First direct measurement of positronium hyperfine splitting with sub-THz light

TL;DR

This paper reports the first direct measurement of positronium hyperfine splitting using sub-THz radiation, overcoming previous indirect methods and potential systematic errors, with a high-power optical system.

Contribution

It introduces a novel experimental setup to directly observe positronium hyperfine transition using high-power sub-THz radiation.

Findings

First direct measurement of Ps-HFS at about 203 GHz

Development of a high-power sub-THz optical system

Observation of positronium hyperfine transition

Abstract

Positronium is an ideal system for the research of the bound state QED. The hyperfine splitting of positronium (Ps-HFS, about 203 GHz) is an important observable but all previous measurements of Ps-HFS had been measured indirectly using Zeeman splitting. There might be the unknown systematic errors on the uniformity of magnetic field. We are trying to measure Ps-HFS directly using sub-THz radiation. We developed an optical system to accumulate high power (about 10 kW) radiation in a Fabry-P\'erot resonant cavity and observed the positronium hyperfine transition for the first time.