Probing the Energy Structure of Positronium with a 203 GHz Fabry-Perot Cavity

Positronium is an ideal system for the research of the bound state QED. The hyperfine splitting of positronium (Ps-HFS: about 203 GHz) is sensitive to new physics beyond the Standard Model via a vacuum oscillation between an ortho-Ps and a virtual photon. Previous experimental results of the Ps-HFS show 3.9 sigma(15 ppm) discrepancy from the QED calculation. All previous experiments used an indirect method with static magnetic field to cause Zeeman splitting (a few GHz) between triplet states of ortho-Ps, from which the HFS value was derived. One possible systematic error source of the indirect method is the static magnetic field. We are developing a new direct measurement system of the Ps-HFS without static magnetic field. In this measurement we use a gyrotron, a novel sub-THz light source, with a high-Finesse Fabry-Perot cavity to obtain enough radiation power at 203 GHz. The present status of the optimization studies and current design of the experiment are described.