Role of Four-Quark Operators in the Inclusive $\Lambda_b$ Decays
  We compute by QCD sum rules the matrix elements of the relevant four-quark operators appearing in the expression of the $\Lambda_b$ inclusive decay rates at the order $1/m_b^3$. The results suggest that $1/m_b^3$ corrections are not responsible of the observed difference between the lifetime of $\Lambda_b$ and $B_d$. 
P.Colangelo, F.De Fazio