Nuclear charge radius of ${}^{26m}$Al and its implication for V${}_{ud}$ in the quark-mixing matrix

Collinear laser spectroscopy was performed on the isomer of the aluminium isotope ${}^{26m}$Al. The measured isotope shift to ${}^{27}$Al in the $3s^{2}3p{}^2\text{\hspace{0.17em}⁣}{P}_{3\mathrm{/}2}^{\circ }\to 3s^{2}4s{}^2\text{\hspace{0.17em}⁣}{S}_{1\mathrm{/}2}$ atomic transition enabled the first experimental determination of the nuclear charge radius of ${}^{26m}$Al, resulting in $R_c$=\qty{3.130\pm.015}{\femto\meter}. 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 ${}^{26m}$Al. Its corrected $\mathcal{F}t$ value, important for the estimation of $V_{ud}$ in the CKM matrix, is thus shifted by one standard deviation to \qty{3071.4\pm1.0}{\second}.