Past exam of the particle and nuclear physics course of the University of Cambridge / 2014 / Paper 1 / 3 / a / ii / Solution

The second decay has ${\mathbf{p}}_A=-{\mathbf{p}}_s$ in the B rest frame. Using $p_A^2=m_A^2$ and $p_s^2=0$ givesThis is an ordinary two-body decay in its parent's rest frame.