Static Properties Of Nucleon Research Articles

Doubly-polarised pion photoproduction on the nucleon at MAMI

The A2 Collaboration at MAMI (Mainz) carried out new measurements of the helicity dependence of the total inclusive photo-absorption cross section and of the partial cross sections for several reaction channels on the proton and on the neutron in the photon energy region 200 He targets. Hadronic reaction products were detected by the large acceptance Crystal Ball-TAPS spectrometer, complemented by plastic scintillators and vertex detectors for charged particle tracking and identification.These new, high-quality doubly-polarised pion-photoproduction data sets provide a valuable input to the study of the nucleon structure and excitation spectrum by significantly constraining the electromagnetic multipole evaluation performed by the different available partial wave analysis models. Furthermore, the helicity dependent observables provide the main ingredient for the verification of the well-known Gerasimov-Drell-Hearn (GDH) sum rule, which relates the helicity-dependent photoasborption process to the main static nucleon properties (mass, charge, spin).

Classic calculations of static properties of nucleons reexamined

Classic calculations of the magnetic moments mu_p and mu_n of the nucleons using the traditional exponential kernel show instability with respect to variations of the Borel mass as well as arbitrariness with respect to the choice of the onset of perturbative QCD. The use of a polynomial kernel, the coefficients of which are determined by the masses of the nucleon resonances stabilizes the calculation and provides much better damping of the unknown contribution of the nucleon continuum. The method is also applied to the evaluation of the coupling gA of proton to the axial current and to the strong part of the neutron-proton mass difference Delta M_np. All these quantities depend sensitively on the value of the 4-quark condensate < 0 | qqqq | 0 > and the value < 0 | qqqq | 0 > ~ 1.5< 0 | qq | 0 >^2 reproduces the experimental results.

Comparison of the Linear Sigma Model and Chiral Perturbation Theory for Nucleon Properties

We compare the static nucleon properties in the Chiral Perturbation Theory (χPT) and the Linear Sigma Model (LSM). We consider a chiral model for the nucleon which is based on the linear sigma model with scalar-isoscalar and scalarisovector mesons coupled to quarks. We have solved the field equations in the mean field approximation for the hedgehog baryon state with different sets of model parameters. A good investigation of some static nucleon properties is obtained by the LSM.

QUARK AND SIGMA MASS DEPENDENCE OF NUCLEON PROPERTIES FROM LINEAR SIGMA MODEL

The sensitivity of static nucleon properties (magnetic moment, axial-vector coupling constant gA, pion–nucleon coupling constant gπNN and sigma commutator term σπN) to the quark and sigma masses have been investigated in the mean-field approximation. We have solved the field equations in the mean-field approximation with different sets of model parameters. Good results have been obtained in comparison with the other models and experimental data.

THE UNIFIED SKYRMION PROFILES AND STATIC PROPERTIES OF NUCLEONS

An unified approximated solution for symmetric Skyrmions was proposed for the SU(2) Skyrme model for baryon numbers up to 8, which take the hybrid form of a kink-like solution and that given by the instanton method. The Skyrmion profiles are examined by computing lowest soliton energy as well as the static properties of nucleons within the framework of collective quantization, with a good agreement with the exact numerical results. The comparisons with the previous computations as well as the experimental data are also presented.

Relativistic Collective Coordinate System of Solitons and Spinning Skyrmion

We consider constructing the relativistic system of collective coordinates of a field theory soliton on the basis of a simple principle: The collective coordinates must be introduced into the static solution in such a way that the equation of motion of the collective coordinates ensures that of the original field theory. As an illustration, we apply this principle to the quantization of spinning motion of the Skyrmion by incorporating the leading relativistic correction to the rigid body approximation. We calculate the decay constant and various static properties of nucleons, and find that the relativistic corrections are in the range of 5% -- 20%. We also examine how the baryons deform due to the spinning motion.

Analytical Solution for the SU(2) Hedgehog Skyrmion and Static Properties of Nucleons

An analytical solution for symmetric Skyrmion is proposed for the SU(2) Skyrme model, which takes the form of the hybrid form of a kink-like solution, given by the instanton method. The static properties of nucleons is then computed within the framework of collective quantization of the Skyrme model, in a good agreement with that given by the exact numeric solution. The comparisons with the previous results as well as the experimental values are also presented.

Relativistic collective coordinate quantization of solitons: Spinning Skyrmion

We develop a consistent relativistic generalization of collective coordinate quantization of field theory solitons. Our principle of introducing collective coordinates is that the equations of motion of the collective coordinates ensure those of the original field theory. We illustrate this principle with the quantization of spinning degrees of freedom of Skyrmion representing baryons. We calculate the leading relativistic corrections to the static properties of nucleons, and find that the corrections are non-negligible ones of 10% to 20%.

Chiral currents and static properties of nucleons in holographic QCD

We analyze static properties of nucleons in the two flavor holographic QCD model of Sakai and Sugimoto described effectively by a five-dimensional $U(2)$ Yang-Mills theory with the Chern-Simons term on a curved background. The baryons are represented in this model as a soliton, which at a time slice is approximately the Belavin-Polyakov-Shvarts-Tyupkin (BPST) instanton with a fixed size. First, we construct a chiral current in four dimensions from the Noether current of local gauge transformations which are nonvanishing on the boundaries of the extra dimension. We examine this chiral current for nucleons with quantized collective coordinates to compute their charge distribution, charge radii, magnetic moments, and axial-vector coupling. Most of the results are better close to the experimental values than in the Skyrme model. We discuss the problems of our chiral current, nonuniqueness of the local gauge transformation for defining the current, and its gauge-noninvariance.

Comparison of the Extended Linear Sigma Model and Chiral Perturbation Theory for Nucleon Properties

We compare the extended linear sigma model and chiral perturbation theory in describing the static nucleon properties. A good description of some nucleon properties is obtained by the extended linear sigma model.

Comparison of perturbative and non-perturbative models for description of nucleon properties

We compare the static nucleon properties in the perturbative (chiral perturbation theory) and the non-perturbative model. We consider a chiral model for the nucleon based on the linear sigma model with scalar–isoscalar scalar–isovector mesons coupled to quarks, and solved it using the coherent pair approximation.

Comparison of perturbative and non-perturbative models for description of nucleon properties

We compare the static nucleon properties in the perturbative (chiral perturbation theory) and the non-perturbative model. We consider a chiral model for the nucleon based on the linear sigma model with scalar–isoscalar scalar–isovector mesons coupled to quarks, and solved it using the coherent pair approximation.

Spin structure function g 1 (x, Q) and the DHGHY integral I(Q) at low Q: predictions from the GVMD model

Theoretical predictions for polarized nucleon structure function $g_1(x,Q^2)$ at low $Q^2$ are obtained in the framework of the Generalized Vector Meson Dominance model. Contributions from both light and heavy vector mesons are evaluated. In the photoproduction limit the first moment of $g_1$ is related to the static properties of nucleon via the Drell-Hearn-Gerasimov-Hosoda-Yamamoto sum rule. This property is employed to fix the magnitude of the light vector meson contribution to $g_1$, using the recent measurements in the region of baryonic resonances. Results are compared to the data on $g_1(x,Q^2)$. Finally, the DHGHY moment function $I(Q^2)$ is calculated, and our theoretical predictions are confronted with the recent preliminary data obtained at the Jefferson Laboratory.

Electroweak form factors of non-strange baryons

We compute electroweak form factors of the nucleon and photon transition form factors of non-strange baryon resonances up to the third resonance region in a model with instanton-induced interaction. The calculation is based on the Bethe-Salpeter equation for three light constituent quarks and is fully relativistic (U. Löring et al., Eur. Phys. J. A 10, 309 (2001)). Static nucleon properties and photon resonance couplings are in good agreement with experiment and the Q2 behaviour of the experimentally known form factors up to large momentum transfer is accounted for.

Fuzzy bag models

We show how hadronic bag models can be generalized to implement effects of a smooth and extended boundary. Our approach is based on fuzzy set theory and can be straightforwardly applied to any type of bag model. We illustrate the underlying ideas by calculating static nucleon properties in a fuzzy chiral bag model.

Medium modification of nucleon properties in the Skyrme model

A Skyrme-type Lagrangian for a skyrmion imbedded in nuclear matter is proposed. The dependence of the static nucleon properties and the nucleon-nucleon tensor interaction on nuclear density is investigated.

Scaling behavior in soliton models

In the framework of chiral soliton models we study the behavior of static nucleon properties under rescaling of the parameters describing the effective meson theory. In particular we investigate the question of whether the Brown-Rho scaling laws are general features of such models. When going beyond the simple Skyrme model we find that restrictive constraints need to be imposed on the mesonic parameters in order to maintain these scaling laws. Furthermore, in the case when vector mesons are included in the model it turns out that the isoscalar form factor no longer scales according to these laws. Finally we note that, in addition to the exact scaling laws of the model, one may construct approximate local scaling laws, which depend of the particular choice of Lagrangian parameters.

Chiral color-dielectric model with perturbative quantum pions and gluons.

Pionic contributions to static nucleon properties are calculated in a chiral extension of the color-dielectric model. The pion field and residual gluon field are treated perturbatively. It is shown that with a simple choice for the energy of the scalar confining field and assuming the chiral limit, the system of equations describing the bare soliton and the perturbative pion and gluon fields may be cast in a dimensionless, parameter-free form for large glueball masses. This enables a formula for the masses of the nucleon and \ensuremath{\Delta} including leading-order pionic and gluonic contributions and corrections for spurious center-of-mass motion, valid for a wide range of input parameters determining the bare-soliton solutions, to be derived. A further consequence of the scaling behavior is that pionic contributions to nucleon properties, calculated using the methods of the cloudy-bag model, are insensitive to the soliton parameters, once the size of the soliton is fixed. The model results are very similar to those of the cloudy-bag model but the predicted masses are about 20% too large, and the pionic contributions to charge radii are underestimated.

NUCLEON PROPERTIES IN CHIRAL COLOR DIELECTRIC MODEL

The chiral extension of color dielectric model has been used to study the static properties of nucleon. In this calculation we have treated the gluon and the pion fields perturbatively. It is found that the neutron charge rms radius and the pion-nucleon coupling constant are almost insensitive to the parameters used and are in excellent agreement with the experimental values. For better fitting the proton charge rms radius, nucleon magnetic moments and axial coupling constant prefer large quark masses (~100 MeV) and small glueball masses (&lt;1100 MeV). The strong coupling constant is found to be very sensitive to the quark and glueball masses.

Generalized Skyrme model with pion masses.

We study the generalized SU(2)\ifmmode\times\else\texttimes\fi{}SU(2) Skyrme model, proposed by Cheung and G\ursey, with an explicit chiral-symmetry-breaking term---the nonderivative pion mass term. We calculate the static properties of nucleons and compare with results obtained in the chiral limit. We find a peculiar behavior of the calculated proton and neutron charge densities, in the context of these models.