Deep inelastic collisions


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Deep inelastic collisions

Either highly energetic collisions of elementary particles, namely, leptons and nucleons, which probe the nucleons' internal structure; or collisions between two heavy ions in which the two nuclei interact strongly while their nuclear surfaces overlap.

Elementary-particle collisions

Deep inelastic collisions of elementary particles are very energetic collisions between leptons such as electrons or neutrinos and nucleons (that is, protons or neutrons, typically in a nucleus) in which the target nucleon breaks up into many particles and the lepton is scattered through a large angle in the center-of-mass frame. These collisions are akin to the Rutherford scattering experiments in which most alpha particles went through a thin gold foil undeflected but some were deflected through large angles. In both cases, the explanation for large deflections is that the incident particle encounters not a uniform sphere of material but a few hard or pointlike objects inside the target. The alpha particles encounter gold nuclei, while leptons strike quarks inside the nucleons. See Alpha particles, Lepton, Nucleon, Quarks, Scattering experiments (nuclei)

Deep inelastic scattering experiments are conducted to study the structure of protons and neutrons. In each collision the fraction x of the nucleon's momentum carried by the struck quark is measured, and thus the x distributions of quarks inside a proton are directly measured. These are known as structure functions. Studies of these have shown, among other things, that the momentum of a proton is not carried entirely by quarks. In fact, only about half the momentum can be ascribed to quarks. The other half is believed to be carried by gluons, which are carriers of the strong force which binds the quarks within nucleons and other hadrons. See Gluons

Modern-day experiments in deep inelastic scattering often use neutrinos or muons as probes. Neutrinos have the advantage that they are not affected by the electric charge of the target nucleus and hence scatter directly off the quarks. Muons are easy to detect and identify. However, the highest-energy deep inelastic collisions are carried out by using an electron-proton collider. These experiments aim to study structure functions at very low values of x where some models predict new behavior. The highest-energy deep inelastic collisions use electrons as probes to search for quark substructure.

Heavy-ion collisions

Deep inelastic collisions of heavy ions are characterized by features that are intermediate between those of comparatively simple quasielastic, few-nucleon transfer reactions and those of highly complex compound-nucleus reactions. These deep inelastic or damped collisions often occur for heavy-ion reactions at center-of-mass energies less than 5 MeV per nucleon above the Coulomb barrier. During the brief encounter of the two nuclei, large amounts of kinetic energy of radial and orbital motion can be dissipated. The lifetime of the dinuclear complex (analogous to a chemical molecule) corresponds to the time required for the intermediate system to make a partial rotation (10-22 s to 5 × 10-21 s). On separation, the final total kinetic energies of the two reaction fragments can be well below those corresponding to the Coulomb repulsion of spheres, indicating that the fragments are highly deformed in the exit channel, as is known to be the case for fission fragments. See Nuclear fission

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