Muonium

Muonium is an exotic atom formed by a positive muon (mu+) bound to an electron (e−). It is hydrogen-like, consisting of a single negatively charged lepton orbiting a positively charged lepton rather than a proton, making it a purely leptonic bound state. The reduced mass is about 0.995 of the electron mass, so the energy levels and size are very close to those of hydrogen: the Bohr radius is about 0.53 angstroms and the ionization energy about 13.54 eV. The state exists only while the muon persists, so muonium is inherently short-lived.

Formation and detection: Muonium is produced when positive muons implanted into matter capture an electron and form the neutral bound state. It can also be formed in vacuum in muon-beam setups with suitable electron-rich environments. Detection is achieved through observation of muon decay products and, in spectroscopic studies, by measuring transition frequencies between energy levels.

Scientific significance: As a simple bound-state system, muonium provides stringent tests of bound-state quantum electrodynamics and enables precise measurements of the muon mass and magnetic moment. Spectroscopic investigations of the ground-state hyperfine splitting and the 1S–2S transition yield high-precision data that test fundamental theory and contribute to our understanding of muon properties.

Lifetime and beyond: The muonium state is terminated by the decay of the muon, with an effective lifetime of about 2.2 microseconds. Experiments have also explored bound-state antimatter analogs, such as antimuonium (μ− e+), to probe physics beyond the Standard Model.