General
The eta was discovered in pion-nucleon collisions at the Bevatron in 1961 by A. 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 η and that of the η' is larger than the quark model can naturally explain. This "η-η' puzzle" is resolved by instantons.
Read more about this topic: Eta Meson
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