Radiative corrections to the hadronic vacuum polarization contribution to the muonium hyperfine interval

Muonium is a purely leptonic system, but the accuracy of the theoretical QED prediction for its hyperfine interval is limited in part by hadronic effects. Here we consider radiative corrections to the leading hadronic vacuum polarization term, since their size is larger than the uncertainty in the calculation of the leading hadronic term itself. The total hadronic contribution of relative order of α in comparison with the leading hadronic term is found to be 4.97(19) Hz. The hadronic uncertainty sets an "ultimate" limit on the use of the 1s hyperfine interval in muonium as a QED variable.