Exotic atom

An atom-like bound system in which one or more usual constituents are replaced by particles such as muons or antiprotons. Its altered masses, charges, or lifetimes produce distinctive spectra and decay behavior.

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Muonium: A bound state consisting of a positive muon and an electron. It is the simplest exotic atom with a lepton replacing the nucleus.
Lamb shift: A small difference between atomic energy levels that quantum electrodynamics predicts beyond the Dirac equation. Muonic-atom Lamb shifts are highly sensitive to the nuclear charge distribution.
Muon: An elementary lepton with the same electric charge as an electron and about 207 times its mass. Its mass makes muonic orbits much smaller and more sensitive to the nucleus.
Ordinary atom: A bound system with a nucleus and electrons, whose chemical properties are mainly set by its electron configuration. Exotic atoms replace at least one usual constituent, changing their spectra and often their lifetimes.
Muonic atom: An atom in which a negative muon occupies an orbital normally filled by an electron. The muon’s large mass pulls its orbit close to the nucleus and shifts spectral energies.
Proton radius puzzle: The historical discrepancy between proton charge-radius measurements from muonic hydrogen and other methods. Muonic hydrogen spectroscopy made the discrepancy a central test of precision physics.
Antiproton: The negatively charged antiparticle of the proton, with the proton’s mass and opposite quantum numbers. It can replace an electron in a bound atom and later annihilate with nuclear matter.
Positronium: A short-lived bound state of an electron and its antiparticle, a positron. It is an electron–positron atom-like system, rather than an atom with a nucleus.
Antiprotonic atom: A system in which an antiproton is bound to an atomic nucleus after replacing an electron. Its antiproton can cascade inward before annihilating with the nucleus.
Rydberg constant: A physical constant that sets the scale of hydrogen-like atomic spectral energies. Exotic-atom spectra provide independent constraints on this constant.