A Relativistic Quark Model for Mesons with an Instanton-Induced   Interaction
  We present new results of a relativistic quark model based on the Bethe-Salpeter equation in its instantaneous approximation. Assuming a linearly rising confinement potential with an appropriate spinorial structure in Dirac space and adopting a residual interaction based on instanton effects, we can compute masses of the light mesons up to highest observed angular momenta with a natural solution of the $U_A(1)$ problem. The calculated ground states masses and the radial excitations describe the experimental results well. In this paper, we will also discuss our results concerning numerous meson decay properties. For processes like $\pi^+/K^+\to e^+\nu_e\gamma$ and $0^-\to\gamma\gamma$ at various photon virtualities, we find a good agreement with experimental data. We will also comment on the form factors of the $K_{\ell 3}$ decay and on the decay constants of the $\pi$, $K$ and $\eta$ mesons. For the sake of completeness, we will furthermore present the electromagnetic form factors of the charged $\pi$ and $K$ mesons as well as a comparison of the radiative meson decay widths with the most recent experimental data. 
Matthias Koll, Ralf Ricken, Dirk Merten, Bernard Metsch, Herbert Petry