An onium is the bound state of a particle and its antiparticle. They are usually named by adding the suffix -onium to the name of the constituting particle. However, despite its name, 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...

 is not a muon–antimuon onium because IUPAC assigned the name to the system of an antimuon bound with an electron. (Muon–antimuon onium would be named muononium.)

Examples

Positronium

Positronium

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...

 is an onium 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

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...

 (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

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...

 (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...

.

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 molecule

Mesonic molecule

A mesonic molecule is a set of two or more mesons bound together by the strong force. Unlike baryonic molecules, which form the nuclei of all elements in nature save hydrogen-1, a mesonic molecule has yet to be definitively observed. The X discovered in 2003 and the Z discovered in 2007 by the...

s 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.