansion in the parameter Z α. We confirm the analytic results of order α^2 (Zα)^5 but disagree with Karshenboim's calculation of the α^2 (Z α)^6 \ln^3(Z α)^{-2} contribution.