First Millimeter-wave Spectroscopy of the Ground-state Positronium
  We report on the first measurement of the Breit-Wigner resonance of the transition from {\it ortho-}positronium to {\it para-}positronium. We have developed an optical system to accumulate a power of over 20 kW using a frequency-tunable gyrotron and a Fabry-P\'{e}rot cavity. This system opens a new era of millimeter-wave spectroscopy, and enables us to directly determine both the hyperfine interval and the decay width of {\it p-}Ps. 
A. Miyazaki, T. Yamazaki, T. Suehara, T. Namba, S. Asai, T. Kobayashi,   H. Saito, Y. Tatematsu, I. Ogawa, and T. Idehara
hep-ex physics.atom-ph physics.ins-det