Structure Dependence in $\pi \to \bf\ell\nu, \pi\ to \ell\nu\gamma$ and   $\pi^0\ to \gamma\gamma$ Decays
  The amplitudes of $\pi$-meson decays are calculated in a relativistic nonperturbative quark model which assumes that mesons are made of a quark antiquark pair and of a scalar neutral component representing the contribution of the nonelementary fluctuations of the quark gluonic field. The experimental data can be fitted in a satisfactory manner using current quark masses $m_u\approx1.5\ MeV, m_d\approx9\ MeV$. 
D. Ghilencea, L. Micu