We compute the matrix element of the four-quark operator \Op_S =
bˉ(1−γ5)sbˉ(1−γ5)s in the Bs−Bˉs mixing on
the lattice using NRQCD, which gives a leading contribution in the calculation
of the width difference ΔΓs in the 1/mb expansion. Using the
next-to-leading formula of Beneke \textit{et al.}, we obtain
(ΔΓ/Γ)s = 0.151(37)(36)(17), where the the first error
reflects the uncertainty of fBs = 245(30) MeV, the second error comes
from our calculation of the matrix element of \Op_S, and the third represents
unknown 1/mb correction.Comment: 17 pages, 6 figures, uses REVTeX, corrected typos, conservative
systematic erro