A possible resolution of the proton spin problem

Myhrer, F.; Thomas, A. W.

doi:10.1016/j.physletb.2008.04.034

Cited by 75 publications

(79 citation statements)

References 38 publications

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“…However, the experimental value is ∆Σ ≃ 0.33 [8,9], which raised the well known proton spin crises [8][9][10][11]. The above values of ∆Σ q correspond to F = 2 3 and D = 1 [12,13,50,57,123].…”

Section: Su (3) Baryon-meson Modelmentioning

confidence: 92%

“…The form factors associated with the weak interaction axial current for the transitions B ′ → B ℓν ℓ , with B, B ′ being spin 1/2 baryons, ℓ = e, µ, τ andν ℓ is a antineutrino, can be decomposed into the axial-vector G A and induced pseudoscalar G P form factors [6,7]. In the limit Q 2 = 0 the octet baryon form factors G A can be related with the polarized deep inelastic scattering data, and used to estimate the spin fraction of the baryon carried by the quarks (valence and sea) [3,[8][9][10][11][12][13].…”

Section: Introductionmentioning

confidence: 99%

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Axial form factors of the octet baryons in a covariant quark model

Ramalho

Tsushima

2016

Phys. Rev. D

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We study the weak interaction axial form factors of the octet baryons, within the covariant spectator quark model, focusing on the dependence of four-momentum transfer squared, Q 2 . In our model the axial form factors GA(Q 2 ) (axial-vector form factor) and GP (Q 2 ) (induced pseudoscalar form factor), are calculated based on the constituent quark axial form factors and the octet baryon wave functions. The quark axial current is parametrized by the two constituent quark form factors, the axial-vector form factor g q A (Q 2 ), and the induced pseudoscalar form factor g q P (Q 2 ). The baryon wave functions are composed of a dominant S-state and a P -state mixture for the relative angular momentum of the quarks. First, we study in detail the nucleon case. We assume that the quark axial-vector form factor g q A (Q 2 ) has the same function form as that of the quark electromagnetic isovector form factor. The remaining parameters of the model, the P -state mixture and the Q 2 -dependence of g q P (Q 2 ), are determined by a fit to the nucleon axial form factor data obtained by lattice QCD simulations with large pion masses. In this lattice QCD regime the meson cloud effects are small, and the physics associated with the valence quarks can be better calibrated. Once the valence quark model is calibrated, we extend the model to the physical regime, and use the low Q 2 experimental data to estimate the meson cloud contributions for GA(Q 2 ) and GP (Q 2 ). Using the calibrated quark axial form factors, and the generalization of the nucleon wave function for the other octet baryon members, we make predictions for all the possible weak interaction axial form factors GA(Q 2 ) and GP (Q 2 ) of the octet baryons. The results are compared with the corresponding experimental data for GA(0), and with the estimates of baryon-meson models based on SU (6) symmetry.

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Section: Su (3) Baryon-meson Modelmentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Axial form factors of the octet baryons in a covariant quark model

Ramalho

Tsushima

2016

Phys. Rev. D

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“…The sizeable contributions from all other diagrams (see Fig. 38) indicate that there is significant quark orbital angular momentum in the proton wave function [93,94]. Table X give results for the neutron form factors, broken down into the total vector and tensor coupling contributions from the Feynman diagrams of Fig.…”

Section: Gevmentioning

confidence: 99%

Role of diquark correlations and the pion cloud in nucleon elastic form factors

2014

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Electromagnetic form factors of the nucleon in the space-like region are investigated within the framework of a covariant and confining Nambu-Jona-Lasinio model. The bound state amplitude of the nucleon is obtained as the solution of a relativistic Faddeev equation, where diquark correlations appear naturally as a consequence of the strong coupling in the colour3 qq channel. Pion degrees of freedom are included as a perturbation to the "quark-core" contribution obtained using the Poincaré covariant Faddeev amplitude. While no model parameters are fit to form factor data, excellent agreement is obtained with the empirical nucleon form factors (including the magnetic moments and radii) where pion loop corrections play a critical role for Q 2 1 GeV 2 . Using charge symmetry, the nucleon form factors can be expressed as proton quark sector form factors. The latter are studied in detail, leading, for example, to the conclusion that the d-quark sector of the Dirac form factor is much softer than the u-quark sector, a consequence of the dominance of scalar diquark correlations in the proton wave function. On the other hand, for the proton quark sector Pauli form factors we find that the effect of the pion cloud and axialvector diquark correlations overcomes the effect of scalar diquark dominance, leading to a larger d-quark anomalous magnetic moment and a form factor in the u-quark sector that is slightly softer than in the d-quark sector.

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“…This same interaction required by spectroscopy naturally reduces the fraction of spin carried by the quarks [29]. Combining these two effects fully accounts for the current experimental value [30] and lattice results [31].…”

Section: Spinmentioning

confidence: 65%