Revealing Dressed Quarks via the Proton’s Charge Distribution

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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“…30), which do not appear in the model described herein. The running of the quark mass function in QCD may also play an important role [77].…”

Section: Nucleon Form Factor Resultsmentioning

confidence: 99%

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

2014

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“…[502] where the parameters were adjusted to reproduce the nucleon and ∆ masses (potentially leaving room for pion cloud effects) and to some extent also their electromagnetic form factors. This makes it possible to study the effects of the individual diquark components and investigate the response to changing ingredients such as the quark mass function [503,504]. Here the quarks and diquarks are still momentum-dependent (i.e., non-pointlike) and modelled using realistic parametrizations.…”

Section: B Kinematics Andmentioning

confidence: 99%

Baryons as relativistic three-quark bound states

Eichmann

Sanchis-Alepuz

Williams

et al. 2016

Progress in Particle and Nuclear Physics

436

530

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We review the spectrum and electromagnetic properties of baryons described as relativistic three-quark bound states within QCD. The composite nature of baryons results in a rich excitation spectrum, whilst leading to highly non-trivial structural properties explored by the coupling to external (electromagnetic and other) currents. Both present many unsolved problems despite decades of experimental and theoretical research. We discuss the progress in these fields from a theoretical perspective, focusing on nonperturbative QCD as encoded in the functional approach via Dyson-Schwinger and Bethe-Salpeter equations. We give a systematic overview as to how results are obtained in this framework and explain technical connections to lattice QCD. We also discuss the mutual relations to the quark model, which still serves as a reference to distinguish 'expected' from 'unexpected' physics. We confront recent results on the spectrum of non-strange and strange baryons, their form factors and the issues of two-photon processes and Compton scattering determined in the DysonSchwinger framework with those of lattice QCD and the available experimental data. The general aim is to identify the underlying physical mechanisms behind the plethora of observable phenomena in terms of the underlying quark and gluon degrees of freedom.

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“…In the diquark model, the singly occurring down-quark in the proton is more likely to be associated with an axial-vector diquark than a scalar diquark, and the contribution of the axial-vector diquark yields a more rapid falloff of the form factors. The up quarks are generally associated with the more tightly bound scalar diquarks, yielding a harder form factors [10,81,82,85,[95][96][97][98][99][100][101][102]. Recent calculations [99] suggest that pion loop corrections play a crucial rule for Q 2 < ∼ 1.0 (GeV/c) 2 .…”

Section: Flavor-dependent Contributions To σRmentioning

confidence: 99%

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

2015

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Background The spatial distribution of charge and magnetization within the proton is encoded in the elastic form factors.These have been precisely measured in elastic electron scattering, and the combination of proton and neutron form factors allows for the separation of the up-and down-quark contributions.Purpose In this work, we extract the proton and neutron form factors from world's data with an emphasis on precise new data covering the low-momentum region, which is sensitive to the large-scale structure of the nucleon. From these, we separate the up-and down-quark contributions to the proton form factors.Method We combine cross section and polarization measurements of elastic electron-proton scattering to separate the proton form factors and two-photon exchange (TPE) contributions. We combine the proton form factors with parameterization of the neutron form factor data and uncertainties to separate the up-and down-quark contributions to the proton's charge and magnetic form factors.Results The extracted TPE corrections are compared to previous phenomenological extractions, TPE calculations, and direct measurements from the comparison of electron and positron scattering. The flavor-separated form factors are extracted and compared to models of the nucleon structure.Conclusions With the inclusion of the precise new data, the extracted TPE contributions show a clear change of sign at low Q 2 , necessary to explain the high-Q 2 form factor discrepancy while being consistent with the known Q 2 → 0 limit. We find that the new Mainz data yield a significantly different result for the proton magnetic form factor and its flavorseparated contributions. We also observe that the RMS radius of both the up-and down-quark distributions are smaller than the RMS charge radius of the proton.

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Revealing Dressed Quarks via the Proton’s Charge Distribution

Cited by 64 publications

References 58 publications

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

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

Baryons as relativistic three-quark bound states

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

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