Generation of a high-intensity, superthermal muonium beam for gravity and laser spectroscopy experiments

Abstract The universality of free fall, a cornerstone of Einstein’s theory of gravity, has so far only been tested with neutral composite states of first-generation standard model particles, such as atoms, neutrons and antihydrogen. Muonium, the bound state of a positively charged anti-muon and an electron, offers the possibility to probe gravity in the absence of the strong interaction with second-generation standard model particles. However, the short muon lifetime and the existing diffuse thermal muonium sources rendered such measurements unfeasible. Here we report the generation of a high-brightness muonium beam, which we extract from a thin layer of superfluid helium. The mean longitudinal velocity and narrow spread of the velocity distribution indicate a superthermal beam, and yields are similar to the highest-intensity diffuse sources. This beam is expected to enable muonium interferometry and a per-cent-level measurement of its gravitational acceleration. In addition, its unprecedented brightness opens the way to sub-kilohertz 1S–2S spectroscopy, enabling the precise determination of the muon mass and stringent tests of bound-state quantum electrodynamics.