 Cascade-decay invariant-mass endpoint

For sequential on-shell two-body decays $C\to B+t$ and $B\to A+s$, with massless $s,t$, evaluate their energies in the B rest frame. Their pair mass is $2E_s{E}_t(1-\cos \theta )$, so its squared invariant mass ranges from zero to the displayed endpoint. The mass ordering is $m_C>m_B>m_A>0$. spin correlations can alter the distribution inside these limits, but cannot move the kinematic endpoints while the same on-shell decay chain applies.