Exotic atom

An exotic atom is the anologue of a normal atom in which one or more of the electrons are replaced by other negative particles, such as a muon or a pion, or the positively charged nucleus is replaced by other positively charged elementary particles, or both.

Muonic atoms

In a muonic atom, an electron is replaced by a muon (which belongs to the same family — the leptons — as the electron). Since the muon only is 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.

Due to the relatively high mass of the muon as compared to an electron, the Bohr orbits are closer to the nucleus, and corrections due to quantum electrodynamics are more important than in ordinary atoms. Study of muonic atoms' energy levels as well as transition rates from excited states to the ground state therefore provide experimental probes to elements of QED.

Mesonic atoms

A mesonic atom is an atom in which the nucleus remains as it is, but one or more of the orbital electrons is replaced by a meson (which are not leptons like the electron or muon). Mesons are particles which can interact via the strong interaction. Therefore the energy levels of such atoms are influenced by the strong force between the nucleus and the meson.

In a mesonic atom, nuclear force effects are comparable to the effects of quantum electrodynamics, since the atomic orbitals are close enough to the nucleus for these short-range interactions to be important. These tend to decrease the lifetime of these atoms to the point where transitions between different atomic levels are not observable. Thus pionic hydrogen and kaonic hydrogen provide interesting experimental probes of the theory of strong interactions, quantum chromodynamics.

Onia

An onium is the bound state of a particle and its antiparticle. The paradigmatic onium is positronium which consists of an electron and a positron bound together as a long-lived metastable state. Studies of positronium were undertaken in the 1950s in the hope that they would lead to a detailed understanding of bound states in quantum field theory. They still used for this purpose: a recent development called non-relativistic quantum electrodynamics (NRQED) used this system as a proving ground. A muonic atom of muonium would also be an onium, containing a muon and an antimuon.

Pionium, a bound state of two oppositely charged pions, is interesting from the point of view of exploration of the strong interaction. This would also true of protonium if it could be produced. However, the true analogues of the positronium in the theory of strong interactions are the quarkonium states made of heavy quarks such as the charm or bottom. (Top quarks are so heavy that they decay through the weak force before they are able form bound states.) Exploration of these states through non-relativistic quantum chromodynamics (NRQCD) and lattice QCD are increasingly important tests of quantum chromodynamics.

Understanding bound states of hadrons such as pionium and protonium are also important in order to clarify notions related to hadronic exotics such as mesonic molecules and pentaquark states.

Hypernuclear atoms

Atoms may be composed of electrons orbiting a nucleus that includes strange particles called hyperons. Such hypernuclear atoms are generally studied for their nuclear behaviour, falling into the realm of nuclear physics rather than atomic physics.

Quasiparticle atoms

In condensed matter systems, specifically in some semiconductors, there are states called excitons which are bound states of an electron and a electron hole.

Extended objects

Although a neutron star could logically be classed as an exotic atom, since the bulk of the star is one huge atomic nucleus and electrical neutrality forces it to have a thin shell of electrons surrounding it, it is generally more useful to consider such objects as stars. Similarly, stars made of other forms of quark matter are also best considered as systems distinct from such exotic atoms in most circumstances.