[Paper Review] Evidence of Two-Source King Plot Nonlinearity in Spectroscopic Search for New Boson

Joonseok Hur, Diana Prado Lopes Aude Craik+13 more|arXiv (Cornell University)|Jan 10, 2022

Nuclear physics research studies81 references66 citations

TL;DR

This study reports a 240σ nonlinearity in a King plot from spectroscopic measurements of the 2S₁/₂ → 2F₇/₂ octupole transition in trapped ¹⁶⁸–¹⁷⁶Yb⁺ ions, indicating a strong deviation from linearity in isotope shift data. The nonlinearity is attributed to nuclear effects via nuclear density functional theory with the Fayans functional, and a second, smaller nonlinearity source is identified at 4.3σ significance, suggesting possible new physics beyond the Standard Model.

ABSTRACT

Optical precision spectroscopy of isotope shifts can be used to test for new forces beyond the Standard Model, and to determine basic properties of atomic nuclei. We measure isotope shifts on the highly forbidden ${}^2S_{1/2} ightarrow {}^2F_{7/2}$ octupole transition of trapped $^{168,170,172,174,176}$Yb ions. When combined with previous measurements in Yb$^+$ and very recent measurements in Yb, the data reveal a King plot nonlinearity of up to 240$\sigma$. The trends exhibited by experimental data are explained by nuclear density functional theory calculations with the Fayans functional. We also find, with 4.3$\sigma$ confidence, that there is a second distinct source of nonlinearity, and discuss its possible origin.

Motivation & Objective

• To test for new physics beyond the Standard Model by searching for deviations in isotope shifts that could signal a new boson mediating interactions between quarks and electrons.
• To resolve the origin of nonlinearity in King plots from optical spectroscopy of Yb+ ions, distinguishing between nuclear effects and new physics contributions.
• To extract precise nuclear properties from isotope shift data to improve nuclear energy density functionals.
• To investigate whether the observed nonlinearity in King plots arises from higher-order nuclear effects or from a hypothetical new boson.

Proposed method

• Performed high-precision laser spectroscopy on the highly forbidden 2S₁/₂ → 2F₇/₂ octupole transition in single trapped ¹⁶⁸–¹⁷⁶Yb⁺ ions with ~500 Hz resolution.
• Measured isotope shifts (νAA′) for five even Yb isotopes and combined them with prior data from Yb⁺ and neutral Yb to construct frequency-normalized King plots.
• Used a normalized King plot formalism (νAA′_γ = fγτ + KγτµAA′ + G(4)_γτ δ⟨r⁴⟩AA′ + G(2)_γτ [δ⟨r²⟩²]AA′ + υneDγτaAA′) to isolate nonlinear contributions.
• Applied nuclear density functional theory (DFT) with the Fayans functional to model nuclear charge radius and higher moments, matching observed isotope shift trends.
• Conducted 3D King plot fits using three transitions (α, β, γ, δ, ε) to test for additional nonlinearities beyond the dominant nuclear effect.
• Quantified sensitivity to a new boson via the coupling parameter υneDγτaAA′, with D factors calculated using advanced atomic structure codes (GRASP2018, ambit).

Experimental results

Research questions

• RQ1 Can isotope shift spectroscopy detect deviations from linearity in King plots that signal a new boson mediating quark-electron interactions?
• RQ2 What is the origin of the observed 240σ nonlinearity in the King plot constructed from Yb⁺ and neutral Yb data?
• RQ3 Is the nonlinearity consistent with nuclear effects predicted by nuclear density functional theory, or does it point to new physics?
• RQ4 Does a second, smaller nonlinearity source exist, and if so, what could be its physical origin?
• RQ5 How can precise isotope shift measurements improve the calibration of nuclear energy density functionals?

Key findings

• A 240σ nonlinearity was observed in the King plot when combining data from Yb⁺ (2S₁/₂ → 2F₇/₂) and neutral Yb, indicating a highly significant deviation from linearity.
• The observed nonlinearity pattern is consistently explained by nuclear density functional theory using the Fayans functional, matching ground-state properties of deformed Yb isotopes.
• A second, smaller nonlinearity source was identified at the 4.3σ confidence level, suggesting a possible contribution from beyond-Standard-Model physics.
• The 2S₁/₂ → 2F₇/₂ transition produced a 20-fold increase in nonlinearity compared to previous quadrupole transitions, due to the large change in electronic configuration.
• The extracted absolute transition frequencies for ¹⁶⁸Yb⁺ to ¹⁷⁶Yb⁺ are reported with uncertainties below 37 Hz, enabling high-precision nuclear and atomic physics tests.
• The sensitivity to a new boson (φ) was quantified across a range of masses (1 eV to 100 MeV/c²), with D factors calculated using multiple atomic structure codes for robustness.