An exotic atom is an otherwise normal

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

in which one or more sub-atomic particles have been replaced by other particles. For example,

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s may be replaced by other negatively charged particles such as

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of ''ħ'', but with a much greater mass. It is classified as a ...

s (muonic atoms) or

pion In particle physics, a pion (, ) or pi meson, denoted with the Greek alphabet, Greek letter pi (letter), pi (), is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the ...

s (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes and no exotic atom observed so far can persist under normal conditions.

Muonic atoms

In a ''muonic atom'' (previously called a ''mu-mesic'' atom, now known to be a misnomer as muons are not

meson In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...

s), an electron is replaced by a muon, which, like the electron, is a

lepton In particle physics, a lepton is an elementary particle of half-integer spin (Spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-li ...

. Since

s are only sensitive to

weak Weak may refer to: Songs * Weak (AJR song), "Weak" (AJR song), 2016 * Weak (Melanie C song), "Weak" (Melanie C song), 2011 * Weak (SWV song), "Weak" (SWV song), 1993 * Weak (Skunk Anansie song), "Weak" (Skunk Anansie song), 1995 * "Weak", a son ...

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

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 In particle physics, quantum electrodynamics (QED) is the Theory of relativity, relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quant ...

are more important. Study of muonic atoms'

energy level A quantum mechanics, quantum mechanical system or particle that is bound state, bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical mechanics, classical pa ...

s as well as transition rates from

excited state In quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...

s to the

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state ...

therefore provide experimental tests of quantum electrodynamics. Other muonic atoms can be formed when negative muons interact with ordinary matter. The muon in muonic atoms can either decay or get captured by a proton. Muon capture is very important in heavier muonic atoms, but shortens the muon's lifetime from 2.2 μs to only 0.08 μs.

Muonic hydrogen

Muonic hydrogen is like normal hydrogen with the electron replaced by a negative muon—that is, a proton orbited by a muon. It is important in addressing the proton radius puzzle. Muonic hydrogen atoms can form muonic hydrogen molecules. The spacing between the nuclei in such a molecule is hundreds of times smaller than in a normal hydrogen molecule – so close that the nuclei can spontaneously fuse together. This is known as ''

muon-catalyzed fusion Muon-catalyzed fusion (abbreviated as μCF or MCF) is a process that allows nuclear fusion to take place at temperatures that are significantly lower than those required for thermonuclear fusion, even at room temperature or lower. It is one of the ...

'', and was first observed between hydrogen-1 and deuterium nuclei in 1957. Muon-catalyzed fusion has been proposed as a means of generating energy using fusion reactions in a room-temperature reactor.

Muonic helium (Hydrogen-4.1)

The symbol ^4.1H (Hydrogen-4.1) has been used to describe the exotic atom muonic helium (⁴He-μ), which is like

helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consi ...

in having two

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

s and two

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s. However one of its

s is replaced by a

, which also has charge –1. Because the muon's orbital radius is less than the electron's orbital radius (due to the mass ratio), the muon can be considered as a part of the nucleus. The atom then has a nucleus with two protons, two neutrons and one muon, with total nuclear charge +1 (from two protons and one muon) and only one electron outside, so that it is effectively an isotope of hydrogen instead of an isotope of helium. A muon's weight is approximately 0.1 Da so the isotopic mass is 4.1. Since there is only one electron outside the nucleus, the hydrogen-4.1 atom can react with other atoms. Its chemical behavior behaves more like a hydrogen atom than an inert helium atom.

Hadronic atoms

A ''hadronic atom'' is an atom in which one or more of the orbital electrons are replaced by a negatively charged

hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

. Possible hadrons include mesons such as the

kaon In particle physics, a kaon, also called a K meson and denoted , is any of a group of four mesons distinguished by a quantum number called strangeness. In the quark model they are understood to be bound states of a strange quark (or antiquark ...

, yielding a ''pionic atom'' or a ''kaonic atom'' (see Kaonic hydrogen), collectively called ''mesonic atoms'';

antiproton The antiproton, , (pronounced ''p-bar'') is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy. The exis ...

s, yielding an ''antiprotonic atom''; and the particle, yielding a or ''sigmaonic atom''.The strange world of the exotic atom
Roger Barrett, Daphne Jackson and Habatwa Mweene, ''New Scientist'', August 4, 1990. accessdate=September 26, 2007. Unlike leptons, hadrons can interact via the

strong force In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interactions. It confines quarks into protons, neutrons, and other hadron particles, an ...

, so the orbitals of hadronic atoms are influenced by

nuclear force The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between hadrons, most commonly observed between protons and neutrons of atoms. Neutrons and protons, both ...

s 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 In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...

Onium

An ''onium'' (plural: ''onia'') is the bound state of a particle and its antiparticle. The classic onium is

positronium Positronium (Ps) is a system consisting of an electron and its antimatter, anti-particle, a positron, bound together into an exotic atom, specifically an onium. Unlike hydrogen, the system has no protons. The system is unstable: the two part ...

, which consists of an electron and a positron bound together as a

metastable In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is onl ...

state, with a relatively long lifetime of 142 ns in the triplet 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 Pionium is a composite particle consisting of one and one meson. It can be created, for instance, by interaction of a proton beam accelerated by a particle accelerator and a target atomic nucleus, nucleus. Pionium has a short lifetime, predicted ...

, a bound state of two oppositely charged

s, is useful for exploring the

strong interaction In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interaction, fundamental interactions. It confines Quark, quarks into proton, protons, n ...

. 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 A pentaquark is a human-made subatomic particle, consisting of four quarks and one antiquark bound together; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them. As quarks have a bar ...

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

s, the '' quarkonium states'', which are made of a heavy quark such as the charm or

bottom quark The bottom quark, beauty quark, or b quark, is an elementary particle of the third generation. It is a heavy quark with a charge of − ''e''. All quarks are described in a similar way by electroweak interaction and quantum chromodynamic ...

and its antiquark. (

Top quark The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs field. This coupling is very close to unity; in the Standard ...

s 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 Lattice QCD is a well-established non- perturbative approach to solving the quantum chromodynamics (QCD) theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time. When the size of the ...

are increasingly important tests of

. Muonium, despite its name, is ''not'' an onium state 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 (called true muonium), has been theorized. The same applies to the ditauonium (or "true tauonium") exotic QED atom.

Hypernuclear atoms

Atoms may be composed of electrons orbiting a hypernucleus that includes

strange Strange may refer to: Fiction * Strange (comic book), a comic book limited series by Marvel Comics * Strange (Marvel Comics), one of a pair of Marvel Comics characters known as The Strangers * Adam Strange, a DC Comics superhero * The title c ...

particles called hyperons. Such hypernuclear atoms are generally studied for their nuclear behaviour, falling into the realm of

nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies th ...

rather than

atomic physics Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. Atomic physics typically refers to the study of atomic structure and the interaction between atoms. It is primarily concerned wit ...

Quasiparticle atoms

In condensed matter systems, specifically in some

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

s, there are states called

exciton An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb's law, Coulomb force resulting from their opposite charges. It is an electrically neutral quasiparticle regarded as ...

s, which are bound states of an electron and an

electron hole In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle denoting the lack of an electron at a position where one could exist in an atom or crystal structure, atomic lattice. Since in ...

Exotic molecules

An exotic molecule contains one or more exotic atoms. * Di-positronium, two bound positronium atoms * Positronium hydride, a positronium atom bound to a hydrogen atom "Exotic molecule" can also refer to a molecule having some other uncommon property such as pyramidal hexamethylbenzene and a Rydberg atom.

References

{{Authority control Quantum chromodynamics