Quark Composition
The η particles belong to the "pseudo-scalar" nonet of mesons which have spin J = 0 and negative parity, and are characterized by having zero total isospin, I, and zero strangeness and hypercharge. Each quark which appears in an η particle is accompanied by its antiquark and all the main quantum numbers are zero.
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 refer to the fact that η1 belongs to a singlet (which is fully antisymmetrical) and η8 is part of an octet. However in this case the weak and electromagnetic forces, which can transform one flavour of quark into another, cause a significant, though small, amount of "mixing" (with mixing angle -11.5 degrees ), so that the actual quark composition is a linear combination of these formulae. The unsubscripted name η refers to the real particle which is actually observed and which is close to the η8. The η′ is the observed particle close to η1.
Read more about this topic: Eta Meson
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