Muonic hyperfine structure and the Bohr-Weisskopf effect

TL;DR

The paper addresses the challenge of precisely determining the Bohr-Weisskopf (BW) effect in muonic atoms to reveal nuclear magnetization distributions and refine magnetic moment extractions. It updates experimental $A$-constants for muonic $1s_{1/2}$ states, discusses extracting the BW contribution via $A_{exp} = A_0 (1+\epsilon_{BR})(1+\epsilon_{BW})$ and $\epsilon_{BW,exp} = \frac{A_{exp}}{A_{0,p.d.}} - 1$, and surveys multiple nuclear magnetization models against the data. Key findings show reasonable agreement within large errors but notable discrepancies for some nuclei, with homogeneous magnetization often failing (e.g., Cs, Bi) and single-particle–based approaches generally performing better; the results underscore the need for systematic calculations across isotopes and more measurements. The work suggests that muonic BW measurements can discriminate among magnetization distributions, improving BW corrections used in atomic QED and parity-violation contexts, and highlights the muX project as a path to new data in the lead and trans-lead regions.

Abstract

An update is given on the experimental values of the magnetic hyperfine structure and the Bohr-Weisskopf effect in muonic atoms. The need for more measurements and systematic calculations is discussed to allow the differentiation of different models of the Bohr-Weisskopf effect in nuclei.