Particle-production threshold
= Particle-production threshold
{title2=$\sqrt{s_{\rm th}}=\sum_i m_i$}
= Production threshold
{synonym}
The lowest initial <centre-of-mass frame> <energy> at which a specified final state can be made. For free massive final particles in <natural units>, the minimum is the sum of their <masses>, attained when their relative <momenta> vanish. The <cross section> at the exact threshold can vanish even though the channel becomes kinematically accessible there.