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Register a new userThe Pion-Cloud Contribution to the Electromagnetic Nucleon Form Factors
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We study the electromagnetic structure of the nucleon within a hybrid constituent-quark model that comprises, in addition to the $3q$ valence component, also a $3q$+$pi$ non-valence component. To this aim we employ a Poincare-invariant multichannel formulation based on the point-form of relativistic quantum mechanics. With a simple 3-quark wave function for the bare nucleon, i.e. the $3q$-component, we obtain reasonable results for the nucleon form factors and predict the meson-cloud contribution to be significant only below $Q^2lesssim 0.5$,GeV$^2$ amounting to about 10% for $Q^2rightarrow 0$, in accordance with the findings of other authors.
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We examine the contribution of the pion cloud to the electromagnetic $N rightarrow Delta$ transition form factors within a relativistic hybrid constituent-quark model. In this model baryons consist not only of the $3q$ valence component, but contain, in addition, a $3 q pi$ non-valence component. We start with constituent quarks which are subject to a scalar, isoscalar confining force. This leads to an $SU(6)$ spin-flavor symmetric spectrum with degenerate nucleon and Delta masses. Mass splitting is caused by pions which are assumed to couple directly to the quarks. The point-form of relativistic quantum mechanics is employed to achieve a relativistically invariant description of this system. The $N rightarrow Delta$ transition current is then determined from the one-photon exchange contribution to the $Delta$ electroproduction amplitude. We will give predictions for the ratios $R_{EM}$ and $R_{SM}$ of electric to magnetic and Coulomb to magnetic form factors, which are supposed to be most sensitive to pion-cloud effects.
Precise proton and neutron form factor measurements at Jefferson Lab, using spin observables, have recently made a significant contribution to the unraveling of the internal structure of the nucleon. Accurate experimental measurements of the nucleon form factors are a test-bed for understanding how the nucleons static properties and dynamical behavior emerge from QCD, the theory of the strong interactions between quarks. There has been enormous theoretical progress, since the publication of the Jefferson Lab proton form factor ratio data, aiming at reevaluating the picture of the nucleon. We will review the experimental and theoretical developments in this field and discuss the outlook for the future.
A group theoretical derivation of a relation between the N --> Delta charge quadrupole transition and neutron charge form factors is presented.
By the analysis of the world data base of elastic electron scattering on the proton and the neutron (for the latter, in fact, on $^2H$ and $^3He$) important experimental insights have recently been gained into the flavor compositions of nucleon electromagnetic form factors. We report on testing the Graz Goldstone-boson-exchange relativistic constituent-quark model in comparison to the flavor contents in low-energy nucleons, as revealed from electron-scattering phenomenology. It is found that a satisfactory agreement is achieved between theory and experiment for momentum transfers up to $Q^2sim$ 4 GeV$^2$, relying on three-quark configurations only. Analogous studies have been extended to the $Delta$ and the hyperon electromagnetic form factors. For them we here show only some sample results in comparison to data from lattice quantum chromodynamics.
The u- and d-quark contributions to the elastic nucleon electromagnetic form factors have been determined using experimental data on GEn, GMn, GpE, and GpM. Such a flavor separation of the form factors became possible up to 3.4 GeV2 with recent data on GEn from Hall A at JLab. At a negative four-momentum transfer squared Q2 above 1 GeV2, for both the u- and d-quark components, the ratio of the Pauli form factor to the Dirac form factor, F2/F1, was found to be almost constant, and for each of F2 and F1 individually, the d-quark portions of both form factors drop continuously with increasing Q2.
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