Abstract
Positronium is an ideal system for precision tests of bound-state quantum electrodynamics (QED). One of the most precisely tested quantities of positronium is the ground-state hyperfine splitting (HFS). Recent progress on theoretical calculation revealed that there was a 16.0 ± 3.5 ppm (4.5 σ) discrepancy between the old experimental value and the theoretical calculation. We performed a new measurement which took into account the positronium thermalization effect for the first time. The result was HFS = 203.3942 ± 0.0016 (stat., 8.0 ppm) ± 0.0013 (syst., 6.4 ppm) GHz, which was consistent with the QED calculation within 1.1 σ, whereas it disfavored the old experimental values by 2.6 σ. It also showed that the positronium thermalization effect on HFS was as large as 10 ± 2 ppm, which was consistent with the discrepancy level within 1.5 σ, which could be the reason of the discrepancy. We are planning to preform a new experiment which uses a slow positron beam and perform HFS measurement in vacuum, instead of using gas as all of the other precision measurements. It will be completely free from material effect on HFS, including the thermalization effect. In this proceeding, summary of our previous work and details of the future new experiment are discussed.
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