Diquarks in elementary particle physics

Many phenomena in elementary particle physics show indications of clustering of two quarks inside baryons. Although the existence of such diquark systems cannot be presently rigourously proven in quantum field theory of strong interactions, phenomenological models require some quark-quark binding to explain effects ranging from the baryon mass spectrum to large pT proton production in high energy pp collisions. This review confronts diquark models predictions with experimental results in low and high transverse momentum hadron-hadron collisions, lepton-nucleon scattering and e+e- annihilations. The recent data from the Split Field Magnet detector on high pT proton production in pp, dd and αα collisions at ISR energies are particularly emphasized. 83 refs., 59 figs. (author)