In particle physics, mesons ( /ˈmiːzɒnz/ or /ˈmɛzɒnz/) are hadronic 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 femtometre, which is about 2⁄3 the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. Charged mesons decay (sometimes through intermediate particles) to form electrons and neutrinos. Uncharged mesons may decay to photons.

Mesons are not produced by radioactive decay, but appear in nature only as short-lived products of very high-energy interactions in matter, between particles made of quarks. In cosmic ray interactions, for example, such particles are ordinary protons and neutrons. Mesons are also frequently produced artificially in high-energy particle accelerators that collide protons, anti-protons, or other particles containing quarks.

In nature, the importance of lighter mesons is that they are the associated quantum-field particles that transmit the nuclear force, in the same way that photons are the particles that transmit the electromagnetic force. The higher energy (more massive) mesons were created momentarily in the Big Bang but are not thought to play a role in nature today. However, such particles are regularly created in experiments, in order to understand the nature of the heavier types of quark which compose the heavier mesons.

Mesons are part of the hadron particle family, defined simply as particles composed of quarks. The other members of the hadron family are the baryons: subatomic particles composed of three quarks rather than two. Some experiments show evidence of tetraquarks—"exotic" mesons made of two quarks and two antiquarks; the particle physics community regards their existence as unlikely, although possible. Since quarks have a spin of 1⁄2, the difference in quark-number between mesons and baryons results in mesons being bosons while baryons are fermions.

Each type of meson has a corresponding antiparticle (antimeson) in which quarks are replaced by their corresponding antiquarks and vice-versa. For example, a positive pion (π+) is made of one up quark and one down antiquark; and its corresponding antiparticle, the negative pion (π−), is made of one up antiquark and one down quark.

Since mesons are composed of quarks, they participate in both the weak and strong interactions. Mesons with net electric charge also participate in the electromagnetic interaction. They are classified according to their quark content, total angular momentum, parity, and various other properties such as C-parity and G-parity. While no meson is stable, those of lower mass are nonetheless more stable than the most massive mesons, and are easier to observe and study in particle accelerators or in cosmic ray experiments. They are also typically less massive than baryons, meaning that they are more easily produced in experiments, and thus exhibit certain higher energy phenomena more readily than baryons composed of the same quarks would. For example, the charm quark was first seen in the J/Psi meson (J/ψ) in 1974, and the bottom quark in the upsilon meson (ϒ) in 1977.

Read more about Meson:  History, Classification

Other articles related to "meson, mesons":

Classification - Nomenclature - Flavourful Mesons
... Flavourful mesons are mesons made of pair of quark and antiquarks of different flavours ... In table form, they are Nomenclature of flavourful mesons antiquark → quark ↓ up down charm strange top bottom up — D0 K+ T0 B+ down — D− K0 T− B0 charm D0 D+ — D+ s T0 c B ... If the meson is not pseudoscalar (JP = 0−) or vector (JP = 1−), J is added as a subscript ...
Eta Meson
... The eta (η) and eta prime meson (η′) are mesons made of a mixture of up, down and strange quarks and their antiquarks ... The charmed eta meson (η c) and bottom eta meson (η b) are forms of quarkonium they have the same spin and parity as the light eta but are made of charm quarks and bottom quarks respectively ... The top quark is too heavy to form a similar meson (top eta meson, symbol η t), due to its very fast decay ...
T Meson
... T mesons are hypothetical mesons composed of a top quark and either an up (T0), down (T+), strange (T+ s) or charm antiquark (T0 c) ... Because of the top quark's short lifetime, T mesons are not expected to be found in nature ... The combination of a top quark and top antiquark is not a T meson, but rather toponium ...
Theta Meson
... The theta meson (θ) is a hypothetical form of quarkonium (i.e ... a flavourless meson) formed by a top quark and top antiquark ... antiquark) and upsilon (bottom quark, bottom antiquark) mesons ...
Phi Meson
... In particle physics, the phi meson is a vector meson formed of a strange quark and a strange antiquark ...