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-Pérot cavity. The hyperfine transition has been observed with a significance of 5.4 standard deviations. The transition probability is measured to be $A={3.1}_{-1.2}^{+1.6}\times {10}^{-8}{\mathrm{s}}^{-1}$ for the first time, which is in good agreement with the theoretical value of $3.37\times {10}^{-8}{\mathrm{s}}^{-1}$.

• Received 5 April 2012

DOI:https://doi.org/10.1103/PhysRevLett.108.253401