Particle-production threshold ($\sqrt{s_{\mathrm{th}}}={\sum }_i{m}_i$)

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.