LEMING

LEptons in Muonium Interacting with Gravity

Two muonium atoms made of an antimuon as the nucleus and an electron. The left one is hit by two photons, illustrating spectroscopy. The right atom is falling in an unknown gravitational potential.

Muonium (M) is a two-body exotic atom consisting of a positive antimuon (μ⁺) and an electron (e⁻). This exotic atom contains only elementary (anti)particles, hence strong interaction does not play a central role: the system is free of finite-size or nuclear effects. The mass of M is dominated by (~200:1) the positive antimuon, an elementary antiparticle from the second generation of fermions. Direct measurements of the gravitational interaction, and hence tests of the weak equivalence principle, for such exotic particles, have not yet been realised. Antihydrogen and positronium have been proposed as laboratory candidates for antimatter gravity experiments but only M could demonstrate how gravity affects charged leptons with different flavours.

A direct gravity experiment with M is inherently challenging due to its short lifetime (2.2 μs) and the fact that M atoms are created in matter, while the experiment must be performed in vacuo. The experiment will use atom interferometry, a sensitive tool to measure inertial forces. During the passage through the interferometer the M atoms can acquire during their short lifetime a small phase shift due to their free fall in the gravitational field of Earth.

Principle of the muonium gravity experiment: On the left is a plot of the velocity distribution of a thermal (very broad) and a superfluid helium (very narrow) muonium beam. The right shows a superfluid helium muonium beam passing through an interferometer made of three gratings then entering a detector.

We are investigating novel methods to create a high-quality atomic beam of muonium, based on μ⁺ → M conversion in superfluid helium (SFHe). Using the new cold muonium source, we are also proposing a new atom interferometry experiment with M atoms that would enable us to carry out an exotic test of the weak equivalence principle, in our newly approved collaboration, LEMING at external page PSI.