Collinear laser spectroscopy was performed on the isomer of the aluminium isotope ${}^{26m}\mathrm{Al}$. The measured isotope shift to ${}^{27}\mathrm{Al}$ in the $3s^{2}3p{}^2{P}_{3/2}^{○}\to 3s^{2}4s{}^2{S}_{1/2}$ atomic transition enabled the first experimental determination of the nuclear charge radius of ${}^{26m}\mathrm{Al}$, resulting in $R_c=3.130(15)\mathrm{fm}$. This differs by 4.5 standard deviations from the extrapolated value used to calculate the isospin-symmetry breaking corrections in the superallowed $\beta$ decay of ${}^{26m}\mathrm{Al}$. Its corrected $\mathcal{F}t$ value, important for the estimation of $V_{ud}$ in the Cabibbo-Kobayashi-Maskawa matrix, is thus shifted by 1 standard deviation to 3071.4(1.0) s.

• Received 25 July 2023
• Accepted 9 October 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.222502

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Published by the American Physical Society