= Cascade-decay invariant-mass endpoint
{title2=$(m_{st}^2)_{\max}=(m_C^2-m_B^2)(m_B^2-m_A^2)/m_B^2$}
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 $2E_sE_t(1-\cos\theta)$, 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.
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