It is becoming increasingly clear that the concept of a diquark (a two-quark system) is important for understanding hadron structure and high-energy particle reactions. According to our present knowledge of quantum chromodynamics (QCD), diquark correlations arise in part from spin-dependent interactions between two quarks, from quark radial or orbital excitations, and from quark mass differences. Diquark substructures affect the static properties of baryons and the mechanisms of baryon decay. Diquarks also play a role in hadron production in hadron-initiated reactions, deep-inelastic lepton scattering by hadrons, and in e + e~ reactions. Diquarks are important in the formation and properties of baryonium and mesonlike semistable states. Many spin effects observed in high-energy exclusive reactions pose severe problems for the pure quark picture of baryons and might be explained by the introduction of diquarks as hadronic constituents. There is considerable controversy, not about the existence of diquarks in hadrons, but about their properties and their effects. In this work a broad selection of the main ideas about diquarks is reviewed.

The Feynman-Hellmann theorem and semiempirical mass formulas are used to predict the masses of baryons containing one or two heavy quarks. In particular, the mass of the Λ b is predicted to be 5620 ± 40 MeV, a value consistent with measurements.PACS numbers: 12.15. Ff, 12.40.Yx, In a recent paper [1], the Feynman-Hellmann theorem [2,3] and semiempirical mass formulas [4,5,1] were used as tools enabling the prediction of the masses of a

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There are striking regularities in the masses and mass differences of known hadrons. Some of these regularities can be understood from known general properties of the interactions of quarks without a need to specify the explicit form of the Hamiltonian. The Feynman-Hellmann theorem is one of the tools providing this understanding. If the mass regularities are exploited, predictions can be made of the masses of as yet undiscovered hadrons. In particular, it is found that the mass of the B * c is 6320 ± 20 MeV. Predictions concerning i) excited vector mesons, ii) pseudoscalar mesons, iii) P -wave mesons, and iv) ground-state spin 1/2 and 3/2 baryons are also made.

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