Pertinent Dirac structure for QCD sum rules of meson-baryon coupling constants

Using general baryon interpolating fields $J_B$ for $B= N, Xi, Sigma, $ without derivative, we study QCD sum rules for meson-baryon couplings and their dependence on Dirac structures for the two-point correlation function with a meson $iint d^4x e^{iqx} bra 0|{rm T}[J_B(x)bar{J}_B(0)] |{cal M}(p)ket$. Three distinct Dirac structures are compared: $igamma_5$, $igamma_5fslash{p}$, and $gamma_5sigma_{mu u}q^mu p^ u$ structures. From the dependence of the OPE on general baryon interpolating fields, we propose criteria for choosing an appropriate Dirac structure for the coupling sum rules. The $gamma_5sigma_{mu u}q^mu p^ u$ sum rules satisfy the criteria while the $igamma_5$ sum rules beyond the chiral limit do not. For the $igamma_5fslash{p}$ sum rules, the large continuum contributions prohibit reliable prediction for the couplings. Thus, the $gamma_5sigma_{mu u}q^mu p^ u$ structure seems pertinent for realistic predictions. In the SU(3) limit, we identify the OPE terms responsible for the $F/D$ ratio. We then study the dependence of the ratio on the baryon interpolating fields. We conclude the ratio $F/D sim 0.6-0.8$ for appropriate choice of the interpolating fields.