THE QUARK AXIAL-VECTOR COUPLING AND HEAVY MESON DECAYS

Form factors and decay widths for D --> D gamma and D* --> D pi decay s are estimated in a relativistic constituent quark model. Relativisti c corrections due to tight quarks are found to be substantial and to s uppress the vector and axial vector form factors. The CLEO experimenta l value of R(gamma)(0) = Gamma (D(0) --> D-0 gamma/Gamma(D*(0) --> D- 0 pi(0)) = 0.572 +/- 0.057 +/- 0.081 is used to determine the quark ax ial vector coupling g(A), which is found to be 0.6-0.8 for m(u), (350- 200) MeV correspondingly, as compared with the chiral model result g(A ) = 0.8-0.9. The heavy meson-pion strong coupling g is found to be 0.4 -0.6, much smaller than g = 1 which is expected in the large N-c and n onrelativistic limit, but consistent with some heavy hadron chiral the ory and QCD sum rule results.