The eta (_eta) and eta prime meson (_eta prime) are isosinglet mesons made of a mixture of up, down and strange quarks and their antiquarks. The charmed eta meson (_charmed eta) and bottom eta meson (_bottom eta) are similar forms of quarkonium; they have the same spin and parity as the (light) _eta defined, but are made of charm quarks and bottom quarks respectively. The top quark is too heavy to form a similar meson, due to its very fast decay. The eta was discovered in pion–nucleon collisions at the Bevatron in 1961 by Aihud Pevsner et al. at a time when the proposal of the Eightfold Way was leading to predictions and discoveries of new particles from symmetry considerations. The difference between the mass of the _eta and that of the _eta prime is larger than the quark model can naturally explain. This "_eta–_eta prime puzzle" can be resolved by the 't Hooft instanton mechanism, whose 1/ N realization is also known as the Witten–Veneziano mechanism. Specifically, in QCD, the higher mass of the _eta prime is very significant, since it is associated with the axial U_A(1) classical symmetry, which is explicitly broken through the chiral anomaly upon quantization; thus, although the "protected" _eta mass is small, the _eta prime is not. The _Eta particles belong to the "pseudo-scalar" nonet of mesons which have spin J = 0 and negative parity, and _eta and _eta prime have zero total isospin, I, and zero strangeness, and hypercharge. Each quark which appears in an _eta particle is accompanied by its antiquark, hence all the main quantum numbers are zero, and the particle overall is "flavourless". The basic SU(3) symmetry theory of quarks for the three lightest quarks, which only takes into account the strong force, predicts corresponding particles and The subscripts are labels that refer to the fact that η_1 belongs to a singlet (which is fully antisymmetrical) and η_8 is part of an octet.

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Concepts associés (3)
Quarkonium
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. Vector meson Light quarks (up, down, and strange) are much less massive than the heavier quarks, and so the physical states actually seen in experiments (η, η′, and π0 mesons) are quantum mechanical mixtures of the light quark states.
Quark model
In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Eightfold Way, the successful classification scheme organizing the large number of lighter hadrons that were being discovered starting in the 1950s and continuing through the 1960s. It received experimental verification beginning in the late 1960s and is a valid effective classification of them to date.
Pion (particule)
Un pion ou méson pi est une des trois particules : π, π+ ou π−. Ce sont les particules les plus légères de la famille des mésons. Elles jouent un rôle important dans l'explication des propriétés à basse énergie de la force nucléaire forte ; notamment, la cohésion du noyau atomique est assurée par l'échange de pions entre les nucléons (protons et neutrons). Le substantif masculin pion (prononcé en français standard) est composé de pi, transcription de la lettre grecque π, et de -on, tiré de électron.

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