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Hard is an adjective often used to describe the energy and or momentum of (an object involved in) a scattering process in collider physics. The energy is usually large enough that in calculations of such processes a perturbative expansion in the coupling strength may be employed. This distinguishes such a process or object from something which is soft, or has low energy and or momentum.

For example, in a general-purpose event generator the hard event is usually calculated at fixed order using a matrix-element generator and evolved to lower energies (typically the hadronisation scale \Lambda_{QCD}) using a parton shower.