Direct measurement of the hyperfine structure of the ground state positronium using high power sub-THz radiation

We plan to directly measure the hyperfine structure of the ground-state positronium. The hyperfine structure between ortho-positronium and para-positronium is about 203 GHz. We develop a new optical system to accumulate about 10 kW power using a gyrotron and a Fabry-Pérot cavity, and have already observed the hyperfine transition at a frequency point of 202.9 GHz. The transition probability is measured to be A = 3.1_-1.2^+1.6×10^-8 s^-1, which is in good agreement with the theoretical value of 3.37×10^-8 s^-1. In order to measure the hyperfine structure, we have to repeat transition measurements at different frequencies around 203 GHz. We are now refining the optical system to quickly change output frequency and obtain more power.