Extrapolating SU(3) breaking effects from D to B decays

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 $\gamma$. 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. With a cautious remark about possible nonfactorizable SU(3) breaking in $B$ decays, we proceed to calculate factorizable SU(3) breaking corrections. Applying heavy quark symmetry to semileptonic $D$ and B decay form factors, we find that 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.