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

Since ${\mathbf{p}}_A=-{\mathbf{p}}_s$, the same angle gives $m_{At}^2=m_A^2+2q(E_A+p\cos \theta )$. ThereforeThe minimum now occurs when $s$ and $t$ are antiparallel. The eventwise identity $m_{At}^2+m_{st}^2=m_C^2+m_A^2-m_B^2$ independently checks the opposite ordering of the two endpoints.