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Positronium Hyperfine Splitting

Positronium is an ideal system for the research of QED in the bound state. The hyperfine splitting of positronium (Ps-HFS: about 203 GHz) is a good tool to test QED and also sensitive to new physics beyond the Standard Model. Previous experimental results show 3.9\,$σ$ (15 ppm) discrepancy from the QED $\mathrm{O}\left(α^3 \ln{1/α}\right)$ prediction. We point out probable common systematic errors in all previous experiments. I measure the Ps-HFS in two different ways. (1) A prototype run without RF system is described first. (2) I explain a new direct Ps-HFS measurement without static magnetic field. The present status of the optimization studies and current design of the experiment are described. We are now taking data of a test experiment for the observation of the direct transition.