  In this work we study QCD corrections to the top quark doubly decay rate with
a detected $B$ hadron containing a $b$ quark. We focus on the regime among
which the emitted $W$ boson nearly carries its maxim energy. The tool that we
use here is the soft-collinear effective theory (SCET). The factorization
theorem based on SCET indicates a novel fragmenting jet function. We calculate
this function to next-to-leading order in $\alpha_s$. Large logarithms due to
several well separated scale are summed up using the renormalization group
equation (RGE). Finally we reach an analytic formula for the distribution which
could easily be generalized to other heavy hadron decay.
