Positronium $2^3\mathrm{S}_1$ to $2^3\mathrm{P}_0$ interval disagrees with QED
In plain words
Positronium, an electron bound to its antiparticle, tests quantum electrodynamics (QED, the quantum theory of light and charges) with no proton involved. One of its microwave transitions came out 4.5 standard errors away from theory.
Precise statement
The UCL measurement of the positronium $2^3S_1$ to $2^3P_0$ interval gave $18501.02(61)\,\mathrm{MHz}$ versus the QED prediction $18498.25(8)\,\mathrm{MHz}$ (Gurung et al 2020), a $4.5\,\sigma$ difference; later work by the same group traced line-shape distortions in related intervals to microwave reflections, but the $2^3P_0$ tension has not been removed. Determine whether the $2^3P_0$ discrepancy survives an independent method such as Ramsey separated-field spectroscopy.
What would settle it
An independent $2^{3}\mathrm{S}_{1}$ to $2^{3}\mathrm{P}_{0}$ measurement at sub-0.3 MHz uncertainty with a different microwave geometry.
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