Parent momentum in a daughter's rest frame ($|{\mathbf{p}}_C|=(m_C^2-m_B^2)/(2m_B)$)

For $C\to B+t$ with massless $t$, use $p_C=p_B+p_t$ in the B rest frame. The mass-shell identity gives $E_t=(m_C^2-m_B^2)/(2m_B)$ and ${\mathbf{p}}_C={\mathbf{p}}_t$. The denominator differs from the familiar daughter-momentum formula in the parent's rest frame. Units here have $c=1$.