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In particle physics, mesons are subatomic particles composed of one quark and one antiquark. All mesons are unstable, with the longest-lived lasting for only a few 100-millionths (10−8) of a second. Mesons have a physical size, with a radius a little smaller than the one femtometre (10−15 m) that characterizes the size of protons and neutrons. Charged mesons decay (sometimes through intermediate particles) ultimately to 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 cosmic ray interaction with matter—a typical high-energy interaction between particles made of quarks (in cosmic ray interactions, these are ordinary protons and neutrons). Mesons are also frequently produced in high-energy particle accelerators that collide protons, antiprotons, or other particles that contain 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. Higher energy 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 and types of the heavier types of quarks 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. 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—that is, mesons have integer spin while baryons have half-integer spin. This means that the Pauli exclusion principle applies to each type of baryon, but does not apply to mesons.

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 will exhibit higher energy phenomena sooner than baryons would. For example, the charm quark was first seen in the J/Psi meson (J/ψ) in 1974,[1][2] and the bottom quark in the upsilon meson (ϒ) in 1977.[3]

Each meson has a corresponding antiparticle (antimeson) where 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. Some experiments show evidence of tetraquarks—"exotic" mesons made of two quarks and two antiquarks—but the particle physics community regards their existence as unlikely, although possible.[4]

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