a collision of particles that is accompanied by a change in the internal state of the particles, conversion into other particles, or the additional production of new particles. Examples of inelastic scattering are the excitation or ionization of atoms during collision, nuclear reactions, the transformations of elementary particles by collision, and the multiple production of particles. Each type (channel) of inelastic scattering has its own minimum (threshold) collision energy, below which a given process is impossible. The total scattering probability when particles collide, which is characterized by a total effective scattering cross section, is the sum of the probabilities of elastic scattering and inelastic scattering; in this case, there exists a relation, given by the so-called optical theorem, between elastic and inelastic processes.