Three-decade quest backs physics' 'Standard Model'

Scientists on Wednesday said that after a nearly three-decade bid they had detected a telltale change in a sub-atomic particle, further backing a key theory about the Universe.

LHCb experiment observes new matter-antimatter difference

(Phys.org) —The LHCb collaboration at CERN today submitted a paper to Physical Review Letters on the first observation of matter-antimatter asymmetry in the decays of the particle known as the B0s. It is only the fourth ...

Glasgow scientists predict mass of new particle

(PhysOrg.com) -- A team of physicists from the University of Glasgow has predicted the mass of a new particle which would help explain one of the fundamental forces of the universe.

Belle Finds a Hint of New Physics in Extremely Rare B Decays

(PhysOrg.com) -- Quarks, the most fundamental constituents of matters, are classified into six species grouped into three generations as predicted by Professors Kobayashi and Maskawa. The purpose of the B factory experiment ...

Particle oddball surprises physicists

(PhysOrg.com) -- Scientists of the CDF experiment at the Department of Energy's Fermi National Accelerator Laboratory announced yesterday that they have found evidence of an unexpected particle whose curious characteristics ...

Searching for new asymmetry between matter and antimatter

Once a particle of matter, always a particle of matter. Or not. Thanks to a quirk of quantum physics, four known particles made up of two different quarks—such as the electrically neutral D meson composed of a charm quark ...

LHCb observes a new decay mode of the charmed beauty meson

The LHCb collaboration recently reported on the arXiv preprint server the first observation of the decay of the Bc+ meson (composed of two heavy quarks, b and c) into a J/ψ charm-anticharm quark bound state and a pair of ...

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Meson

In particle physics, mesons (pronounced /ˈmiːzɒnz, ˈmɛzɒnz/) 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 2⁄3 the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few 100-millionths (10−8) of a second. 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.

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