Oscillating nuclear charge radii as sensors for ultralight dark matter

TL;DR

This paper proposes using oscillations in nuclear charge radii caused by ultralight dark matter to detect such dark matter via shifts in atomic clock frequencies, offering a novel experimental approach.

Contribution

It introduces a new method to probe ultralight dark matter-nuclear couplings through atomic clock measurements of nuclear charge radius oscillations.

Findings

Set bounds on scalar UDM-nuclear couplings.

Constrained QCD axion decay constant.

Demonstrated sensitivity with ${}^{171}Yb^+}$ transitions.

Abstract

We show that coupling of ultralight dark matter (UDM) to quarks and gluons would lead to an oscillation of the nuclear charge radius for both the quantum chromodynamics (QCD) axion and scalar dark matter. Consequently, the resulting oscillation of electronic energy levels could be resolved with optical atomic clocks, and their comparisons can be used to investigate UDM-nuclear couplings, which were previously only accessible with other platforms. We demonstrate this idea using the ${}^2S_{1/2} (F=0)\leftrightarrow {}^2F_{7/2} (F=3)$ electric octupole and ${}^2S_{1/2} (F=0)\leftrightarrow \,{}^2D_{3/2} (F=2)$ electric quadrupole transitions in ${}^{171}Yb^+$. Based on the derived sensitivity coefficients for these two transitions and a long-term comparison of their frequencies using a single trapped ${}^{171}Yb^+$ ion, we find bounds on the scalar UDM-nuclear couplings and the QCD axion decay constant. These results are at a similar level compared to the tightest spectroscopic limits, and future investigations, also with other optical clocks, promise significant improvements.