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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“In communist society, where nobody has one exclusive sphere of activity but each can become accomplished in any branch he wishes, society regulates the general production and thus makes it possible for me to do one thing today and another tomorrow, to hunt in the morning, fish in the afternoon, rear cattle in the evening, criticize after dinner, just as I have a mind, without ever becoming hunter, fisherman, shepherd or critic.”
—Karl Marx (1818–1883)