Direct Observation of the Hyperfine Transition of the Ground State Positronium

TL;DR

This paper reports the first direct observation of the hyperfine transition in ground state positronium, measuring its transition probability with high significance and confirming theoretical predictions.

Contribution

It presents the first direct measurement of the positronium hyperfine transition, utilizing a novel high-power optical system to achieve the observation.

Findings

Hyperfine transition observed with 5.4 sigma significance.

Transition probability measured as 3.1^{+1.6}_{-1.2} x 10^{-8} s^{-1}.

Measured value agrees with theoretical prediction.

Abstract

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 $A = 3.1^{+1.6}_{-1.2} \times 10^{-8}$ s$^{-1}$ for the first time, which is in good agreement with the theoretical value of $3.37 \times 10^{-8}$ s$^{-1}$.