Exotic atom
Encyclopedia
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in which one or more sub-atomic particles have been replaced by other particles of the same charge.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, electron
s may be replaced by other negatively charged particles such as muon
s (muonic atoms) or pion
s (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes.
, 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. The description of these atoms is not complicated by 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 level
s as well as transition rates from excited state
s to the ground state
therefore provide experimental tests of quantum electrodynamics.
Muon-catalyzed fusion
is a technical application of muonic atoms.
is replaced by a charged hadron
. Possible hadrons include meson
s such as the pion
or kaon
, yielding a mesonic atom; antiproton
s, yielding an antiprotonic atom; and the particle, yielding a or sigmaonic atom.
Unlike leptons, hadrons can interact via the strong force, so the energy levels of hadronic atoms are influenced by nuclear force
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 interesting experimental probes of the theory of strong interactions, quantum chromodynamics
.
, 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 pion
s, is interesting for exploring the strong interaction
. This should also be true of protonium
. The true analogs of positronium in the theory of strong interactions, however, are not exotic atoms but certain meson
s, the quarkonium
states, which are made of a heavy quark such as the charm
or bottom quark
and its antiquark. (Top quark
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
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 true muonium, which is an onium, has been theorized.
Understanding bound states of hadron
s such as pionium
and protonium
is also important in order to clarify notions related to exotic hadron
s such as mesonic molecules and pentaquark
states.
that includes strange
particles called hyperon
s. Such hypernuclear atoms
are generally studied for their nuclear behaviour, falling into the realm of nuclear physics
rather than atomic physics
.
systems, specifically in some semiconductor
s, there are states called exciton
s which are bound states of an electron and an electron hole
.
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s may be replaced by other negatively charged particles such as muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
s (muonic atoms) or pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
s (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes.
Muonic atoms
In a muonic atom (also called a mu-mesic atom), an electron is replaced by a muonMuon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
, which, like the electron, is a lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...
. Since leptons are only sensitive to weak
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...
, electromagnetic and gravitational forces, muonic atoms are governed to very high precision by the electromagnetic interaction. The description of these atoms is not complicated by strong forces
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
between the lepton and the nucleus.
Since a muon is more massive than an electron, the Bohr orbits
Bohr model
In atomic physics, the Bohr model, introduced by Niels Bohr in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction,...
are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to quantum electrodynamics
Quantum electrodynamics
Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...
are more important. Study of muonic atoms' energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
s as well as transition rates from excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
s to the ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; 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.
Muon-catalyzed fusion
Muon-catalyzed fusion
Muon-catalyzed fusion is a process allowing nuclear fusion to take place at temperatures significantly lower than the temperatures required for thermonuclear fusion, even at room temperature or lower...
is a technical application of muonic atoms.
Hadronic atoms
A hadronic atom is an atom in which one or more of the orbital electronsAtomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
is replaced by a charged hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
. Possible hadrons include meson
Meson
In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s such as the pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
or kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...
, yielding a mesonic atom; antiproton
Antiproton
The antiproton 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....
s, yielding an antiprotonic atom; and the particle, yielding a or sigmaonic atom.
Unlike leptons, hadrons can interact via the strong force, so the energy levels of hadronic atoms are influenced by nuclear force
Nuclear force
The nuclear force is the force between two or more nucleons. It is responsible for binding of protons and neutrons into atomic nuclei. The energy released causes the masses of nuclei to be less than the total mass of the protons and neutrons which form them...
s between the nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
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
Kaonic hydrogen
Kaonic hydrogen is an exotic atom consisting of a negatively charged kaon orbiting a proton.Such particles were first seen, through their X-ray spectrum, at the KEK in Tsukuba, Japan. More detailed studies have been performed at DAFNE in Frascati, Italy. Kaonium has been created in very low energy...
, thus provide interesting experimental probes of the theory of strong interactions, quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
.
Onium
An onium (plural: onia) is the bound state of a particle and its antiparticle. The classic onium is positroniumPositronium
Positronium is a system consisting of an electron and its anti-particle, a positron, bound together into an "exotic atom". Being unstable, the two particles annihilate each other to produce two gamma ray photons after an average lifetime of 125 ps or three gamma ray photons after 142 ns in...
, which consists of an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
and a positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
bound together as a long-lived metastable state. Positronium has been studied since the 1950s to understand bound states in quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...
. A recent development called non-relativistic quantum electrodynamics (NRQED) used this system as a proving ground.
Pionium
Pionium
Pionium is an exotic atom consisting of one and one mesons. It can be created, for instance, by interaction of a proton beam accelerated by a particle accelerator and a target nucleus....
, a bound state of two oppositely-charged pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
s, is interesting for exploring the strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
. This should also be true of protonium
Protonium
Protonium is an exotic atom in which a proton and an antiproton orbit each other. Protonium is an electrically neutral boson with zero baryon number....
. The true analogs of positronium in the theory of strong interactions, however, are not exotic atoms but certain meson
Meson
In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s, the quarkonium
Quarkonium
In particle physics, quarkonium designates a flavorless meson whose constituents are a quark and its own antiquark. Examples of quarkonia are the J/ψ meson and the meson...
states, which are made of a heavy quark such as the charm
Charm quark
The charm quark or c quark is the third most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks...
or bottom quark
Bottom quark
The bottom quark, also known as the beauty quark, is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large bare mass , combined with low values of the CKM matrix elements Vub and Vcb, gives it a...
and its antiquark. (Top quark
Top quark
The top quark, also known as the t quark or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and...
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
Lattice QCD is a well-established non-perturbative approach to solving the quantum chromodynamics theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time....
are increasingly important tests of quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
.
Muonium
Muonium
Muonium is an exotic atom made up of an antimuon and an electron, which was discovered in 1960 and is given the chemical symbol . During the muon's lifetime, muonium can enter into compounds such as muonium chloride or sodium muonide . Due to the mass difference between the antimuon and the...
, 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 true muonium, which is an onium, has been theorized.
Understanding bound states of hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s such as pionium
Pionium
Pionium is an exotic atom consisting of one and one mesons. It can be created, for instance, by interaction of a proton beam accelerated by a particle accelerator and a target nucleus....
and protonium
Protonium
Protonium is an exotic atom in which a proton and an antiproton orbit each other. Protonium is an electrically neutral boson with zero baryon number....
is also important in order to clarify notions related to exotic hadron
Exotic hadron
Exotic hadrons are subatomic particles made of quarks , but which do not fit into the usual scheme of hadrons. While bound by the strong interaction they are not predicted by the simple quark model...
s such as mesonic molecules and pentaquark
Pentaquark
A pentaquark is a hypothetical subatomic particle consisting of four quarks and one antiquark bound together . As quarks have a baryon number of +, and antiquarks of −, it would have a total baryon number of 1, thus being classified as an exotic baryon...
states.
Hypernuclear atoms
Atoms may be composed of electrons orbiting a hypernucleusHypernucleus
A hypernucleus is a nucleus which contains at least one hyperon in addition to nucleons. The first was discovered by Marian Danysz and Jerzy Pniewski in 1952....
that includes strange
Strange quark
The strange quark or s quark is the third-lightest of all quarks, a type of elementary particle. Strange quarks are found in hadrons, which are subatomic particles. Example of hadrons containing strange quarks include kaons , strange D mesons , Sigma baryons , and other strange particles...
particles called hyperon
Hyperon
In particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm quarks or bottom quarks.-Properties and behavior of hyperons:...
s. Such hypernuclear atoms
Hypernucleus
A hypernucleus is a nucleus which contains at least one hyperon in addition to nucleons. The first was discovered by Marian Danysz and Jerzy Pniewski in 1952....
are generally studied for their nuclear behaviour, falling into the realm of nuclear physics
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
rather than atomic physics
Atomic physics
Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and...
.
Quasiparticle atoms
In condensed matterCondensed Matter
Condensed matter may refer to several things*Condensed matter physics, the study of the physical properties of condensed phases of matter*European Physical Journal B: Condensed Matter and Complex Systems, a scientific journal published by EDP sciences...
systems, specifically in some semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s, there are states called exciton
Exciton
An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids...
s which are bound states of an electron and an electron hole
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
.
See also
- PositroniumPositroniumPositronium is a system consisting of an electron and its anti-particle, a positron, bound together into an "exotic atom". Being unstable, the two particles annihilate each other to produce two gamma ray photons after an average lifetime of 125 ps or three gamma ray photons after 142 ns in...
- Di-positroniumDi-positroniumDi-positronium, or dipositronium, is a molecule consisting of two atoms of positronium. It was predicted to exist in 1946 by John Archibald Wheeler, and subsequently studied theoretically, but was not observed until 2007 in an experiment done by David Cassidy and Allen Mills at the University of...
- Kaonic hydrogenKaonic hydrogenKaonic hydrogen is an exotic atom consisting of a negatively charged kaon orbiting a proton.Such particles were first seen, through their X-ray spectrum, at the KEK in Tsukuba, Japan. More detailed studies have been performed at DAFNE in Frascati, Italy. Kaonium has been created in very low energy...
- NeutroniumNeutroniumNeutronium is a proposed name for a substance composed purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 for the conjectured "element of atomic number zero" that he placed at the head of the periodic table...
- MuoniumMuoniumMuonium is an exotic atom made up of an antimuon and an electron, which was discovered in 1960 and is given the chemical symbol . During the muon's lifetime, muonium can enter into compounds such as muonium chloride or sodium muonide . Due to the mass difference between the antimuon and the...
- tests of quantum electrodynamicsQuantum electrodynamicsQuantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...
- tests of quantum electrodynamics
- Mumesic atom
- QuarkoniumQuarkoniumIn particle physics, quarkonium designates a flavorless meson whose constituents are a quark and its own antiquark. Examples of quarkonia are the J/ψ meson and the meson...
- Quantum chromodynamicsQuantum chromodynamicsIn theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
, including lattice QCDLattice QCDLattice QCD is a well-established non-perturbative approach to solving the quantum chromodynamics theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time....
- Quantum chromodynamics
- AntihydrogenAntihydrogenAntihydrogen is the antimatter counterpart of hydrogen. Whereas the common hydrogen atom is composed of an electron and proton, the antihydrogen atom is made up of a positron and antiproton...
- Antiprotonic heliumAntiprotonic heliumAntiprotonic helium is a three-body atom composed of an antiproton and an electron orbiting around a helium nucleus. It is thus made partly of matter, and partly of antimatter. The atom is electrically neutral, since both electrons and antiprotons have a charge of -1, whereas helium nuclei have a...