AMO
In the literature: open
Antihydrogen ground-state hyperfine splitting at the ppm level
In plain words
The tiny splitting caused by the magnetic interaction of the antiproton and positron is sensitive to different possible symmetry violations than the optical line. It has been measured only to a few parts in 10000.
Precise statement
Measure the zero-field ground-state hyperfine splitting of antihydrogen (hydrogen: 1420.405751768 MHz) with relative uncertainty below 1e-6, using either an in-beam Rabi method (ASACUSA) or trapped-atom microwave spectroscopy (ALPHA), and determine whether it differs from hydrogen.
What would settle it
A beam or trap measurement of the Hbar hyperfine splitting with ppm uncertainty compared to the hydrogen maser value.