UC Riverside

Positronium (Ps) is an exotic hydrogen-like atom consisting of an electron and its antiparticle—positron. As a pure leptonic atom, positronium is a perfect source to test quantum electrodynamics (QED). The leptonic hydrogen-like structure also makes it an ideal test bench for the proton radius puzzle—the discrepancy in the measurement of proton charge radius using different methods and atoms. Recently, hydrogen spectroscopy has been conducted to relative uncertainties of 10-15. Benefitting from the unique structure of Ps atoms, Ps spectroscopy only needs a precision three orders of magnitude lower for the same level of impact. However, the path to this target is still full of challenges. Due to annihilation, Ps’s lifetime is relatively short. The limited lifetime of Ps significantly increased the challenge for Ps spectroscopy. Excitation of Ps atoms into Rydberg states can effectively extend the lifetime of Ps atoms for detection. However, many other challenges exist, such as the uncertainty in the energy level induced by its transient lifetime.The last positronium 13S1-23S1 spectroscopy was performed by Fee, Mills, Chu, et al. almost thirty years ago. It reached a relative uncertainty of 3.2 ppb, and the measured 13S1-23S1 interval was 1.7σ away from the theoretical calculation. In 2020, Gurungs et al. measured the Ps n=2 fine structure and also found a disagreement with the theoretical value. This paper presents the development of the instrument we built for positronium precision spectroscopy and our current experiments. The apparatus includes an ultrahigh finesse optical interferometer for positronium two-photon excitation, a frequency stabilizing system, a frequency comb for ultra-precise spectroscopy, and pulsed lasers for 2P/2S to Rydberg excitation. We have achieved a positronium 1S-2P excitation with a 7× improvement in yield compared with the last experiment in 2018. Ps 13S1-23S1 spectroscopy experiments are under operation, and we expect a 10-1000 improvement in precision from the previous experiment.