Radiative α7m QED contribution to the helium Lamb shift

Abstract

We present a derivation of the last unknown part of the $\alpha^7m$ contribution to the Lamb shift of a two-electron atom, induced by the radiative QED effects beyond the Bethe logarithm. The derivation is performed in the framework of nonrelativistic quantum electrodynamics and is valid for the triplet (spin $S = 1$) atomic states. This derivation opens a way for a complete calculation of the $\alpha^7m$ QED effects in helium, which will allow an accurate determination of the nuclear charge radius from measurements of helium transition frequencies. Together with the complementary investigations of muonic helium, this will provide a stringent test of universality of electromagnetic interactions of leptons in the Standard Model.