Eur. Phys. J. D (2021) 75: 49
https://doi.org/10.1140/epjd/s10053-021-00052-4

Regular Article - Atomic Physics

Hadronic vacuum-polarization contribution to various QED observables

¹ Ludwig-Maximilians-Universität, Fakultät für Physik, 80799, München, Germany
² Max-Planck-Institut für Quantenoptik, 85748, Garching, Germany
³ Pulkovo Observatory, 196140, St.Petersburg, Russia
⁴ D. I. Mendeleev Institute for Metrology, 190005, St.Petersburg, Russia

^a savely.karshenboim@mpq.mpg.de

Received: 14 September 2020
Accepted: 15 January 2021
Published online: 8 February 2021

Abstract

Due to precision tests of quantum electrodynamics (QED), determination of accurate values of fundamental constants, and constraints on new physics, it is important in a consistent way to evaluate a number of QED observables such as the Lamb shift in hydrogen-like atomic systems. Even in a pure leptonic case, those QED variables are in fact not pure QED ones since hadronic effects are involved through intermediate states while accounting for higher-order effects. One of them is hadronic vacuum polarization (hVP). Complex evaluations often involve a number of QED quantities, for which treatment of hVP is not consistent. The highest accuracy for a calculation of the hVP term is required for the anomalous magnetic moment of a muon. However, a standard data-driven treatment of hVP, based on a dispersion integration of experimental data on electron-positron annihilation to hadrons and some other phenomena, leads to a contradiction with the experimental value of $a_{\mu }$. This experimental value can be considered as an indirect determination of the hVP contribution to $a_{\mu }$ and the scatter of theory and experiment allows one to obtain a conservative estimation of the related hVP contribution. In this paper, we derive exact and approximate relations between the leading-order (LO) hVP contributions to various observables. Using those relations, we obtain for them a consistent set of the results, based on the scatter of $a_{\mu }$ values. While calculating the LO hVP term, we have to remember that next-to-LO (NLO) hVP corrections are often comparable with the uncertainty of the LO term. Special attention is payed to hVP contribution to simple atoms. In particular, we discuss the NLO contribution to the Lamb shift in ordinary and muonic hydrogen and other two-body atoms for $Z\le 10$. We also consider the NLO contribution of the muonic vacuum polarization to the Lamb shift in hydrogen-like atoms. With the $a_{\mu }$ puzzle unresolved, one may still require present-days values of the hVP contributions to various observable for comparison to experiment etc. the presence of contradicting values and a lack of consistency means an additional uncertainty for $a_{\mu }$ and for key contributions to it, including the LO hVP one. We present here an estimation of such a propagated uncertainty in hVP contributions to different QED observables and recommend a consistent set of the related LO hVP contributions.