Electroproduction of nucleon resonances

Tiator, L.; Drechsel, D.; Kamalov, S.S.; Giannini, M.; Santopinto, E.; Vassallo, A.

doi:10.1140/epjad/s2004-03-009-9

Cited by 112 publications

(151 citation statements)

References 25 publications

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“…Yet as the magnetic term increases with Q 2 , the helicity difference runs through a zero at Q 2 ≈ 0.4 GeV 2 , and eventually the cross section σ 1/2 dominates. The latter finding is a prediction of perturbative QCD since only states with helicity 1/2 can be produced by absorption on an individual free quark, and it is also corroborated by the analysis of various experiments [100,102,103]. Figure 7 also shows an overall decrease of the cross sections with increasing momentum transfer Q 2 .…”

Section: Figsupporting

confidence: 67%

The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon

Drechsel

Tiator

2004

Annu. Rev. Nucl. Part. Sci.

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The Gerasimov-Drell-Hearn sum rule is one of several dispersive sum rules that connect the Compton scattering amplitudes to the inclusive photoproduction cross sections of the target under investigation. Being based on such universal principles as causality, unitarity, and gauge invariance, these sum rules provide a unique testing ground to study the internal degrees of freedom that hold the system together. The present article reviews these sum rules for the spin-dependent cross sections of the nucleon by presenting an overview of recent experiments and theoretical approaches. The generalization from real to virtual photons provides a microscope of variable resolution: At small virtuality of the photon, the data sample information about the long range phenomena, which are described by effective degrees of freedom (Goldstone bosons and collective resonances), whereas the primary degrees of freedom (quarks and gluons) become visible at the larger virtualities. Through a rich body of new data and several theoretical developments, a unified picture of virtual Compton scattering emerges, which ranges from coherent to incoherent processes, and from the generalized spin polarizabilities on the low-energy side to higher twist effects in deep inelastic lepton scattering.

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Section: Figsupporting

confidence: 67%

The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon

Drechsel

Tiator

2004

Annu. Rev. Nucl. Part. Sci.

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“…At low Q 2 , the agreement is not so good, since the meson cloud contributions are expected to be important and even dominant in some transitions [1,2,[18][19][20][21].…”

Section: Introductionmentioning

confidence: 99%

Semirelativistic approximation to the γN→N(1520) and γN
Ramalho
¹

2017
Phys. Rev. D
18
2
12
0
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The representation of the wave functions of the nucleon resonances within a relativistic framework is a complex task. In a nonrelativistic framework the orthogonality between states can be imposed naturally. In a relativistic generalization, however, the derivation of the orthogonality condition between states can be problematic, particularly when the states have different masses. In this work we study the N (1520) and N (1535) states using a relativistic framework. We considered wave functions derived in previous works, but impose the orthogonality between the nucleon and resonance states using the properties of the nucleon, ignoring the difference of masses between the states (semirelativistic approximation). The N (1520) and N (1535) wave functions are then defined without any adjustable parameters and are used to make predictions for the valence quark contributions to the transition form factors. The predictions compare well with the data particularly for high momentum transfer, where the dominance of the quark degrees of freedom is expected. I. INTRODUCTIONIn the last century we learned that the hadrons, including the nucleon (N ) and the nucleon excitations (N * ) are not pointlike particles and have their own internal structure. The structure of those states is the result of the internal constituents, quarks and gluons, and the interactions ruled by Quantum Chromodynamics (QCD). In the last decades experimental facilities such as Jefferson Lab (JLab), MAMI (Mainz) and MIT-Bates have accumulated information (data) about the electromagnetic structure of the nucleon resonances, parametrized in terms of structure form factors for masses up to 3 GeV [1,2].Several theoretical models have been proposed to interpret the nucleon resonance spectrum and the information associated with its internal structure [1][2][3]. Different models provide different parametrizations of the internal structure in terms of the effective degrees of freedom. Some of the more successful models are the constituent quark models (CQM) based on nonrelativistic kinematics like the Karl-Isgur model [3,4] and the Light Front quark models (LFQM) defined in the infinite momentum frame [5][6][7]. In those extreme cases, nonrelativistic models or LFQM, the kinematics is simplified. In general, however, the transition between the nonrelativistic and relativistic regimes is not a trivial task.In this work we discuss the γ * N → N * transition form factors for resonances N * with negative parity. The definition of the wave functions of the nucleon (mass M N ) and a nucleon excitation (mass M R ), in terms of the internal quark degrees of freedom, can be done first in the rest frame of the particle, and extended later for a moving frame using a Lorentz transformation. In a nonrelativistic framework the mass and energy of the state are not relevant for the definition of the states. Moreover, the orthogonality between the nucleon and the resonance N * is ensured, since the wave functions are independent of their masses. To understand the complexity of ...

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“…For the Ã N ! Á reaction, the meson cloud contributions are shown to be crucial [1,2,16,32,33]. Then, it is natural to investigate the role of the meson cloud also for the other octet to decuplet electromagnetic transitions.…”

Section: Introductionmentioning

confidence: 99%

Octet to decuplet electromagnetic transition in a relativistic quark model

Ramalho

Tsushima

2013

Phys. Rev. D

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We study the octet to decuplet baryon electromagnetic transitions using the covariant spectator quark model and predict the transition magnetic dipole form factors for those involving the strange baryons. Utilizing SU (3) symmetry, the valence quark contributions are supplemented by the pion cloud dressing based on the one estimated in the γ * N → ∆ reaction. Although the valence quark contributions are dominant in general, the pion cloud effects turn out to be very important to describe the experimental data. We also show that other mesons besides the pion in particular the kaon, may be relevant for some reactions such as γ * Σ + → Σ * + , based on our analysis for the radiative decay widths of the strange decuplet baryons.

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Electroproduction of nucleon resonances

Cited by 112 publications

References 25 publications

The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon

The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon

Semirelativistic approximation to the γN→N(1520) and γN
Ramalho
¹

2017
Phys. Rev. D
18
2
12
0
View full text Add to dashboard Cite

Octet to decuplet electromagnetic transition in a relativistic quark model

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Electroproduction of nucleon resonances

Cited by 112 publications

References 25 publications

The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon

The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon

Semirelativistic approximation to the γ*N→N(1520) and γ*NRamalho1 2017Phys. Rev. D182120View full textAdd to dashboardCite

Octet to decuplet electromagnetic transition in a relativistic quark model

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Semirelativistic approximation to the γN→N(1520) and γN
Ramalho
¹

2017
Phys. Rev. D
18
2
12
0
View full text Add to dashboard Cite