True muonium

Antimatter
Antiparticles Positron Antiproton Antineutron Antihydrogen Antihelium Onia Antiprotonic hydrogen Antiprotonic helium Muonium True muonium Pionium Positronium Quarkonium
Concepts and phenomena Annihilation Baryogenesis Baryon asymmetry Comet CP violation Gravitational interaction Positron emission
Devices Cloud chamber Particle accelerator Antiproton decelerator Relativistic Heavy Ion Collider Penning trap
Uses Positron emission tomography Fuel Weapon
People and bodies Carl David Anderson Paul Dirac Andrei Sakharov CERN
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For atoms where muons have replaced one or more electrons, see Muonic atom. For the onium of an electron and an antimuon, see muonium.

In particle physics, true muonium is a theoretically predicted exotic atom representing a bound state of an muon and an antimuon (μ⁺μ⁻). The existence of true muonium is well established theoretically within the Standard Model. Its properties within the Standard Model are determined by quantum electrodynamics, and may be modified by physics beyond the Standard Model.

True muonium is yet to be observed experimentally, though it may have been produced in experiments involving collisions of electron and positron beams.^[1][2] The ortho-state of true muonium (i.e. the state with parallel alignment of the muon and antimuon spins) is expected to be relatively long-lived (with a lifetime of 1.8×10⁻¹² s), and decay predominantly to an e⁺e⁻ pair, which makes it possible for LHCb experiment at CERN to observe it with the dataset collected by 2025.^[3]