= Reaction threshold
{title2=$E_{\rm CM}^{\min}=\sum_fm_fc^2$}
The kinematic minimum energy needed to produce an allowed final state. In the <centre-of-momentum frame>, the threshold for freely separated massive final particles is the sum of their <rest energies>, with no relative kinetic energy. Conservation of total <energy> and <momentum> then determines the corresponding laboratory threshold. Charges and other selection rules must also permit the reaction; the mass bound alone does not guarantee an observable transition.
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