Electromagnetic nucleon form factors in instant and point form

Melde, T.; Berger, Katharina; Canton, L.; Plessas, W.; Wagenbrunn, R. F.

doi:10.1103/physrevd.76.074020

Cited by 44 publications

(38 citation statements)

References 82 publications

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“…Here ρ is the probability density associated with an intermediate spectator state of invariant 7 It appears that this class includes the model of (Glozman et al, 1998) if a point-form spectator model (PFSM) is employed for the electromagnetic current operator (Melde et al, 2007). Of the currents considered in this connection, only the PFSM can produce results in fair agreement with experiment.…”

Section: Pion Cloudsupporting

confidence: 64%

Nucleon and pion distribution functions in the valence region

2010

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We provide an experimental and theoretical perspective on the behavior of unpolarized distribution functions for the nucleon and pion on the valence-quark domain; namely, Bjorken-x > ∼ 0.4. This domain is key to much of hadron physics; e.g., a hadron is defined by its flavor content and that is a valence-quark property. Furthermore, its accurate parametrization is crucial to the provision of reliable input for large collider experiments. We focus on experimental extractions of distribution functions via electron and muon inelastic scattering, and from Drell-Yan interactions; and on theoretical treatments that emphasize an explanation of the distribution functions, providing an overview of major contemporary approaches and issues. Valence-quark physics is a compelling subject, which probes at the heart of our understanding of the Standard Model. There are numerous outstanding and unresolved challenges, which experiment and theory must confront. In connection with experiment, we explain that an upgraded Jefferson Lab facility is well-suited to provide new data on the nucleon, while a future electron ion collider could provide essential new data for the mesons. There is also great potential in using Drell-Yan interactions, at FNAL, J-PARC and GSI, to push into the large-x domain for both mesons and nucleons. We argue furthermore that explanation, in contrast to modeling and parametrization, requires a widespread acceptance of the need to adapt theory: to the lessons learnt already from the methods of nonperturbative quantum field theory; and a fuller exploitation of those methods. * Electronic address: holt@anl.gov † Electronic address: cdroberts@anl.gov

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Section: Pion Cloudsupporting

confidence: 64%

Nucleon and pion distribution functions in the valence region

2010

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“…An ambiguity in defining such a spectator current, however, enters through a normalization factor N that has to be introduced in order to recover the hadron charge from the electric form factor in the limit Q 2 → 0 [30]. Because both quantities,Q and N depend effectively on all quark momenta and not only on those of the active ones, the model current constructed in this way cannot be considered as a pure one-body current [31]. It has therefore been termed "point-form spectator model" (PFSM) to distinguish it from the usual impulse approximation.…”

Section: B Comparison With the Point-form Spectator Modelmentioning

confidence: 99%

“…These differences resemble the situation for the electromagnetic nucleon form factors. In the latter case the stronger falloff produced by the PFSM is a welcome feature that brings the theoretical predictions from constituent quark models close to experiments [31]. For the usual frontform spectator current in the q + = 0 frame agreement with experiment is achieved only by introducing electromagnetic form factors for the constituent quarks [33].…”

Section: B Comparison With the Point-form Spectator Modelmentioning

confidence: 99%

Electromagnetic meson form factor from a relativistic coupled-channels approach

et al. 2009

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Point-form relativistic quantum mechanics is used to derive an expression for the electromagnetic form factor of a pseudoscalar meson for spacelike momentum transfers. The elastic scattering of an electron by a confined quark-antiquark pair is treated as a relativistic two-channel problem for the qqe and qqeγ states. With the approximation that the total velocity of the qqe system is conserved at (electromagnetic) interaction vertices this simplifies to an eigenvalue problem for a Bakamjian-Thomas type mass operator. After elimination of the qqeγ channel the electromagnetic meson current and form factor can be directly read off from the one-photon-exchange optical potential. By choosing the invariant mass of the electron-meson system large enough, cluster separability violations become negligible. An equivalence with the usual front-form expression, resulting from a spectator current in the q + = 0 reference frame, is established. The generalization of this multichannel approach to electroweak form factors for an arbitrary bound few-body system is quite obvious. By an appropriate extension of the Hilbert space this approach is also able to accommodate exchange-current effects.

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“…The effective one-gluon exchange interaction [1,2,3,4] was replaced by one-boson exchange interactions between constituent quarks [6] or by instanton-induced interactions [5]. The improved baryon wave functions provided the basis for calculations of partial decay widths [7,8,9,10,11], of helicity amplitudes [12,13], and of form factors [14,15,16,17]. The spectrum of nucleon and ∆ resonances calculated on the lattice [18] shows striking agreement with expectations based on quark models: with increasing mass, there are alternating clusters of states with positive and with negative parity.…”

Section: Introductionmentioning

confidence: 99%

“…It has been suggested that the transition from constituent quarks to current quarks can be followed by precise measurements of the masses of excited hadron resonances [30]. The splitting in (squared) mass between two states forming a parity doublet (like ∆(1950) F 17 and ∆(2200)G 17 , 1.04 GeV 2 ) is slightly smaller than the mean mass square difference per unit of angular momentum (the string tension, 1.1 GeV 2 ). This effect can possibly be interpreted as weak attraction between parity partners in the 2 GeV mass region and as onset of a regime in which chiral symmetry is restored [31].…”

Section: Introductionmentioning

confidence: 99%

Nucleon resonances in the fourth-resonance region

et al. 2011

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Abstract.Nucleon and ∆ resonances in the fourth resonance region are studied in a multichannel partial-wave analysis which includes nearly all available data on pion-and photo-induced reactions off protons. In the high-mass range, above 1850 MeV, several alternative solutions yield a good description of the data. For these solutions, masses, widths, pole residues and photo-couplings are given. In particular, we find evidence for nucleon resonances with spin-paritiesFor one set of solutions, there are four resonances forming naturally a spin-quartet of resonances with orbital angular momentum L = 2 and spin S = 3/2 coupling to J = 1/2, · · · , 7/2. Just below 1.9 GeV we find a spin doublet of resonances with J P = 1/2 − and 3/2 − . Since a spin partner with J P = 5/2 − is missing at this mass, the two resonances form a spin doublet which must have a symmetric orbital-angular-momentum wave function with L = 1. For another set of solutions, the four positive-parity resonances are accompanied by mass-degenerate negativeparity partners -as suggested by the conjecture of chiral symmetry restoration. The possibility of a J P = 1/2 + , 3/2 + spin doublet at 1900 MeV belonging to a 20-plet is discussed.

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Electromagnetic nucleon form factors in instant and point form

Cited by 44 publications

References 82 publications

Nucleon and pion distribution functions in the valence region

Nucleon and pion distribution functions in the valence region

Electromagnetic meson form factor from a relativistic coupled-channels approach

Nucleon resonances in the fourth-resonance region

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