Up+Down Scalar Meson: Isospin SU(3) & Low Energies

[ddimensoes]

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?

 

[mathman]

All mesons are made up of quark plus antiquark. You can't have a two quark particle since the color charge must add up to white.

I can't comment on the rest of your question - beyond my expertise.

 

[Vanadium 50]

Particles mix, and the closer they are in mass, the more they mix. So you don't have a pure uubar or ddbar state because they mix, and the eta adds some ssbar, etc. The same thing happens with the scalars.