Pseudoscalar meson

[1][a] Pseudoscalar mesons are commonly seen in proton-proton scattering and proton-antiproton annihilation, and include the pion (π), kaon (K), eta (η), and eta prime (η′) particles, whose masses are known with great precision.

In the 1950s and 1960s, the pseudoscalar mesons began to proliferate, and were eventually organized into a multiplet according to Murray Gell-Mann's so-called "Eightfold Way".

[3] Gell-Mann further predicted the existence of a ninth resonance in the pseudoscalar multiplet, which he originally called X.

This failure of the quark model to explain this mass difference is called the "η-η′ puzzle".

The presence of an η(1405) state also brings glueball mixing into the discussion.