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arXiv:2605.22318

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

R. Aramyan, D. Budker, V. A. Dzuba, V. V. Flambaum, S. G. Porsev, M. S. Safronova, O. Tretiak, K. Zhang

physics.atom-phnucl-thphysics.opticsquant-phSubmitted May 21, 2026• Announced May 22, 2026
v1 · May 21, 2026v2 · May 28, 2026v3 · Sep 8, 2026
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Abstract

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.

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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 54f^{6}6s^{2}\,{}^{5}4f66s25D0_{0}0​ level of neutral samarium at 14 564.90(2) cm−114\,564.90(2)\,\mathrm{cm}^{-1}14564.90(2)cm−1, opening the 7{}^{7}7F0→5_{0}\rightarrow{}^{5}0​→5D0_{0}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) MHz/torr0.12(2)\,\mathrm{MHz/torr}0.12(2)MHz/torr, and pressure shift, 0.145(4) MHz/torr0.145(4)\,\mathrm{MHz/torr}0.145(4)MHz/torr, indicate an inner-shell 4f4f4f-transition shielded from external perturbations. Many-body calculations predict a ∼ ⁣120 ms\sim\!120\,\mathrm{ms}∼120ms metastable lifetime (quality factor Q∼3×1014\mathcal{Q}\sim 3\times 10^{14}Q∼3×1014), 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=0J=0\rightarrow J=0J=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-7/27/27/2 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.
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Precision spectroscopy of simple atomic systems has refined our understanding of the fundamental laws of quantum physics. In particular, helium spectroscopy has played a crucial role in describing two-electron interactions, determining the fine-structure constant and extracting the size of the helium nucleus. Here we present a measurement of the doubly forbidden 1557-nanometer transition connecting the two metastable states of helium (the lowest energy triplet state 2 (3)S(1) and first excited singlet state 2 (1)S(0)), for which quantum electrodynamic and nuclear size effects are very strong. This transition is weaker by 14 orders of magnitude than the most predominantly measured transition in helium. Ultracold, submicrokelvin, fermionic (3)He and bosonic (4)He atoms are used to obtain a precision of 8 × 10(-12), providing a stringent test of two-electron quantum electrodynamic theory and of nuclear few-body theory.
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