= Eta and eta prime mesons
{title2=$\eta,\eta^\prime$}
These neutral isospin-zero <pseudoscalar mesons> have $J^{PC}=0^{-+}$. Their light-quark components mix the <Eta octet state> and <eta singlet state>. The singlet axial anomaly contributes to the unusually large eta-prime mass, while flavour breaking also affects both masses and the mixing. The simple orthogonal one-angle flavour model is useful for explaining the nonet; detailed decay-constant descriptions can require more than one parameter.
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