Quantum Monte Carlo Study Resolves Nuclear Structure Discrepancies in Light Nuclei Zemach Moments
Researchers have used quantum Monte Carlo methods within modern ab initio nuclear theory to compute Zemach radii and related electromagnetic moments for light nuclei (A≤9). These quantities are critical for interpreting high-precision atomic spectroscopy, including hyperfine splittings and the Lamb shift in muonic atoms. The work helps resolve or contextualize existing discrepancies between theory and experiment for nuclei such as lithium-6 and beryllium-9.
A new study posted to arXiv applies quantum Monte Carlo techniques, using Norfolk two- and three-body interactions derived from chiral effective field theory, to calculate Zemach radii and other electromagnetic moments for light nuclei up to mass number 9. The Zemach radius encodes the convolution of a nucleus's charge and magnetization distributions and is essential for accurately modeling hyperfine splittings in atomic spectra; the third electric Zemach moment also enters the elastic two-photon-exchange contribution to the Lamb shift in muonic atoms. For lithium-6, the calculations yield a Zemach radius larger than values extracted from atomic measurements, a result consistent with recent independent calculations and confirming that the discrepancy is not an artifact of the particular nuclear model used. For beryllium-9, the new results agree with experiment, and the authors trace a previously reported discrepancy in phenomenological evaluations to a model-dependent choice of magnetic radius input. The study also assesses model dependence by comparing different interaction choices and examines the role of two-body currents, providing a more systematic uncertainty quantification than prior phenomenological approaches.
What's missing
The paper is a preprint and has not yet undergone peer review. The authors note model dependence is assessed but do not fully resolve the lithium-6 discrepancy with atomic measurements, leaving open whether the tension points to missing nuclear physics, issues in the atomic theory, or experimental uncertainties.
What different sources said
- arXiv physicsCenter
Quantum Monte Carlo calculations of Zemach moments in $A\leq 9$ nuclei
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