= Decay threshold
{title2=$M\geq\sum_i m_i$}
A parent of <rest mass> $M$ can decay to on-shell daughters of <rest masses> $m_i$ only if $M\geq\sum_i m_i$. In the parent <rest frame>, each daughter <energy> is at least its <rest energy>, so <four-momentum conservation> proves necessity. The excess <energy> controls the available <Lorentz-invariant phase space>. A <two-body decay threshold> is the two-daughter special case; spin and other <selection rules> can still forbid an energetically allowed channel.
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