Hyperfine structure in hydrogen and helium ion

Abstract

QED theory of the hyperfine splitting of the $1s$ and $2s$ state in hydrogen isotopes and helium-3 ion is considered. We develop an accurate theory of a specific difference 8E_HFS(2s)-E_HFS(1s). We take into account fourth order corrections and nuclear structure effects. The theoretical prediction is now of a higher accuracy than the experiment is. The study of the difference provides the most accurate test (on a level of a part in 10⁸) of the QED theory of $1s$ HFS up to date. The theory agrees with most of the experimental data.