atom in which one electron is replaced by a negative muon or a negative pion (pi meson). The muon or pion, after being slowed down in matter, is captured in a high atomic orbit and cascades down, ejecting electrons by the Auger effect or radiating visible light or X-ray photons, to an orbit with a radius less than ¹/₂₀₀ that of the corresponding electron orbit. From studies of mesic atoms, scientists have learned much about nuclear size, distribution of electric charge within nuclei, and the mass of pions.