Relativistic recoil as a key to the fine-structure puzzle in muonic 90Zr

Abstract:

The long-standing fine-structure anomaly in muonic    Zr is resolved through a rigorous treatment of the relativistic-recoil effect. From a fit of ab initio QED calculations of the muonic    Zr spectrum to precision measurements performed four decades ago, we extract a significantly more precise root-mean-square (rms) charge radius with 6-fold improvement in quality of the fit. A 2-parameter Fermi (2pF) distribution is assumed to model the nuclear charge density and yields a best-fit value of rms charge radius of       fm (   ), in agreement with the previous muonic spectroscopy value, but a factor   more precise, and 3  larger than the accepted literature value. Additionally, the same analysis has been performed for    Sn, where the extracted value of       fm (   ) is consistent with the accepted value. These results confirm our assumption that the muonic fine-structure puzzle arose from an incomplete treatment of QED effects and their uncertainties.

K. A. Beyer, I. A. Valuev, Z. A. Mandrykina, Z. Sun, N. S. Oreshkina, „Relativistic recoil as
a key to the fine-structure puzzle in muonic 90Zr“, 27. Nov. 2025, arXiv:2511.22298 (2025).

https://arxiv.org/abs/2511.22298

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