Chiral Bag Research Articles

MESONS, SKYRMIONS AND BARYONS

We review the construction of low energy effective actions for QCD expressed in terms of mesons and baryons rather than quarks and gluons. The mesons consist of quark-antiquark pairs bound by effective gluon exchange, while the baryons contain three valence quarks, also bound by effective gluon exchange, but enveloped in a meson cloud (the “sea quarks”), which may take the form of a chiral soliton. Such a “chiral quark soliton model” offers a unified approach to the various superficially dissimilar models of baryons currently popular, in particular constituent quark models, chiral bags, and Skyrmions, permitting an objective assessment of which is likely to be the most useful for a given physical problem.

Neutral pion photoproduction at threshold and explicit chiral symmetry breaking

We investigate effects of explicit chiral symmetry breaking on the ( γ, π 0) amplitude at threshold (the photoproduction “sigma”- term) using a chiral bag model of the nucleon. We point out a strong connection between this term and the flavor singlet axial coupling constant g A 0 which measures the spin fraction carried by the quarks inside the nucleon. A comparison is made with recent experimental data which show deviations from the standard low energy theorem for photoproduction. The possible origin of this discrepancy is discussed on the basis of our calculations.

Vector and axial strangeness of the proton in a topological chiral bag model

A method for estimating the presently interesting vector and axial-vector current form factors of the proton in a topological chiral bag model (CBM) is presented. It is found that the observables depending on the local strange densities in the proton break the Cheshire cat principle, otherwise expected in the CBM. Both the constraints on the vector hypercharge form factor (imposed by nucleon anomalous magnetic momenta) and on the axial hypercharge form factor (from semileptonic hyperon decays) favor the bag radius R ⋍ 0.8–0.85 fm . On the one hand, this in turn predicts the baryomagnetic moment or, equivalently, the flavor-singlet form factor F 0) 2(0) ⋍ −0.16 and the strange form factor F (s) 2(0) ⋍ −0.04 . On the other hand, the axial form factors enable us to discriminate between different proposals to incorporate the axial anomaly effects in the CBM.

Gluonic contributions in the chiral hyperbag

We incorporate into a non-perturbative chiral bag model scheme the gluons and the η' in a perturbative fashion. We analyze in this context the proton matrix element for the flavor singlet axial current, where due account is taken of the anomaly, and the delta-nucleon mass difference. Our results show that the contribution due to the gluons is significant for large bag radii and that they are crucial in order to establish the Cheshire cat principle.

Static Interaction between Two Spherical Chiral Bags

Static Interaction between Two Spherical Chiral Bags

Static Interaction between Two Spherical Chiral Bags

Static Interaction between Two Spherical Chiral Bags

The second EMC effect, semileptonic baryon decays and the chiral bag

TheU(1) anomaly gives a massiveη0 in the chiralSU(3) limit. We show in this limit how a chiral bag model givesboth theF andD amplitudes consistent with all baryon semileptonic decays,and the integral over the spin structure function measured by the EMC collaboration.

BAG INDEPENDENT RELATION FOR SPIN-ISOSPIN PROJECTED BARYONS IN THE CHIRAL BAG PLUS SKYRMION HYBRID MODEL

We propose an approximate spin-isospin projection method with collective coordinates. We perform the collective rotation in the coordinate space instead of the isospin space and construct a local Lorentz boosted quark field as an approximated solution of the collective rotation. In this projection scheme, we find that the moment of inertia of the quark is expressed by one body operator as the baryon number and the Casimir energy. After the quantization of the rotation, we find for the chiral bag model also the bag independent relations between the magnetic moments and the nucleon-delta mass difference, which have been derived for the Skyrmion.

Chiral bag with vector mesons

We investigate nucleon structure in a (non-linear) chiral bag model with vector mesons. The model incorporates two different degrees of freedom: mesons outside the bag at long and intermediate ranges, and quarks inside the bag at short distances. The ρ, a 1 and ω mesons outside the bag are included in a chiral effective lagrangian based on the non-linear sigma model. The classical solution is obtained using the hedgehog ansatz, and the cranking method is applied to construct the physical nucleon states. Static properties of the nucleon such as its mass, axial vector coupling constant, magnetic moments and charge radii are studied in detail as functions of the bag radius. Quark and meson contributions to these quantities are calculated separately. In particular, we discuss the extent to which the vector-meson dominance picture holds in the chiral bag.

Comments on "Neutron-proton mass difference in the chiral solitonic bag model"

It is pointed out that the topological soliton bag is incompletely quantized in the papers of Durgut, Pak, and Yilmaz and of Wittman and Woloshyn, leading to results on the neutron-proton mass difference and other phenomena that are not implied by the model. The purpose of this paper is to clarify how and where their scheme goes wrong and to propose an alternative consistent scheme of quantization.

Spin content of the nucleon in the chiral bag

The spin content of the nucleon is studied in the chiral bag model. U(1) A symmetry and its breaking is incorporated by the massive η' meson outside the bag. The Coriolis coupling between the η' and the quarks is treated in the framework of the semi-classical cranking method. We confirm previously obtained results that the quark spin content of the nucleon depends rather strongly on the bag radius R. The EMC data indicate a preference for small bag radii in the chiral bag.

Skyrmions and effective Lagrangians

The need to model quantum chromodynamics (QCD) at low energy is emphasized. An outline of the1/NC expansion of QCD, for large NC, shows the deep link between the Skyrme effective Lagrangian and QCD. The Skyrme model, built from the nonlinear σ model plus a stabilizer term related to vector dominance, is briefly described. The model satisfies like QCD chiral symmetry. We illustrate how the gauged Wess–Zumino action demonstrates that the topological current can be identified with the baryon current. We recall how one can show that the topological soliton is a fermion for odd NC. An example of an effective Lagrangian, built from π and low-mass vector mesons, ω, ρ, and A1 fields, is given. It describes rather well low-energy meson and baryon physics. Predictions of effective Lagrangians of the Skyrme type on meson–meson, meson–baryon, and baryon–baryon scatterings at low energy are depicted. A two-phase chiral symmetric model, the chiral bag, is introduced. It contains an inner core of confined quarks and gluons surrounded by meson fields in the topological configuration of a Skyrmion. It can describe nuclei from the low- to the high-energy range.

Proton spin, axial anomaly and the cheshire cat principle

The proton matrix element of the flavor-singlet axial current (FSAC), often referred to as “proton spin”, is discussed in terms of the chiral bag model with due account of the axial anomaly and quantum number fractionization. We argue that independently of the “confinement” or bag radius R, the FSAC matrix element remains small, say, less than 15%. We suggest this as an intricate manner in which the Cheshire cat principle manifests itself or can break down.

Baryomagnetic moment and strangeness of the proton in a topological chiral bag model

A method for estimating the presently interesting proton vector-current form factors in the topological chiral bag model ( χBM) is presented. It is found that the observables depending on the local strange densities in the proton break the “Cheshire cat” principle, otherwise expected in the χBM. The experimental constraint on the hypercharge form factor favors the bag radius R≈0.85 fm. It in turn predicts the flavor-singlet form factor F 2 (0) (0) ≅ −0.16, related to the baryomagnetic moment, and the strange form factor F 2 ( s) (0)≅−0.04.

Deep inelastic structure functions in the chiral bag model

Abstract We calculate the structure functions for deep inelastic scattering on baryons in the cavity approximation to the chiral bag model. The behavior of these structure functions is analyzed in the Bjorken limit. We conclude that scaling is satisfied, but not Regge behavior. A trivial extension as a parton model can be achieved by introducing the structure function for the pion in a convolution picture. In this extended version of the model not only scaling but also Regge behavior is satisfied. Conclusions are drawn from the comparison of our results with experimental data.

Positive and convergent self-energy in a chiral potential model

In the framework of a chiral potential model, relativistic many-body theory is used to study the effects of the pion cloud perturbatively. The second order quark self-energy is derived and studied numerically by inclusion of the full model space in the quark sector of the calculation. The self-energy shift of the three-quark ground state is found to be positive and convergent for a linearly confining potential in contrast to the negative and divergent result of the chiral bag.

Two and Three Cocycles in a Hybrid Chiral Bag Model (A Comment on the Bosonization of Fermions)11This work was supported by the Science Fund of the Chinese Academy of Sciences.

In this paper, a 3-cocycle is realized in a hybrid chiral bag model as quantities at the boundary of the bag. The equivalence between charges of gauged boson currents in Skyrme model and full gauge transformation generators is established both in dimensions and dimensions.

Baryon properties in chiral models

In the Skyrme model based on an effective Lagrangian for Goldstone bosons corresponding to chiral symmetry breaking, baryons emerge as topological solitons. A class of intermediate models with quarks and Goldstone bosons interacting in a chirally invariant way is discussed. In this context a chiral bag model is briefly reviewed. Finally we discuss problems connected with introducing strangeness into the chiral models.

Structure functions in the chiral bag model

We calculate the structure functions of an isoscalar nuclear target for the deep inelastic scattering by leptons in an extended version of the chiral bag model which incorporates the qq̄ structure of the pions in the cloud. Bjorken scaling and Regge behavior are satisfied. The model calculation reproduces the low- x behavior of the data but fails to explain the medium- to large- x behavior. Evolution of the quark structure functions seem inevitable to attempt a connection between the low-energy models and the high-energy behavior of quantum chromodynamics.

A modified massive Thirring model and new solitonic coats on the (1 + 1)-dimensional chiral bag

The bosonisation of a modified massive Thirring model Lagrangian leads to a system with a solitary-wave solution. This is used as a bosonic coating (together with other examples) in a (1 + 1)-dimensional chiral bag. With appropriate choice of parameters, the baryon energy is insensitive to the bag radius within a certain range, irrespective of the bosonic coating.