# $g$ Factor of Lithiumlike Silicon: New Challenge to Bound-State QED

## TL;DR

This paper reports a significant improvement in the measurement and calculation of the $g$ factor of lithiumlike silicon, providing a stringent test of bound-state QED effects in a magnetic field.

## Contribution

It presents the most precise theoretical and experimental $g$ factor values for lithiumlike silicon, challenging bound-state QED calculations at a new level of accuracy.

## Key findings

- Theoretical $g$ factor precision is limited by two-loop QED contributions.
- Experimental $g$ factor matches theory within a few percent.
- Results provide a new benchmark for bound-state QED tests.

## Abstract

The recently established agreement between experiment and theory for the $g$ factors of lithiumlike silicon and calcium ions manifests the most stringent test of the many-electron bound-state quantum electrodynamics (QED) effects in the presence of a magnetic field. In this Letter, we present a significant simultaneous improvement of both theoretical $g_\text{th} = 2.000\,889\,894\,4\,(34)$ and experimental $g_\text{exp} = 2.000\,889\,888\,45\,(14)$ values of the $g$ factor of lithiumlike silicon $^{28}$Si$^{11+}$. The theoretical precision now is limited by the many-electron two-loop contributions of the bound-state QED. The experimental value is accurate enough to test these contributions on a few percent level.