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=3.1−1.2+1.6×10−8 s−1 for the first time, which is in good agreement with the theoretical value of 3.37×10−8 s−1.
@article{arxiv.1204.1129,
title = {Direct Observation of the Hyperfine Transition of the Ground State Positronium},
author = {T. Yamazaki and A. Miyazaki and T. Suehara and T. Namba and S. Asai and T. Kobayashi and H. Saito and I. Ogawa and T. Idehara and S. Sabchevski},
journal= {arXiv preprint arXiv:1204.1129},
year = {2013}
}