Meson

In particle physics, mesons (/ˈmiːzɒnz/ or /ˈmɛzɒnz/) are hadronic subatomic particles composed of one quark and one antiquark, bound together by strong interactions. Because mesons are composed of quark subparticles, they have physical size, notably a diameter of roughly one femtometer, which is about 1.2 times 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 mediating particles) to form electrons and neutrinos. Uncharged mesons may decay to photons. Both of these decays imply that color is no longer a property of the byproducts.† ^ The C parity is only relevant to neutral mesons.†† ^ For JPC=1−−, the ψ is called the J/ψ(>5% of decays)π+ + π0 + π−(ρ0 + γ) / (π+ + π− + γ) orπ0 + π0 + ηπ+ + π0 or π0 + e+ + νe orπ+ + π0π0 + π0π0 + π0 + π0 or π+ + π0 + π−(>5% of decays)π0 + γ In particle physics, mesons (/ˈmiːzɒnz/ or /ˈmɛzɒnz/) are hadronic subatomic particles composed of one quark and one antiquark, bound together by strong interactions. Because mesons are composed of quark subparticles, they have physical size, notably a diameter of roughly one femtometer, which is about 1.2 times 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 mediating particles) to form electrons and neutrinos. Uncharged mesons may decay to photons. Both of these decays imply that color is no longer a property of the byproducts. Outside the nucleus, mesons appear in nature only as short-lived products of very high-energy collisions between particles made of quarks, such as cosmic rays (high-energy protons and neutrons) and ordinary matter. Mesons are also frequently produced artificially in cyclotron in the collisions of protons, antiprotons, or other particles. 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 heavy mesons are regularly created in particle accelerator experiments, in order to understand the nature of the heavier types of quark that compose the heavier mesons. Mesons are part of the hadron particle family, and are defined simply as particles composed of an even number of quarks. The other members of the hadron family are the baryons: subatomic particles composed of odd numbers of valence quarks (at least 3), and some experiments show evidence of exotic mesons, which do not have the conventional valence quark content of two quarks (one quark and one antiquark), but 4 or more. Because quarks have a spin of ​1⁄2, the difference in quark number between mesons and baryons results in conventional two-quark mesons being bosons, whereas 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. Because 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. Mesons are classified according to their quark content, total angular momentum, parity and various other properties, such as C-parity and G-parity. Although no meson is stable, those of lower mass are nonetheless more stable than the more massive, and hence are easier to observe and study in particle accelerators or in cosmic ray experiments. Mesons 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 do baryons. 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. From theoretical considerations, in 1934 Hideki Yukawa predicted the existence and the approximate mass of the 'meson' as the carrier of the nuclear force that holds atomic nuclei together. If there were no nuclear force, all nuclei with two or more protons would fly apart due to electromagnetic repulsion. Yukawa called his carrier particle the meson, from μέσος mesos, the Greek word for 'intermediate', because its predicted mass was between that of the electron and that of the proton, which has about 1,836 times the mass of the electron. Yukawa had originally named his particle the 'mesotron', but he was corrected by the physicist Werner Heisenberg (whose father was a professor of Greek at the University of Munich). Heisenberg pointed out that there is no 'tr' in the Greek word 'mesos'. The first candidate for Yukawa's meson, now known in modern terminology as the muon, was discovered in 1936 by Carl David Anderson and others in the decay products of cosmic ray interactions. The mu meson had about the right mass to be Yukawa's carrier of the strong nuclear force, but over the course of the next decade, it became evident that it was not the right particle. It was eventually found that the 'mu meson' did not participate in the strong nuclear interaction at all, but rather behaved like a heavy version of the electron, and was eventually classed as a lepton like the electron, rather than a meson. Physicists in making this choice decided that properties other than particle mass should control their classification.