Quark model analysis of strangeness form factors of the proton

The present empirical information on the proton strangeness form factors suggests that the uuds subsystem of the uudsˉ configuration has the flavor spin symmetry [4]_FS[22]_F[22]_S , mixed orbital symmetry [31]_X and that the ˉ is in the ground state. This uudsˉ configuration yields the empirical signs for both the form factors G _E ^s and G _M ^s . Transition matrix elements between the uud and uudsˉ components are significant.     

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.     

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.     

The nucleon-nucleon potential in the nonrelativistic quark model

The effective nucleon-nucleon potential is presented in the model with two three-quark clusters. The potential is nonlocal and nonadiabatic.The shape of the local adiabatic part strongly depends on the definition of the effective potential and on the choice of the subspace used in the calculation. To get some information about the repulsive core and the weak attractive part, one has also to take into account the nonlocal terms. Then for commonly used quark-quark interactions, a repulsive core is obtained, which is not very sensitive to the choice of the parameters of the quark-quark interaction; beyond the core there is a weak attraction.The trial function is constructed as a mixture of NN, ΔΔ and different coloured baryon-coloured baryon configurations. The trial function is an antisymmetric colour singlet with isospin T = 0, spin S = 1 and orbital angular momentum L = 0.     

Baryon spectra and electroweak nucleon form factors in constituent quark models

We discuss the spectra of light and strange baryons as well as the electroweak structure of the nucleons as described by relativistic constituent quark models. Special attention is paid to the performance of different types of quark dynamics and to the role of relativistic effects. It is found that the concept of constituent quark models, set up in a covariant framework, represents a promising tool to deal with low-energy hadron phenomena.     

Nucleon-nucleon forces in a nonrelativistic quark model

Conclusion We have considered allNN-partial waves simultaneously. The central part of the one gluon exchange is always repulsive, the tensor part can be neglected and the spin-orbit part is too weak for this choice of parameters. An additional colourless V^MEP potential allows us to reproduce the experimental data. However, this potential cannot be related to a long range one-pion exchange potential.Presented at the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.     

The Drell-Hearn-Gerasimov sum rule and the constituent quark model

The helicity structure function of the nucleon has been calculated for the constituent quark model and compared to the prediction of the Drell-Hearn-Gerasimov sum rule. The multipole decomposition of the sum rule shows large cancellations between different resonances. The small isoscalar-isovector contribution is related to the admixture of aD-state (bag deformation) in the nucleon's wave function. The calculations indicate a relatively slow saturation of this part of the sum rule with excitation energy.Work supported by Deutsche Forschungsgemeinschaft and Istituto Nazionale di Fisica Nucleare     

Rigorous bounds on quark masses within a nonrelativistic approach

By applying the Feynman-Hellmann theorem to \(q\bar q\) systems we find the following bounds on quark mass differences from the spectrum ofall quarkonium states $$\begin{gathered} 0.27 \leqq m_s - m_u \leqq 0.45GeV \hfill \\ 1.23 \leqq m_c - m_s \leqq 1.46GeV \hfill \\ 3.30 \leqq m_b - m_c \leqq 3.55GeV. \hfill \\ \end{gathered}$$ As best values we derive $$\begin{gathered} m_u = m_d = 0.31GeV,m_s = 0.62GeV, \hfill \\ m_c = 1.91GeV,m_b = 5.27GeV. \hfill \\ \end{gathered}$$      

Nucleon form factors in a relativistic quark model

Electromagnetic form factors of protons and neutrons are investigated based on a relativistic quark model with the inclusion of a pion cloud. Pseudo-scalar π-quark interaction is employed to study the coupling between the nucleon and the π. The results show the important role of the pion cloud for the neutron charge form factor. Moreover, our numerical analysis indicates a difference between the relativistic and the nonrelativistic treatments. Received: 10 March 1999 / Revised version: 14 June 1999     

Nonperturbative contribution to the quark form factor within the instanton model

The nonperturbative effects in the quark form factor are considered in the Wilson loop formalism, within the framework of the instanton liquid model. For the integration path corresponding to this form factor, the explicit expression for the vacuum expectation value of the Wilson operator is found to the leading order. It is shown that the instantons produce the power-like corrections to the perturbative result, which are comparable in magnitude with the perturbative part at the scale of order of the inverse average instanton size. It is demonstrated that the instanton contributions to the quark form factor are exponentiated to high orders in the small instanton density parameter.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.38.-t Quantum chromodynamics - 11.15.Tk Other nonperturbative techniques