The first direct observation of positronium hyperfine splitting

The direct transition from ortho-positronium to para-positronium with high-power sub-THz radiation has been observed with 5.1 σ significance. The observed transition rate is consistent with the theoretical calculation. A sub-THz gyrotron is used as the light source, and a quasi-optical Fabry-Perot cavity is utilized to accumulate the radiation. This is the first observation of the positronium hyperfine splitting (Ps-HFS), which leads to the first direct measurement of Ps-HFS with a target accuracy of 100 ppm in several years.