Direct Measurement of Positronium HyperFine Structure: ∼ A New Horizon of Precision Spectroscopy Using Gyrotrons ∼

Positronium is an ideal system for research on QED, especially in a bound state. A discrepancy (3.9 sigma) is found recently between measured HFS values and the QED prediction (including up-to O(alpha (3) log alpha (-aEuro parts per thousand 1)), where alpha is the fine-structure constant.). It might be due to a contribution of unknown new physics or common systematic problems in all the previous measurements. A new method to measure HFS directly is performed using a high power gyrotron. The transition from ortho-positronium to para-positronium has been observed with 5 sigma CL, which is the first observation of M1 transition in (sub)Terahertz region. New technologies of high power gyrotrons are developed for precision spectroscopy.