Width difference in the B_s-bar{B}_s system with lattice NRQCD

We compute the matrix element of the four-quark operator $\Op_S$ = $\bar{b}(1-\gamma_5) s \bar{b} (1-\gamma_5) s$ in the $B_s-\bar{B}_s$ mixing on the lattice using NRQCD, which gives a leading contribution in the calculation of the width difference $\Delta\Gamma_s$ in the $1/m_b$ expansion. Using the next-to-leading formula of Beneke \textit{et al.}, we obtain $(\Delta\Gamma/\Gamma)_s$ = 0.151(37)(36)(17), where the the first error reflects the uncertainty of $f_{B_s}$ = 245(30) MeV, the second error comes from our calculation of the matrix element of $\Op_S$, and the third represents unknown $1/m_b$ correction.