Probing Nuclear Interactions Through Isotope Shift Spectroscopy of Mercury

TL;DR

This study uses high-precision isotope shift spectroscopy of mercury atoms to explore nuclear structure and test for new physics beyond the Standard Model, revealing nonlinearities in isotope shifts with significant implications.

Contribution

It provides the first high-precision isotope shift measurements in mercury, revealing nonlinearities that suggest contributions from higher-order nuclear effects and potential new forces.

Findings

Detected nonlinearities in King plot analysis with 4.9σ significance.

Resolved isotope shifts with uncertainties as low as 20 kHz.

Provided new constraints on nuclear deformation and hypothetical forces.

Abstract

We present precision isotope shift spectroscopy of the $\mathrm{6s^{2}}\, {}^{1}\mathrm{S}_{0}{\rightarrow\,}\mathrm{6s\, 6p}\, {}^{3}\mathrm{P}_{1}$ intercombination line and the $\mathrm{6s\,6p}\, {}^{3}\mathrm{P}_{1}{\rightarrow\,}\mathrm{6s\,6d}\, {}^{3}\mathrm{D}_{J}$ ($J=1,2$) transitions in neutral mercury, performed on the five naturally abundant even isotopes, including the low-abundant isotope ${}^{196}\mathrm{Hg}$. Using laser-cooled atoms in a magneto-optical trap, we achieve uncertainties down to $20\,\mathrm{kHz}$, resolving the isotope shift to a fractional uncertainty of ${\sim}\,2{\,\times\,}10^{-6}$. A King plot analysis comparing our ${}^{1}\mathrm{S}_{0}{\rightarrow}{}^{3}\mathrm{P}_{1}$ data to previous results on the $\mathrm{6s\,6p}\,{}^{3}\mathrm{P}_{2}{\rightarrow\,}\mathrm{6s\,7s}\,{}^{3}\mathrm{S}_{1}$ line reveals a nonlinearity with $4.9\,\sigma$ significance. Our generalized King plot nonlinear decomposition analysis discusses potential contributions from quadratic ($\propto \delta\langle r^{2}\rangle^{2}$) and higher order field shifts ($\propto \delta\langle r^{4}\rangle$) also induced by nuclear deformation. These measurements yield new insights into the structure of the $\mathrm{Hg}$ nucleus and provide benchmarks for nuclear-structure models. They further establish mercury as a potential platform to search for hypothetical Yukawa-type boson-mediated forces coupling electrons to neutrons.