The penguin operators in nonresonant $B^- \to M \bar M \pi^-$ $(M   =\pi^-, K^-, K^0)$ decays
  We investigate the contributions coming from the penguin operators in the nonresonant $B^- \to M \bar M \pi^-$ ($M =\pi^-, K^-, K^0$) decays. The effective Wilson coefficients of the the strong penguin operators $O_{4,6}$ are found to be relatively larger. We calculate the contributions arising from the $O_4$ and $O_6$ operators in the nonresonant decays $B^- \to M \bar M\pi^-$ ($M = \pi^-, K^-, \bar K^0$) using a model combining heavy quark symmetry and the chiral symmetry, developed previously. We find that the forbidden nonresonant $B^- \to K^0 \bar K^0 \pi^-$ decay occurs through the strong penguin operators. These penguin contributions affect the branching ratios for $B^- \to M \bar M\pi^-$ ($M = \pi^-, K^-$) by only a few percent. The branching ratio for $B^- \to K^0 \bar K^0 \pi^-$ is estimated to be of the order $ 10^{-6}$. 
S. Fajfer, R. J. Oakes and T. N. Pham