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Concept
Eta and eta prime mesons
Summary
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.
Official source
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