Quark-diquark approximation of the three-quark structure of baryons in the quark confinement model

Octet (1/2⁺) and decuplet (3/2⁺) baryons as relativistic three-quark states are investigated using the quark confinement model (QCM), the relativistic quark model, based on some assumptions about hadronization and quark confinement. The quark-diquark approximation of the three-quark, structure of baryons is proposed. In the framework of this approach, the main low-energy characteristics of baryons, such as magnetic moments, electromagnetic radii and form factors, the ratio of axial and vector constants in semileptonic baryon decays, strong form factors and decay widths, are described. The obtained results agree with experimental data.     

Nucleon polarization and the nuclear charge operator

Effects of nucleon polarization on the nuclear charge operator have been evaluated in a constituent quark model. At momentum transfer q ≈ 4 fm^?1 monopole, dipole and quadrupole excitations are of equal importance. In a harmonic oscillator model for ³He all multipolarities give negative contributions, leading to an overall contribution comparable to the relativistic pair effect. The influence of realistic wave functions, coupling constants and off-shell form factors is discussed.     

Heavy to light baryon weak form factors in the lightcone constituent quark model

Heavy to light baryon weak form factors are investigated in a lightcone constituent quark model. In a SU(4) symmetry broken scheme, both charged and neutral weak current-induced form factors are calculated at theq ² = 0 point including the leading relativistic effects in the spin composition of baryons. The corresponding semileptonic decays are described by assuming dipole dependence of form factors onq ².     

Quark model analysis of semileptonicD decays intoK andK *

We derive the form factors governing the decaysD→Kev andD→K ^* ev in the framework of a relativistic constituent quark model. Our results agree with the recently measured form factors and the data for rates at the level of the present experimental uncertainty.     

Masses and electroweak properties of light mesons in the relativistic quark model

The masses, pseudoscalar and vector weak decay constants and electromagnetic form factors of light S-wave mesons are studied in the framework of the relativistic quark model based on the quasipotential approach. We use the same model assumptions and parameters as in our previous investigations of heavy meson and baryon properties. The masses and wave functions of the ground state and radially excited π, ρ, K, K^* and φ mesons, obtained by solving numerically the relativistic Schrödinger-like equation with the complete relativistic qq? potential including both spin-independent and spin-dependent terms, are presented. Novel relativistic expressions for the weak decay constants of the pseudoscalar and vector mesons are derived. It is shown that the intermediate negative-energy quark states give significant contributions which essentially decrease the decay constants bringing them in agreement with experimental data. The electromagnetic form factors of the pion, charged and neutral kaon are calculated in a broad range of the space-like momentum transfer. The corresponding charge radii are determined. All results agree well with the available experimental data.     

Axial exchange currents and nucleon spin

We calculate the axial couplings g_A⁸(0) and g_A⁰(0) related to the spin of the nucleon in a constituent quark model. In addition to the standard one-body axial currents, the model includes two-body axial exchange currents. The latter are necessary to satisfy the Partial Conservation of Axial Current (PCAC) condition. For both axial couplings we find significant corrections to the standard quark model prediction. Exchange currents reduce the valence quark contribution to the nucleon spin and afford an interpretation of the missing nucleon spin as orbital angular momentum carried by nonvalence quark degrees of freedom.     

Semileptonic decays B → (π, )eν in relativistic quark model

Quark model results for the B → π, decays are analysed, making use of the dispersion formulation of the model: The form factors at q² > 0 are expressed as relativistic invariant double spectral representation over invariant masses of the initial and final mesons through their light-cone wave functions. The dependence of the results on the quark model parameters is studied. For various versions of the quark model the ranges

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, and Γ_L/Γ_T = 0.7 ± 0.08 are found. The effects of the constituent quark transition form factor are briefly discussed.     

Radiative decays and transition form factors of strange mesons in the relativistic constituent quark model

Motivated by the recent lattice QCD results indicating that the topological charge contribution to the flavor singlet axial vector current can be traded off by the constituent quark masses, we investigate the radiative decays of pseudoscalar (π,K, η, η′), vector (ρ,K*, ω, ?) and axial vector (A ₁) mesons using a simple relativistic constituent quark model. For both simplicity and relativity, we take advantage of the distinguished features in the light-cone quantization method: (1) the Fock-state expansion of meson wavefunctions are not contaminated by the vacuum fluctuation, (2) the assignment of meson quantum numbers are given by the Melosh transformation. Except the well-known constituent quark masses of (u,d,s) quarks and the spin-averaged meson masses, the only parameter in the model is the gaussian parameter β which determines the broadness (or sharpness) of radial wavefunction. The computed decay widths and the transition form factors of ρ, ω → π(η)γ*,K* →Kγ* andA ₁ → πγ* at 0≤Q ²≤5 GeV² and π⁰(η) → γ*γ at 0≤Q ²≤3 GeV² are in a remarkably good agreement with the experimental data and the result forA ₁ ⁺ → π₊ γ* transition is quite consistent with the experiments of pion scattering on a nucleus using Primakoff effect. This model is potentially useful in the cocktail analyses of the dilepton productions in proton-proton, proton-nucleus and nucleus-nucleus collisions at SPS energies and a little above.     

Electromagnetic and Axial Form Factors of Octet and Decuplet Baryons

We report from a study of the elastic electromagnetic and axial form factors of all lowest baryon states with flavors up, down, and strange along relativistic constituent-quark models. We consider the baryons as relativistic bound states of three constituent quarks and solve the eigenvalue problem of the invariant mass operator. The corresponding eigenstates are employed to calculate manifestly covariant form factors within the point form of Poincaré-invariant quantum mechanics. The electromagnetic and axial current operators are constructed along the spectator model in point-form relativistic dynamics. We have thus obtained covariant predictions for the electroweak form factors, for momentum transfers up to Q ² ~ 4 GeV², as well as the electric radii, magnetic moments, and axial charges. The theoretical results in general agree very well with existing phenomenological data. In cases, where no experimental information is yet available, the results are well compatible with data from lattice quantum chromodynamics.     

Semileptonic B decays into even parity charmed mesons

By using a constituent quark model we compute the form factors relevant to semileptonic transitions of the B mesons into low-lying p-wave charmed mesons. We evaluate the q² dependence of these form factors and compare them with other model calculations. The Isgur–Wise functions τ_1/2 and τ_3/2 are also obtained in the heavy quark limit of our results. PACS 13.25.Hw; 12.39.Hg; 12.39.Jh     

A study of meson-baryon couplings in the constituent quark model

Within the framework of the constituent quark model we discuss the effects of different types of meson-baryon-baryon vertex operators on the form factors and the coupling strengths of the lowest-lying positive and negative parity non-strange baryons. We compare the quark pair creation model (³P₀-model) with the SU(6)-model in which mesons are treated as elementary fields that directly couple to the quarks. The latter model is employed both in the so-called static limit and in a modification motivated by Galilei invariance. It is demonstrated that the inclusion of non-static effects simulates some features of the³P₀ vertex. Especially the reaction πN→ππN is found to be very sensitive to the different assumptions on the dynamics of theq¯q pair creation process. More indirect hints for the internal structure of the mesons might be obtained from the predicted asymmetry for the two form factors ofN→Δ+π andΔ→N+π, which occurs in the³P₀-model, only.     

QCD sum rules analysis of the rare B_{c}\rightarrow X\nu\bar{\nu} decays

Taking into account the gluon correction contributions to the correlation function, the form factors relevant to the rare decays are calculated in the framework of the three-point QCD sum rules, where X stands for axial vector particle, AV(D _s1), and vector particles, V(D ^*,D _s ^*). The total decay width as well as the branching ratio of these decays are evaluated using the q ² dependent expressions of the form factors. A comparison of our results with the predictions of the relativistic constituent quark model is presented.     

Electromagnetic Form Factors of Nucleons in a Relativistic Three-Quark Model

We study electromagnetic form factors of nucleons within a relativistic three-quark model with a Gaussian shape for the nucleon-quark vertex. The allowed regions for two adjustable parameters, the range parameter Λ _N in the Gaussian and the constituent quark mass m _q , are obtained from fitting the data for magnetic moments and electromagnetic radii of nucleons. It is found that these observables, when calculated with m _q = 420 MeV and Λ _N = 1.25 GeV, agree very well with the experimental data. For these parameter values, however, our model underestimates the electromagnetic form factors of the nucleon at high momentum transfers for the calculated range O ≤ Q²≤ 1 GeV². Received April 10, 1996; revised June 20, 1996; accepted for publication July 19, 1996     

A relativistic quark model for mesons with an instanton-induced interaction

We present new results of a relativistic quark model based on the Bethe-Salpeter equation in its instantaneous approximation. Assuming a linearly rising confinement potential with an appropriate spinorial structure in Dirac space and adopting a residual interaction based on instanton effects, we can compute masses of the light mesons up to highest observed angular momenta with a natural solution of the U _A(1) problem. The calculated ground states masses and the radial excitations describe the experimental results well. In this paper, we will also discuss our results concerning numerous meson decay properties. For processes like π⁺/K ⁺↦e ⁺υ_eγ and 0^-↦γγ at various photon virtualities, we find a good agreement with experimental data. We will also comment on the form factors of the K _?3 decay and on the decay constants of the π, K and η mesons. For the sake of completeness, we will furthermore present the electromagnetic form factors of the charged π and K mesons as well as a comparison of the radiative meson decay widths with the most recent experimental data. Received: 28 August 2000 / Accepted: 12 September 2000     

A relativistic study of the nucleon helicity amplitudes

Abstact: We perform a calculation of the relativistic transition form factors for the electromagnetic excitation of the nucleon resonances. We use as input the 3-quark wave functions obtained in a Constituent Quark Model with three-body forces in the hypercentral approach. With respect to the non relativistic calculations a significant contribution is obtained up to Q ²≃ 2 (GeV/c)². However, the low Q ²-behaviour exhibits a lack of strength, which may be connected with the need of taking into account explicitly further degrees of freedom beyond the three constituent quark ones. Received: 16 April 1998     

Covariant nucleon electromagnetic form factors from the Goldstone-boson-exchange quark model

We present a study of proton and neutron electromagnetic form factors for the recently proposed Goldstone-boson-exchange constituent quark model. Results for charge radii, magnetic moments, and electric as well as magnetic form factors are reported. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. All the predictions by the Goldstone-boson-exchange constituent quark model are found to fall remarkably close to existing experimental data.     

Radiative leptonic decays of heavy mesons in heavy quark limit

We study the radiative leptonic decays of heavy mesons within the covariant light-front model. Using this model, both the form factors F_V and F_A have the same form when the heavy quark limit is taken. In addition, the relation between the form factor F_V and the decay constant of a heavy meson F_H is obtained. The hadronic parameter β can be determined by the parameters appearing in the wave function of the heavy meson. We find that the value of β is not only quite smaller than the one in the non-relativistic case, but also insensitive to the value of the light quark mass m_q. These results mean that the relativistic effects are very important in this work. We also obtain that the branching ratio of B→lν_lγ is about (1.40–1.67)×10^-6, in agreement with the general estimates in the literature. PACS 12.39.Hg; 13.40.Gp     

Hadronic decays of N and Δ resonances in a chiral quark model

π and η decay modes of light baryon resonances are investigated within a chiral quark model whose hyperfine interaction is based on Goldstone-boson exchange. For the decay mechanism a modified version of the ³ P ₀ model is employed. Our primary aim is to provide a further test of the recently proposed Goldstone-boson exchange constituent quark model. We compare the predictions for π and η decay widths with experiment and also with results from a traditional one-gluon exchange constituent quark model. The differences between nonrelativistic and semirelativistic versions of the constituent quark models are outlined. We also discuss the sensitivity of the results on the parameterization of the meson wave function entering the ³ P ₀ model. Received: 11 May 2001 / Accepted: 17 September 2001     

Flavor-changing radiativeB decays

We calculate the exclusive rate forB→K ^* (890)γ andB→K ^** (1400)γ arising from the quark subprocessb→sγ. These provide a good test of the standard model because they probe the flavor changing loop enhanced by QCD corrections. We have used relativised constituent quark model to evaluate the different form factors and estimate the recoil effects.