Why is there no meson made up by only up and down quarks but even under parity? Is there something that forbids its existence? The pions are all axial (pseudoscalar) mesons. As we go higher in energy, there are such "flavour-pure" mesons. Is this a consequence of the almost-unbroken isospin SU(3)? If this is so, why can't I find such "flavour-pure" mesons at low energies?