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Title: Nine $\sigma$ discrepancy in the ionization energy of metastable helium
Plain: Helium has two electrons, which makes its energy levels harder to compute than hydrogen's, but theory now claims sub-MHz accuracy. The measured binding energy of a long-lived excited state disagrees with this theory by nine standard errors.
Precise: The measured ionization energy of $\mathrm{He}\,(1\mathrm{s})(2\mathrm{s})\,2^{3}\mathrm{S}_1$ is $1152842742.7082(60)\,\mathrm{MHz}$ (Clausen et al 2025), versus the theoretical $1152842742.231(52)\,\mathrm{MHz}$ (Patkos, Yerokhin, Pachucki), a $9\sigma$ difference; a similar $7\sigma$ deviation affects $2^{1}\mathrm{S}_0$. Identify whether the error lies in the $\alpha^{7} m$ QED contribution, higher-order terms, or the experiments.
Settled by: An independent complete $\alpha^{7} m$ calculation for helium singlet and triplet S states, or an independent measurement of the same ionization energy with a different method.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: The 2025 ETH Zurich Rydberg-series measurement improved precision five-fold and confirmed the discrepancy.
Posed since: (empty)