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Pseudoscalar mesons are commonly seen in proton-proton scattering and proton-antiproton annihilation. The pion was first proposed to exist by Yukawa in the 1930s as the primary force carrying boson of the Yukawa Potential in nuclear interactions, and was later observed at nearly the same mass that he originally predicted for it. 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".
Gell-Mann further predicted the existence of a ninth resonance in the pseudoscalar multiplet, which he originally called X. Indeed, this particle was later found and is now known as the eta prime meson. The structure of the pseudoscalar meson multiplet, and also the ground state baryon multiplets, led Gell-Mann (and Zweig, independently) to create the well known quark model.
Among all of the mesons known to exist, the pseudoscalars are perhaps the most well known in a sense. The masses of the pion, kaon, eta and eta prime particles are known with great precision. However, the decay properties of the pseudoscalar mesons, particularly of eta and eta prime, are somewhat contradictory to the mass hierarchy. While the eta prime meson is much more massive than the eta meson, the eta meson is thought to contain a larger component of strange and anti-strange quarks than the eta prime meson, which appears contradictory. The presence of an eta(1405) state also brings glueball mixing into the discussion. It is possible that the eta and eta prime mesons mix with the pseudoscalar glueball which should occur, in its pure state, somewhere above the scalar glueball in mass. This is one of a few ways in which the unexpectedly large eta prime mass of 957.78 MeV/c2 can be explained, relative to its model-predicted mass around 250 to 300 MeV/c2.
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