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Accurate Charge Radius Measurement of C14 Confronts Ab Initio Theory

Kristian König1,2,*, Patrick Müller1, Tobias Gesser1, Emily Burbach1, Stefano Gandolfi3, Matthias Heinz4,5, Phillip Imgram1, Alessandro Lovato6,7,8,9, Pieter Maris10 et al.

Takayuki Miyagi11, Wilfried Nörtershäuser1,2, Robert Roth1,2, Achim Schwenk1,12,13, and Julien Spahn1

  • *Contact author: kkoenig@ikp.tu-darmstadt.de
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Phys. Rev. Lett. 137, 182501 – Published 30 October, 2026

DOI: https://doi.org/10.1103/f5s7-1t7w

Abstract

Located at the neutron shell closure N=8, the long-lived radioactive isotope C14 plays a critical role in geochronology and nuclear structure studies. Despite its widespread use, the nuclear charge radius of C14 has remained less precisely known than that of its stable counterpart C12. Here, we report a high-precision determination of the C14 charge radius using collinear laser spectroscopy at the COALA setup at TU Darmstadt, improving upon the precision of previous muonic measurements by a factor of 5 and revealing a 1.9σ discrepancy of combined uncertainty, indicating a likely underestimated uncertainty in the muonic determination. This measurement challenges state-of-the-art ab initio nuclear theory calculations, including auxiliary field diffusion Monte Carlo, the valence-space in-medium similarity renormalization group, and the no-core shell model, augmented by neural-network techniques. With C12 and C14 now forming one of the most precisely characterized even-even isotope pairs, these results also enable improved QED tests.

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Phys. Rev. Lett. 137, 182501– Published 30 October, 2026
  • Received 11 March 2026
  • Revised 16 June 2026
  • Accepted 17 August 2026
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