AMO
In the literature: open
Proton and antiproton magnetic moments compared at 1e-12
In plain words
A single proton or antiproton held in an electromagnetic trap can have its magnetism measured by flipping its spin. Matter-antimatter symmetry requires the two magnetic moments to be equal and opposite.
Precise statement
BASE compared proton and antiproton charge-to-mass ratios to 16 ppt (2022) and magnetic moments to about 1.5 ppb (2017). Determine whether $\left|\mu_{\mathrm{pbar}}\right|/\mu_{\mathrm{p}} - 1$ is nonzero at the 1e-12 level, using sympathetic or quantum-logic cooling for faster single-spin-flip detection.
What would settle it
Penning-trap measurements of the proton and antiproton g-factors each at sub-ppt uncertainty.