An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have short lifetimes.
In a muonic atom (also called a mu-mesic atom),, an electron is replaced by a muon, which, like the electron, is a lepton. Since leptons are only sensitive to weak, electromagnetic and gravitational forces, muonic atoms are governed to very high precision by the electromagnetic interaction. There are no complications due to strong forces between the lepton and the nucleus.
Since a muon is more massive than an electron, the Bohr orbits are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to quantum electrodynamics are more important. Study of muonic atoms' energy levels as well as transition rates from excited states to the ground state therefore provide experimental tests of quantum electrodynamics.
Muon-catalyzed fusion is a technical application of muonic atoms.
A hadronic atom is an atom in which one or more of the orbital electrons has been replaced by a hadron. Possible hadrons include mesons such as the pion or kaon, yielding a mesonic atom; antiprotons, yielding a antiprotonic atom; and the Σ−
particle, yielding a Σ−
or sigmaonic atom.
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