Shielded inner-shell transitions in atomic samarium for tests of fundamental physics

Forbidden atomic transitions provide some of the most stringent low-energy tests of physics beyond the Standard Model, with sensitivity set by the interplay between the sought-for signals and systematics suppressed by symmetry. Here we identify the previously unobserved $4f^{6}6s^{2}\,{}^{5}$D$_{0}$ level of neutral samarium at $14\,564.90(2)\,\mathrm{cm}^{-1}$, opening the ${}^{7}$F$_{0}\rightarrow{}^{5}$D$_{0}$ inner-shell transition for precision spectroscopy. Candidate lines extracted from dual-comb absorption spectra were assigned using double-resonance population-depletion and sequential-excitation measurements. The observed pressure broadening, $0.12(2)\,\mathrm{MHz/torr}$, and pressure shift, $0.145(4)\,\mathrm{MHz/torr}$, indicate an inner-shell $4f$-transition shielded from external perturbations. Many-body calculations predict a $\sim\!120\,\mathrm{ms}$ metastable lifetime (quality factor $\mathcal{Q}\sim 3\times 10^{14}$), large sensitivity coefficients for variation of the fine-structure constant, and a nuclear-spin-dependent parity-violation amplitude comparable to that of cesium. Crucially, the $J=0\rightarrow J=0$ selection rule suppresses by symmetry both the nuclear-spin-independent parity-violation channel and the M1 and E2 backgrounds that complicated previous heavy-atom experiments, yielding a uniquely clean window onto the nuclear anapole moment. The two stable spin-$7/2$ isotopes of samarium provide a remarkable opportunity to largely cancel atomic-structure uncertainties by measuring the ratio of parity-violation effects in the two isotopes. These results establish neutral samarium as a platform for inner-shell precision spectroscopy and tests of physics beyond the Standard Model.