We report the first direct measurement of the hyperfine transition 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, a mode converter, and a Fabry-P\'{ e}rot cavity. The hyperfine transition has been observed with a significance of 5.4 standard deviations. The transition probability is measured to be $A = 3.1^{ +1.6}_{ -1.2} \times 10^{ -8}$ s$^{ -1}$ for the first time, which is in good agreement with the theoretical value of $3.37 \times 10^{ -8}$ s$^{ -1}$.
Categories: hep-ex physics.atom-ph