Measurement of hyperfine structure and the Zemach radius in $\rm^6Li^+$ using optical Ramsey technique

TL;DR

This study uses optical Ramsey spectroscopy to precisely measure hyperfine splittings in $ m^6Li^+$, leading to a more accurate determination of its Zemach radius, which challenges existing nuclear models.

Contribution

The paper presents the most precise hyperfine measurements in $ m^6Li^+$ to date and derives a Zemach radius that conflicts with simple nuclear models, highlighting the need for refined nuclear physics calculations.

Findings

Hyperfine splittings measured with 10 kHz uncertainty

Zemach radius of $ m^6Li$ determined as 2.44(2) fm

Discrepancy with simple nuclear models of $ m^6Li$

Abstract

We investigate the $2\,^3\!S_1$--$2\,^3\!P_J$ ($J = 0, 1, 2$) transitions in $\rm^6Li^+$ using the optical Ramsey technique and achieve the most precise values of the hyperfine splittings of the $2\,^3\!S_1$ and $2\,^3\!P_J$ states, with smallest uncertainty of about 10~kHz. The present results reduce the uncertainties of previous experiments by a factor of 5 for the $2\,^3\!S_1$ state and a factor of 50 for the $2\,^3\!P_J$ states, and are in better agreement with theoretical values. Combining our measured hyperfine intervals of the $2\,^3\!S_1$ state with the latest quantum electrodynamic (QED) calculations, the improved Zemach radius of the $\rm^6Li$ nucleus is determined to be 2.44(2)~fm, with the uncertainty entirely due to the uncalculated QED effects of order $m\alpha^7$. The result is in sharp disagreement with the value 3.71(16) fm determined from simple models of the nuclear charge and magnetization distribution. We call for a more definitive nuclear physics value of the $\rm^6Li$ Zemach radius.