Phase space analysis for three and four massive particles in final states

We propose formulae for computing the phase space integrals of $1\to 3$ and $1\to 4$ processes with massive particles in final states. As an application of these formulae we study the final state mass effects in some interesting phenomenological cases, giving fully integrated analytic results for the corresponding phase spaces. We consider also the $B_s-\bar{B}_s$ process at NNLO and calculate one of the most complicated master integrals, which contributes to the $\Delta\Gamma_{B_s}$ at $O(\alpha_s^2)$.