b \to ss\bar{d}$ in a Vector Quark Model
  The rare decay $b\to ss\bar{d}$ is studied in a vector quark model by adding the contributions from exotic vector-like quarks. We find that the contribution from box diagrams amounts to $10^{-9}$ in the branching ratio, while the $Z$-mediated tree level contribution is negligible. 
Hai-Ying Cai and Da-Xin Zhang