Lifetime difference in B_s mixing: Standard Model and beyond

We present a calculation of $1/m_b^2$ corrections to the lifetime differences of $B_s$ mesons in the heavy-quark expansion. We find that they are small to significantly affect $\Delta\Gamma$ $(B_s)$ and present the result for lifetime difference including non-perturbative $1/m_b$ and $1/m_b^2$ corrections. We also analyze the generic $\Delta B$ = 1 New Physics contributions to the lifetime difference of $B_s$ mesons and provide several examples