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

Let $q=(m_C^2-m_B^2)/(2m_B)$ and $p=(m_B^2-m_A^2)/(2m_B)$. In the B frame these are the energies of $t$ and $s$. If $\theta$ is their relative angle,The minimum has parallel massless particles; the maximum has antiparallel ones. The isotropic decay allows both limiting directions. The bounds are Lorentz invariant even though the B frame makes their calculation simple.