Nucleon form factors in terms of resonances (original) (raw)
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Survey of Nucleon Electromagnetic Form Factors
Few-Body Systems, 2009
A dressed-quark core contribution to nucleon electromagnetic form factors is calculated. It is defined by the solution of a Poincaré covariant Faddeev equation in which dressed-quarks provide the elementary degree of freedom and correlations between them are expressed via diquarks. The nucleonphoton vertex involves a single parameter; i.e., a diquark charge radius. It is argued to be commensurate with the pion's charge radius. A comprehensive analysis and explanation of the form factors is built upon this foundation. A particular feature of the study is a separation of form factor contributions into those from different diagram types and correlation sectors, and subsequently a flavour separation for each of these. Amongst the extensive body of results that one could highlight are: r n,u 1 > r n,d 1 , owing to the presence of axial-vector quark-quark correlations; and for both the neutron and proton the ratio of Sachs electric and magnetic form factors possesses a zero.
Electromagnetic Transition Form Factors of Nucleon Resonances
AIP Conference Proceedings, 2008
Recent measurements of nucleon resonance transition form factors with CLAS at Jefferson Lab are discussed. The new data resolve a long-standing puzzle of the nature of the Roper resonance, and confirm the assertion of the symmetric constituent quark model of the Roper as the first radial excitation of the nucleon. The data on high Q 2 nπ + production confirm the slow fall off of the S11(1535) transition form factor with Q 2 , and better constrain the branching ratios βNπ = 0.50 and βNη = 0.45. For the first time, the longitudinal transition amplitude to the S11(1535) was extracted from the nπ + data. Also, new results on the transition amplitudes for the D13(1520) resonance are presented showing a rapid transition from helicity 3/2 dominance seen at the real photon point to helicty 1/2 dominance at higher Q 2 .
Re-analysis of the nucleon space-and time-like electromagnetic form factors in a two-component model
2004
Recent experimental data on space-like and time-like form factors of the nucleon are analyzed in terms of a two-component model with a quark-like intrinsic q 3 structure and qq pairs. PACS numbers: 13.40.Gp, 14.20.Dh Recent experiments on the electromagnetic form factors of the proton [1, 2] and the neutron [3] using the recoil polarization technique have shown a dramatically different picture of the nucleon as compared with a previously accepted picture [4, 5]. Leaving aside the question of whether or not the experimental results are in disagreement with each other [6], which is the subject of many theoretical investigations related to the role of two-photon contributions [7], the new experiments for the proton [1, 2] are in excellent agreement with a model of the nucleon put forward in 1973 [8] wherein the external photon couples both to an intrinsic structure and to a meson cloud through the intermediate vector mesons (ρ, ω, ϕ). On the contrary, the new experiments for the neutron [3] are in agreement with the 1973 model up to Q 2 ∼ 1 (GeV/c) 2 , but not so for higher values of Q 2 [9]. It is of great current interest to understand whether a modification of the 1973 parametrization can bring the calculation in agreement with both proton and neutron data.
A relativistic study of the nucleon form factors
European Physical Journal A
We perform a calculation of the relativistic corrections to the electromagnetic elastic form factors of the nucleon obtained with various Constituent Quark Models. With respect to the non relativistic calculations a substantial improvement is obtained up to Q 2 ≃ 2(GeV /c) 2 .
Electromagnetic form factors of the nucleon
The European Physical Journal A, 2005
We reanalyze the world data on the electromagnetic form factors of the nucleon. The calculations are performed in the framework of an algebraic model of the nucleon combined with vector meson dominance.
Phys Rev C, 2004
A non-relativistic potential-model version of the factorization assumption, used in perturbative QCD calculations of hadronic form factors, is used, along with the Born approximation valid at high energies, to derive a remarkably simple relationship between the impulse approximation contribution to the deuteron form factor at high momentum transfer and the high energy neutron-proton scattering amplitude. The relation states that the form factor at a given value of Q 2 is proportional to the scattering amplitude at a specific energy and scattering angle. This suggests that an accurate computation of the form factors at large Q 2 requires a simultaneous description of the phase-shifts at a related energy, a statement that seems reasonable regardless of any derivation. Our form factor-scattering amplitude relation is shown to be accurate for some examples. However, if the potential consists of a strong short distance repulsive term and a strong longer ranged attractive term, as typically occurs in many realistic potentials, the relation is found to be accurate only for ridiculously large values of Q. More general arguments, using only the Schroedinger equation, suggest a strong, but complicated, relationship between the form factor and scattering amplitude. Furthermore, the use of recently obtained soft potentials, along with an appropriate current operator, may allow calculations of form factors that are consistent with the necessary phase shifts.
Proceedings of 6th International Workshop on Chiral Dynamics — PoS(CD09), 2010
Measurements of the electric and magnetic form factors have been carried out by means of scattering of polarized electrons from vector-polarized hydrogen and deuterium. The experiment used the longitudinally polarized stored electron beam of the MIT-Bates South Hall Ring along with an isotopically pure, highly vector-polarized internal atomic hydrogen and deuterium target provided by an atomic beam source, and the symmetric Bates Large Acceptance Spectrometer Toroid (BLAST) with enhanced neutron detection capability. The measurements are at low momentum transfers (Q 2 < 1 (Gev/c) 2) and results are presented for the proton form factor ratio G E p /G M p and for the electric form factor of the neutron, G E n. To quantify the effect of multiple photon exchange and to address the puzzle of the proton form factor ratio at Q 2 > 1 (GeV/c) 2 , an experiment under development using BLAST and an unpolarized internal hydrogen target will measure electron-proton and positron-proton elastic scattering.
Electromagnetic form factors of bound nucleons revisited
The European Physical Journal A - Hadrons and Nuclei, 2003
We calculate electromagnetic form factors of the proton bound in specified orbits for several closed shell nuclei. The quark structure of the nucleon and the shell structure of the finite nuclei are given by the QMC model. We find that orbital electromagnetic form factors of the bound nucleon deviate significantly from those of the free nucleon.