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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• Revised May 28, 2026 (v2)
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.
Atom interferometry provides a unique platform for testing fundamental physics. Building on the successful realization of the gravitational Aharonov-Bohm (gAB) effect in a large-baseline atom interferometer, I propose a next-generation experiment adapting this precise setup by adding a controllable electric potential. This experiment enables a background-free search for Beyond Standard Model (BSM) physics manifesting as a phenomenological composition-dependent coupling to both a gravitational and an electric potential (LBSM∝qφgφe\mathcal{L}_{BSM} \propto q \varphi_g \varphi_eLBSM​∝qφg​φe​). I show that the analogous Standard Model (SM) effect, a gravitationally-modified Stark shift, vanishes identically due to parity conservation; as the atomic ground state has even parity, the expectation value of this interaction is zero. To detect the target microradian-scale signal beneath milliradian-scale technical noise and gigaradian-scale inertial backgrounds, the experimental design integrates three crucial solutions: (1) a simultaneous dual-isotope (85^{85}85Rb/87^{87}87Rb) interferometer to reject technical common-mode noise, (2) optimal spin-squeezed states providing N−2/3^{-2/3}−2/3 sensitivity scaling to surpass the Standard Quantum Limit, and (3) a four-point differential quadrature (±V0,±keff\pm V_0, \pm k_{\text{eff}}±V0​,±keff​) to algebraically cancel the dominant inertial phase and all k-odd systematics. A Monte Carlo analysis validates this complete protocol's robustness against systematics and projects a realistic path to a 5-sigma discovery within a 3-month integration time. This model-independent search, particularly sensitive to neutron-coupled forces like U(1)B−L_{B-L}B−L​ gauge bosons, constitutes a high-precision null test that further solidifies the Aharonov-Bohm intuition that potentials are more fundamental than fields.
The open inner-shell electronic structure of lanthanides with large magnetic moments gives rise to a rich spectrum of transitions available for laser cooling, trapping, and coherent control. Despite this, the large number of ultraviolet (UV) transitions below 400nm have so far been rarely utilized in dipolar atom experiments. Here, we investigate multiple UV ground state transitions in dysprosium. Several of these UV excited states have the largest decay strengths to the ultralong-lived, low-lying first excited state which are comparable to the most commonly used strongest transitions found in dipolar atoms. Using two-dimensional shelving spectroscopy which improves detection sensitivity and provides a straightforward way to determine the hyperfine-isotope structure and excited state total angular momentum JJJ, we measure isotope shifts, hyperfine coefficients, and create King plots to determine their electronic nature. Such knowledge of these UV transitions which analogously exist in other magnetic atoms is important for optically populating the first excited state and can be used towards creating an optical clock, high resolution imaging in quantum gas microscopy, and probing lanthanide nuclei with enhanced Schiff moments in search of physics beyond the standard model.
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