Extrapolating SU(3) breaking fromDtoBdecays

We consider two SU(3) breaking parameters $R_1{(m}_B)$ and $R_2{(m}_B)$ appearing in a relation between $B^{+}\to K\pi$ and $B^{+}\to \pi \pi$ amplitudes, which plays an important role in determining the weak phase $\gamma .$ In the heavy quark limit, we identify an isospin-related quantity $R_2{(m}_D)$ measured in D decays, exhibiting large SU(3) breaking which is likely due to nonfactorizable effects. Applying heavy quark symmetry to semileptonic D and B decay form factors, we find that factorizable SU(3) breaking in $R_2{(m}_B{)/R}_1{(m}_B)$ may be significantly larger than estimated from certain model calculations of form factors.