Parent momentum in a daughter's rest frame
= Parent momentum in a daughter's rest frame
{title2=$|\mathbf p_C|=(m_C^2-m_B^2)/(2m_B)$}
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 $c=1$.