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have been replaced by other particles of the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 very
short lifetimes.
== Muonic atoms ==
In a muonic atom (previously called a mu-mesic
atom, now known to be a misnomer as muons
are not mesons), 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.
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.
== Hadronic atoms ==
A hadronic atom is an atom in which one or
more of the orbital electrons is replaced
by a negatively charged hadron.
Possible hadrons include mesons such as the
pion or kaon, yielding a pionic atom or a
kaonic atom (see Kaonic hydrogen), collectively
called mesonic atoms; antiprotons, yielding
an antiprotonic atom; and the Σ− particle,
yielding a Σ− or sigmaonic atom.Unlike
leptons, hadrons can interact via the strong
force, so the orbitals of hadronic atoms are
influenced by nuclear forces between the nucleus
and the hadron.
Since the strong force is a short-range interaction,
these effects are strongest if the atomic
orbital involved is close to the nucleus,
when the energy levels involved may broaden
or disappear because of the absorption of
the hadron by the nucleus.
Hadronic atoms, such as pionic hydrogen and
kaonic hydrogen, thus provide experimental
probes of the theory of strong interactions,
quantum chromodynamics.
== Onium ==
An onium (plural: onia) is the bound state
of a particle and its antiparticle.
The classic onium is positronium, which consists
of an electron and a positron bound together
as a long-lived metastable state.
Positronium has been studied since the 1950s
to understand bound states in quantum field
theory.
A recent development called non-relativistic
quantum electrodynamics (NRQED) used this
system as a proving ground.
Pionium, a bound state of two oppositely-charged
pions, is useful for exploring the strong
interaction.
This should also be true of protonium, which
is a proton-antiproton bound state.
Understanding bound states of pionium and
protonium is important in order to clarify
notions related to exotic hadrons such as
mesonic molecules and pentaquark states.
Kaonium, which is a bound state of two oppositely
charged kaons, has not been observed experimentally
yet.
The true analogs of positronium in the theory
of strong interactions, however, are not exotic
atoms but certain mesons, the quarkonium states,
which are made of a heavy quark such as the
charm or bottom quark and its antiquark.
(Top quarks are so heavy that they decay through
the weak force before they can form bound
states.)
Exploration of these states through non-relativistic
quantum chromodynamics (NRQCD) and lattice
QCD are increasingly important tests of quantum
chromodynamics.
Muonium, despite its name, is not an onium
containing a muon and an antimuon, because
IUPAC assigned that name to the system of
an antimuon bound with an electron.
However, the production of a muon/antimuon
bound state, which is an onium, has been theorized.
== Hypernuclear atoms ==
Atoms may be composed of electrons orbiting
a hypernucleus 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 an electron hole.
== See also ==
Antihydrogen
Antiprotonic helium
Di-positronium
Kaonic hydrogen
Lattice QCD
Muonium
Neutronium
Positronium
Quantum chromodynamics
Quantum electrodynamics
Quarkonium
