Perturbative Chiral Quark Model Research Articles

Axial transition form factors of octet baryons in the perturbative chiral quark model

We study the axial transition form factors $G_A^{B\to B'}(Q^2)$ as well as the axial charges $g_A^{B\to B'}$ of the octet baryons in the perturbative chiral quark model~(PCQM) with including both the ground and excited states in the intermediate quark propagators. The PCQM results on the $G_A^{B\to B'}(Q^2)$ and the $g_A^{B\to B'}$ are found in good agreement with the existing experimental data and the lattice-QCD values. The study figures out that the $G_A^{B\to B'}(Q^2)$ for all transitions behave in the dipolelike form, which is dominantly caused by the three-quark core. The meson cloud with the ground-state quark propagator also plays an extremely important role but results in a flat contribution. The excited-state quark propagator contributing to the $G_A^{B\to B'}(Q^2)$ could be regarded as the higher order correction and it is very limited.

Inclusion of the excited states quark propagator to the nucleon electromagnetic form factors in the perturbative chiral quark model

Electromagnetic form factors of the nucleon are studied within the framework of the Perturbative Chiral Quark Model (PCQM). In a previous analysis of the nucleon electromagnetic form factors, the quark propagator used in the calculation was truncated to quark in the ground state only. The attempt to include the excited states quark propagator contribution to the neutron was performed but not on the proton. Therefore, in this work, we try to be consistent in the nucleonic sector by including the excited states quark propagator up to the second excited states to study the electromagnetic form factors of the nucleon. Our results are in good agreement with experimental data and show the significance of the contribution of the quark excited states propagator to the electromagnetic properties of the nucleon.

Analysis of excited quark propagator effects on neutron charge form factor

The charge form factor and charge radius of neutron are investigated in the perturbative chiral quark model (PCQM) with considering both the ground and excited states in the quark propagator. A Cornell-like potential is extracted in accordance with the predetermined ground state quark wavefunction, and the excited quark states are derived by solving the Dirac equation with the extracted PCQM potential numerically. The study reveals that the contributions of the excited quark states are considerably influential in the charge form factor and charge radius of neutron as expected, and the total results are significantly improved and increased by nearly four times by including the excited states in the quark propagator. The theoretical PCQM results are found, including the ground and excited quark propagators, in good agreement with the recent lattice QCD values at pion mass of about 130 MeV.

Axial charges of octet and decuplet baryons in a perturbative chiral quark model

Axial charges of octet and decuplet baryons in a perturbative chiral quark model

Charmed baryon Λc in nuclear matter

Density dependences of the mass and self-energies of ${\mathrm{\ensuremath{\Lambda}}}_{c}$ in nuclear matter are studied in the parity projected QCD sum rule. Effects of nuclear matter are taken into account through the quark and gluon condensates. It is found that the four-quark condensates give dominant contributions. As the density dependences of the four-quark condensates are not known well, we examine two hypotheses. One is based on the factorization hypothesis (F-type) and the other is derived from the perturbative chiral quark model (QM-type). The F-type strongly depends on density, while the QM-type gives a weaker dependence. It is found that, for the F-type dependence, the energy of ${\mathrm{\ensuremath{\Lambda}}}_{c}$ increases as the density of nuclear matter grows, that is, ${\mathrm{\ensuremath{\Lambda}}}_{c}$ feels repulsion. On the other hand, the QM-type predicts a weak attraction ($\ensuremath{\sim}20$ MeV at the normal nuclear density) for ${\mathrm{\ensuremath{\Lambda}}}_{c}$ in nuclear matter. We carry out a similar analysis of the $\mathrm{\ensuremath{\Lambda}}$ hyperon and find that the F-type density dependence is too strong to explain the observed binding energy of $\mathrm{\ensuremath{\Lambda}}$ in nuclei. Thus we conclude that the weak density dependence of the four-quark condensate is more realistic. The scalar and vector self-energies of ${\mathrm{\ensuremath{\Lambda}}}_{c}$ for the QM-type dependence are found to be much smaller than those of the light baryons.

Study of excited quark propagator contributions in perturbative chiral quark model

In this work, masses of octet baryons are evaluated in the framework of the perturbative chiral quark model (PCQM) with considering both the ground and excited states in the quark propagator, in which a Cornell-like potential is first constructed by letting the predetermined ground state quark wavefunction satisfy Dirac equation, and the excited quark states are derived by resolving Dirac equation with the extracted PCQM potential numerically employing Generalized Eigenvalue & Eigenstate Problem method. The results on the octet baryon masses are found in good agreement with the experimental data, and the study reveals that the contributions of the excited quark states are considerably influential in the octet baryon masses as expected.

Comparisons of magnetic charge and axial charge meson cloud distributions in the PCQM

The meson cloud distributions in r-space are extracted from the nucleon electromagnetic and axial form factors which are derived in the perturbative chiral quark model. The theoretical results indicate that the magnetic charge and axial charge distributions of the three-quark core have the similar distributions in r-space, the magnetic charge distributions of the meson cloud and three-quark core are more or less in the same region and peak at distances of around 0.4 fm, which is in good agreement with the finding of works in the framework of chiral perturbation theory, but the axial charge meson cloud distributes mainly inside the three-quark core.

Meson cloud contributions to baryon axial form factors

The axial form factor as well as the axial charges and radii of octet $N$, $\mathrm{\ensuremath{\Sigma}}$, and $\mathrm{\ensuremath{\Xi}}$ baryons are studied in the perturbative chiral quark model with the quark wave functions predetermined by fitting the theoretical results of the proton charge form factor to experimental data. The theoretical results are found, based on the predetermined quark wave functions, in good agreement with experimental data and lattice values. This may indicate that the electric charge and axial charge distributions of the constituent quarks are the same. The study reveals that the meson cloud plays an important role in the axial charge of octet baryons, contributing 30%--40% to the total values, and strange sea quarks have a considerable contribution to the axial charges of the $\mathrm{\ensuremath{\Sigma}}$ and $\mathrm{\ensuremath{\Xi}}$.

Study of baryon octet electromagnetic form factors in perturbative chiral quark model

The electromagnetic properties of baryon octet are studied in the perturbative chiral quark model (PCQM). The relativistic quark wave function is extracted by fitting the theoretical results of the proton charge form factor to experimental data and the predetermined quark wave function is applied to study the electromagnetic form factors of other octet baryons as well as magnetic moments, charge and magnetic radii. The PCQM results are found, based on the predetermined quark wave function, in good agreement with experimental data.

Study of baryon octet charge form factors in perturbative chiral quark model

The charge form factors of baryon octet are studied in the perturbative chiral quark model (PCQM). The relativistic quark wave function is extracted by fitting the theoretical results of the nucleon charge form factors to the experimental data and the predetermined quark wave function is applied to study the charge form factors of other octet baryons. The PCQM results are found, based on the predetermined quark wave function, in good agreement with the experimental data.

Dependence of nucleon properties on pseudoscalar meson masses

We discuss the sensitivity of nucleon properties (mass, magnetic moments and electromagnetic form factors) on the variation of the pseudoscalar meson masses in the context of the perturbative chiral quark model. The obtained results are compared to data and other theoretical predictions.

A pentaquark model for Λ(1405)

We study a pentaquark model of the negative parity hyperon Lambda(1405). We choose a specific quark configuration for trial, and use the perturbative chiral quark model extended to four quarks and one anti-quark system. We calculate a sigma-term and a scalar form factor of the hyperon and demonstrate their usefulness to study the nature of the hyperon.

Strong CP violation and the neutron electric dipole form factor

We calculate the neutron electric dipole form factor induced by the CP violating theta-term of QCD, within a perturbative chiral quark model which includes pion and kaon clouds. On this basis we derive the neutron electric dipole moment and the electron-neutron Schiff moment. From the existing experimental upper limits on the neutron electric dipole moment we extract constraints on the theta-parameter and compare our results with other approaches.

The neutron electric dipole form factor in the perturbative chiral quark model

We calculate the electric dipole form factor of the neutron in a perturbative chiral quark model, parameterizing CP violation of generic origin by means of effective electric dipole moments of the constituent quarks and their CP-violating couplings to the chiral fields. We discuss the relation of these effective parameters to more fundamental ones such as the intrinsic electric and chromoelectric dipole moments of quarks and the Weinberg parameter. From the existing experimental upper limits on the neutron electric dipole moment, we derive constraints on these CP-violating parameters.

GROUND-STATE BARYON MASSES IN THE PERTURBATIVE CHIRAL QUARK MODEL

The mass differences of the flavor octet and decuplet ground-state baryons are studied in the perturbative chiral quark model. We present a way to understand the nontrivial spin- and flavor-dependent mass differences, whereby both pseudoscalar mesons and gluons play a significant role.

Nucleon polarizabilities in the perturbative chiral quark model

The nucleon polarizabilities αE and βM are studied in the context of the perturbative chiral quark model. We demonstrate that meson-cloud effects are sufficient to explain the electric polarizability of the nucleon. Contributions of excited quark states to the paramagnetic polarizability are dominant and cancel the diamagnetic polarizability arising from the chiral field. The obtained results are compared to data and other theoretical predictions.

MASS SPECTRUM OF THE JP=1/2- AND 3/2- PENTAQUARK ANTIDECUPLETS IN THE PERTURBATIVE CHIRAL QUARK MODEL

We study the recently discovered Θ+ baryon in the context of the perturbative chiral quark model. The basic configuration of the Θ+ is a pentaquark bound state, where the single particle wave functions are the ground state solutions of a confining potential. We classify the resulting pentaquark multiplets as the JP=1/2- and 3/2- flavor SU (3) antidecuplet. The full mass spectrum of the multiplets is determined by including the meson and gluon cloud contributions, which induce flavor SU (3) breaking. The resulting 3/2- antidecuplet is about 185 MeV heavier than the 1/2- one, mainly because of the semi-perturbative gluon effects. We assign the observed Θ+ baryon as a member of the 1/2- antidecuplet and discuss in particular its relation to the recent experimental signal for a Ξ-- baryon.

Structure of baryons in the perturbative chiral quark model

We discuss recent applications of the perturbative chiral quark model (PCQM) in the analysis of the structure of baryons. The PCQM is based on an effective Lagrangian, where baryons are described as relativistic valence quarks and a perturbative cloud of Goldstone bosons as required by chiral symmetry.

Axial form factor of the nucleon in the perturbative chiral quark model

We apply the perturbative chiral quark model (PCQM) at one loop to analyse the axial form factor of the nucleon. This chiral quark model is based on an effective Lagrangian, where baryons are described by relativistic valence quarks and a perturbative cloud of Goldstone bosons as dictated by chiral symmetry. We apply the formalism to obtain analytical expressions for the axial form factor of the nucleon, which is given in terms of fundamental parameters of low-energy pion–nucleon physics (weak pion decay constant, strong pion–nucleon form factor) and of only one model parameter (radius of the nucleonic three-quark core).

Updated analysis of meson-nucleonσterms in the perturbative chiral quark model

We present an updated analysis of meson-baryon sigma terms in the perturbative chiral quark model, which is based on effective chiral Lagrangian. The new feature concerns the inclusion of excited states in the quark propagator. Its influence on meson loops is shown to lead, in particular, for the pion-nucleon $\ensuremath{\sigma}$ term to an enhancement relevant for the current evaluation of this quantity. We also determine various flavor combinations of the scalar nucleon form factors and their respective low-momentum transfer limits.