Onia

thumb|200px|right|An electron and [[positron orbiting around their common centre of mass. An s state has zero angular momentum, so orbiting around each other would mean going straight at each other until the pair of particles is either scattered or annihilated, whichever occurs first. This is a bound quantum state known as positronium.]]

neutral meson having charm quark and charm anti quark

In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. The name "quarkonium" is analogous to positronium, the bound state of electron and anti-electron. The particles are short-lived due to matter–antimatter annihilation.

particle

in particle physics

thumb|An illustration of the protonium atom.

thumb|An illustration of the protonium atom. An onium (plural: onia) is a bound state of a particle and its antiparticle. These states are usually named by adding the suffix -onium to the name of one of the constituent particles (replacing an -on suffix when present), with one exception for "muonium"; a muon–antimuon bound pair is called "true muonium" to avoid confusion with old nomenclature.

hypothetical form of quarkonium

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 nucleus. Pionium has a short lifetime, predicted by chiral perturbation theory to be (i.e. 2.89 femtoseconds). It decays mainly into two mesons, and to a smaller extent into two photons.

flavorless vector meson formed from a superposition of an up quark–antiquark and a down quark–antiquark pair

exotic atom consisting of a muon and an antimuon

Kaonium is an exotic atom consisting of a bound state of a positively charged and a negatively charged kaon. Kaonium has not been observed experimentally and is expected to have a short lifetime on the order of 10−18 seconds.