The charmed eta meson (ηc) and bottom eta meson (ηb) are similar forms of quarkonium; they have the same spin and parity as the (light) η defined, but are made of charm quarks and bottom quarks respectively.

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 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 η1 belongs to a singlet (which is fully antisymmetrical) and η8 is part of an octet.

[10] The η and η′ particles are closely related to the better-known neutral pion π0, where In fact, π0, η1, and η8 are three mutually orthogonal, linear combinations of the quark pairs uu, dd, and ss; they are at the centre of the pseudo-scalar nonet of mesons[9][10] with all the main quantum numbers equal to zero.

Fundamentally, it results from the direct sum decomposition of the approximate SU(3) flavor symmetry among the 3 lightest quarks,