Abstract:
In the framework of the relativistic quasipotential quark model the mass spectrum of baryons with two heavy quarks is calculated. The quasipotentials for interactions of two quarks and of a quark with a scalar and axial vector diquark are evaluated. The bound state masses of baryons with $J^P=\frac{1}{2}^+, \frac{3}{2}^+$ are computed.

Abstract:
We present a relativistic quark model for baryons, based on the Bethe-Salpeter equation in instantaneous approximation. Confinement is implemented by an interaction kernel which essentially is a linearly rising potential with a spin-dependence chosen such as to minimize spin-orbit effects. The fine structure of the baryon spectrum follows from an effective quark-interaction based on instanton effects. Results for the spectra of all baryons build from u,d,s-quarks are presented. In particular it is found, that the present relativistic setup can account for the low position of Roper-like resonances in all sectors.

Abstract:
Mass spectra of baryons consisting of two heavy (b or c) and one light quarks are calculated in the framework of the relativistic quark model. The light quark-heavy diquark structure of the baryon is assumed. Under this assumption the ground and excited states of both the diquark and quark-diquark bound system are considered. The quark-diquark potential is constructed. The light quark is treated completely relativistically, while the expansion in the inverse heavy quark mass is used revealing the close similarity with the mass spectra of B and D mesons. We find that the relativistic treatment of the light quark plays an important role. The level inversion of the p-wave excitations of the light quark in doubly heavy baryons is discussed.

Abstract:
Mass spectra of heavy baryons are calculated in the heavy-quark--light-diquark picture in the framework of the QCD-motivated relativistic quark model. The dynamics of light quarks in the diquark as well as the dynamics of the heavy quark and light diquark in the baryon are treated completely relativistically without application of nonrelativistic v/c and heavy quark 1/m_Q expansions. Such approach allows us to get predictions for the heavy baryon masses for rather high orbital and radial excitations. On this basis the Regge trajectories of heavy baryons for orbital and radial excitations are constructed, and their linearity, parallelism, and equidistance are verified. The relations between the slopes and intercepts of heavy baryons are considered and a comparison of the slopes of Regge trajectories for heavy baryons and heavy-light mesons is performed. All available experimental data on heavy baryons fit nicely to the constructed Regge trajectories. The possible assignment of the quantum numbers to the observed excited charmed baryons is discussed.

Abstract:
In this thesis a model framework for describing baryons as diquark-quark bound states is presented which is formulated in an explicitly covariant manner. The relativistic bound state problem for three quarks is considered. Diquarks are introduced as separable correlations in the two-quark correlation function and thereby baryons are described by a Bethe-Salpeter equation for bound states of quark and diquark which interact by quark exchange. In analogy to the meson spectrum, scalar and axialvector diquarks are considered to be the most important two-quark configurations within baryons. The numerical solutions of the covariant Bethe-Salpeter equation are subsequently employed in the calculation of electromagnetic, strong and axial form factors of the nucleons. The construction of the electromagnetic current operator in the diquark-quark model respects gauge invariance. Invariance under chiral symmetry transformations is slightly violated for vector diquarks have been neglected, leading to a modest violation of the Goldberger-Treiman relation. The possibility of an effective parametrization of confinement by suitable modifications of the quark and diquark propagators is investigated. It is shown that the modelling of confinement chosen here is suitable for the calculation of the octet and devuplet spectrum and spacelike nucleon properties, but its applicability breaks down for processes where large energies are transferred to the nucleon as it is the case in certain meson production processes. Model solutions for nucleon wave functions and observables are compared between the full relativistic treatment and a widely used semi-relativistic approximation. The considerable deviations in the results illustrate the inadequacy of employing the semi-relativistic treatment.

Abstract:
Mass spectra of the ground-state baryons consisting of three or two heavy (b or c) and one light (u,d,s) quarks are calculated in the framework of the relativistic quark model and the hyperspherical expansion. The predictions of masses of the triply and doubly heavy baryons are obtained by employing the perturbation theory for the spin-independent and spin-dependent parts of the three-quark Hamiltonian.

Abstract:
On the basis of the three-particle Bethe-Salpeter equation we formulated a relativistic quark model for baryons. Assuming the propagators to be given by their free form with constituent quark masses and the interaction kernel by an instantaneous potential, which contains a string-like parameterization of confinement and a flavor dependent interaction motivated by instanton effects we can account for the major features in the baryon spectrum, such as the low position of the Roper resonance and the occurrence of approximate parity doublets apparent in the Nucleon- and Lambda-spectra.

Abstract:
The electroweak properties of nucleons and hyperons are calculated in a relativistic constituent quark model. The baryons are treated as three quark bound states, and the diagrams of perturbation theory are considered on the light front. The electroweak properties of the baryons are of nonperturbative nature and can be represented by one-loop diagrams. We consider different extensions of the simplest model: quark form factors, configuration mixing of the wave function, asymmetric wave function, wave function different from the one of a harmonic oscillator valid up to energies of more than 30 GeV$^2$. A comprehensive study of various baryonic properties is given: elastic form factors of the nucleon, magnetic moments of the baryon octet, semileptonic weak form factors. This analysis also gives the Kobayashi-Maskawa matrix element $V_{us}$ and a sound symmetry breaking scheme for the Cabibbo theory. A consistent physical picture appears in this work. The nucleon consists of an unmixed, symmetric three quark state, the wave function of the hyperons is however asymmetric with a spin-isospin-0 diquark. Only for the strangeness-changing weak decay do we need nontrivial form factors.

Abstract:
Masses of tetraquarks with two heavy quarks and open charm and bottom are calculated in the framework of the diquark-antidiquark picture in the relativistic quark model. All model parameters were regarded as fixed by previous considerations of various properties of mesons and baryons. The light quarks and diquarks are treated completely relativistically. The c quark is assumed to be heavy enough to make the diquark configurations dominating. The diquarks are considered not to be point-like but to have an internal structure which is taken into account by the calculated diquark form factor entering the diquark-gluon interaction. It is found that all the (cc)(\bar q\bar q') tetraquarks have masses above the thresholds for decays into open charm mesons. Only the I(J^P)=0(1^+) state of (bb)(\bar u\bar d) lies below the BB* threshold and is predicted to be narrow.

Abstract:
We discuss some recent developments in the description of baryons as three-quark systems within relativistic constituent quark models. In particular we address the issues of excitation spectra, electroweak structure, and mesonic resonance decays. The necessities of implementing simultaneously the symmetries of low-energy quantum chromodynamics and of relativistic invariance are emphasized.