On flavor mixing by an effective Light-Cone QCD-Hamiltonian
  The $\uparrow\downarrow$-model has produced the physical masses of all flavor-asymmetric mesons like the $\pi^\pm$ or the $\rho^\pm$. Can the same model also account for the flavor-symmetric mesons like the $\pi^0$ or the $\rho^0$? -- By adjusting one parameter in a $5\times5$-matrix, the $\pi$-$\eta$ degeneracy is lifted and the $\eta-\eta'$ splitting falls into the right ball park. The isospin triplets for pions and rhos are caused the mass-degeneracy of the up and down quark. 
Hans-Christian Pauli (MPI Kernphysik, Heidelberg)