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 4f⁶6s² {}⁵D₀ level of neutral samarium at 14 564.90(2) cm^-1, opening the {}⁷F₀→{}⁵D₀ 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) MHz/torr, and pressure shift, 0.145(4) MHz/torr, indicate an inner-shell 4f-transition shielded from external perturbations. Many-body calculations predict a sim120 ms metastable lifetime quality factor $mathcal{Q}sim 3times 10¹⁴$, 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→ 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.