Evidence of Two-Source King Plot Nonlinearity in Spectroscopic Search for New Boson

TL;DR

This study uses high-precision isotope shift spectroscopy in ytterbium ions to detect nonlinearity in King plots, indicating potential new physics or nuclear effects beyond current models.

Contribution

It provides the first evidence of a second source of nonlinearity in King plots, combining experimental measurements with nuclear theory to explore beyond Standard Model physics.

Findings

Detected up to 240σ nonlinearity in King plots

Identified a second nonlinearity source with 4.3σ confidence

Nuclear density functional theory explains observed trends

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 ${}^2S_{1/2} \rightarrow {}^2F_{7/2}$ 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.