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We extend our investigation of resonance production in the Sakai-Sugimoto model to the case of negative parity baryon resonances. Using holographic techniques we extract the generalized Dirac and Pauli baryon form factors as well as the helicity amplitudes for these baryonic states. Identifying the first negative parity resonance with the experimen...
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We generalize our holographic derivation of spontaneous angular momentum
generation in 2 + 1 dimensions in several directions. We consider cases when a
parity violating perturbation responsible for the angular momentum generation
can be non-marginal (while in our previous paper we restricted to a marginal
perturbation), including all possible two-d...
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... Previous interesting works [21,22] focusing on a class of spin 1/2 resonances, accounted for resonance-proton transition form factors and resonance contributions to the unpolarized proton structure functions in the WSS model. Our analysis extends to all possible WSS spin 1/2 resonances, includes the spin 3/2 ones, 3 includes the neutron as possible target state, and it is meant to explore the polarized structure functions of both nucleons. ...
... Moreover, it is useful to recall the following identity [22] Tr( ...
... Commonly, experimental results on nucleon-resonance transitions 22 ...
We study polarized inelastic electron-nucleon scattering at low momentum transfer in the Witten-Sakai-Sugimoto model of holographic QCD, focusing on resonance production contributions to the nucleon spin structure functions. Our analysis includes both spin 3/2 and spin 1/2 low-lying nucleon resonances with positive and negative parity. We determine, in turn, the helicity amplitudes for nucleon-resonance transitions and the resonance contributions to the neutron and proton generalized spin polarizabilities. Extrapolating the model parameters to realistic QCD data, our analysis, triggered by recent experimental results from Jefferson Lab, agrees with the observation that the ∆(1232) resonance gives the dominant contribution to the forward spin polarizabilities at low momentum transfer. The contribution is negative and tends to zero as the momentum transfer increases. As expected, the contribution of the ∆(1232) to the longitudinal-transverse polarizabilities is instead negligible. The latter, for both nucleons, turn out to be negative functions with zero asymptote. The holographic results, at least for the proton where enough data are available, are in qualitative agreement with the resonance contributions to the spin polarizabilities extracted from experimental data on the helicity amplitudes.
... The transition form factors have also been calculated using light cone sum rules, based on the distribution amplitudes determined by lattice QCD [22,23]. There are also calculations based on coupled-channel models [24][25][26], constituent quark models [27], light-front quark models [28] and AdS/QCD [29,30]. ...
... There are also calculations of transition form factors based on AdS/QCD [28,29]. Reference [28] shows that the data can be described assuming significant contribution of higher order Fock states, namely from qq and ðqqÞðqqÞ contributions. ...
We develop a covariant model for the γ*N→N(1535) transition in the timelike kinematical region, the region where the square momentum transfer q2 is positive. Our starting point is the covariant spectator quark model constrained by data in the spacelike kinematical region (Q2=−q2≥0). The model is used to estimate the contributions of valence quarks to the transition form factors, and one obtains a fair description of the Dirac form factor at intermediate and large Q2. For the Pauli form factor there is evidence that beyond the quark-core contributions there are also significant contributions of meson cloud effects. Combining the quark-core model with an effective description of the meson cloud effects, we derive a parametrization of the spacelike data that can be extended covariantly to the timelike region. This extension enabled us to estimate the Dalitz decay widths of the N(1535) resonance, among other observables. Our calculations can help in the interpretation of the present experiments at HADES (pp collisions and others).
... In the past two decades, there was significant experimental progress in the study of the electromagnetic structure of the nucleon (N) and the nucleon excited states (N Ã ) [1][2][3][4]. From the theoretical point of view, there was some progress in the description of the electromagnetic excitations N Ã using QCD sum rules [5][6][7], approaches based on the Dyson-Schwinger equations [8,9], AdS=QCD [10][11][12][13][14], and different classes of quark models [15][16][17][18][19]. At low Q 2 , the dynamical coupled-channel models which take into account the meson cloud dressing of the baryons are also successful [4,[20][21][22]. ...
The data associated with the electromagnetic excitations of the nucleon (γ*N→N*) are usually parametrized by helicity amplitudes at the resonance N* rest frame. The properties of the γ*N→N* transition current at low Q2 can be, however, better understood when expressed in terms of structure form factors, particularly near the pseudothreshold, when the magnitude of the photon three-momentum vanishes (|q|=0). At the pseudothreshold, the invariant four-momentum square became q2=(MR−MN)2, well in the timelike region Q2=−q2<0 [MN and MR are the mass of the nucleon and of the resonance, respectively]. In the helicity amplitude representation, the amplitudes have well-defined dependences on |q|, near the pseudothreshold, and there are correlations between different amplitudes. Those constraints are often ignored in the empirical parametrizations of the helicity amplitudes. In the present work, we show that the structure of the transition current near the pseudothreshold has an impact on the parametrizations of the data. We present a method which modifies analytic parametrizations of the data at low Q2, in order to take into account the constraints of the transition amplitudes near the pseudothreshold. The model dependence of the parametrizations on the low-Q2 data is studied in detail.
... 15 A computation of nucleon transition form factors has been carried out in the framework of the Sakai and Sugimoto model in Refs. [293] and [294]. the end of Chapter 5, at low energies the strongly correlated bound state of n quarks behaves as a system of an active quark vs the n − 1 spectators. ...
In this Report we explore the remarkable connections between light-front
dynamics, its holographic mapping to gravity in a higher-dimensional anti-de
Sitter (AdS) space, and conformal quantum mechanics. This approach provides new
insights into the origin of a fundamental mass scale and the physics underlying
confinement dynamics in QCD in the limit of massless quarks. The result is a
relativistic light-front wave equation for arbitrary spin with an effective
confinement potential derived from a conformal action and its embedding in AdS
space. This equation allows for the computation of essential features of hadron
spectra in terms of a single scale. The light-front holographic methods
described here gives a precise interpretation of holographic variables and
quantities in AdS space in terms of light-front variables and quantum numbers.
This leads to a relation between the AdS wave functions and the boost-invariant
light-front wavefunctions describing the internal structure of hadronic
bound-states in physical space-time. The pion is massless and the excitation
spectra of relativistic light-quark meson and baryon bound states lie on linear
Regge trajectories with identical slopes in the radial and orbital quantum
numbers. In the light-front holographic approach described here currents are
expressed as an infinite sum of poles, and form factors as a product of poles.
At large $q^2$ the form factor incorporates the correct power-law fall-off for
hard scattering independent of the specific dynamics and is dictated by the
twist. At low $q^2$ the form factor leads to vector dominance. The approach is
also extended to include small quark masses. We briefly review in this Report
other holographic approaches to QCD, in particular top-down and bottom-up
models based on chiral symmetry breaking. We also include a discussion of open
problems and future applications.
... Light-front holographic QCD methods have also been used to obtain general parton distributions (GPDs) in Ref. [273], and a study of the EM nucleon to ∆ transition form factors and nucleon to the S 11 (1535) negative parity nucleon state has been carried out in the framework of the Sakai and Sugimoto model in Refs. [274] and [275] respectively. It is certainly worth to extend the simple computations described here and perform a systematic study of the different transition form factors measured at JLab. ...
Studies of the structure of excited baryons are key to the N* program at
Jefferson Lab. Within the first year of data taking with the Hall B CLAS12
detector following the 12 GeV upgrade, a dedicated experiment will aim to
extract the N* electrocouplings at high photon virtualities Q2. This experiment
will allow exploration of the structure of N* resonances at the highest photon
virtualities ever yet achieved, with a kinematic reach up to Q2 = 12 GeV2. This
high-Q2 reach will make it possible to probe the excited nucleon structures at
distance scales ranging from where effective degrees of freedom, such as
constituent quarks, are dominant through the transition to where nearly
massless bare-quark degrees of freedom are relevant. In this document, we
present a detailed description of the physics that can be addressed through N*
structure studies in exclusive meson electroproduction. The discussion includes
recent advances in reaction theory for extracting N* electrocouplings from
meson electroproduction off protons, along with QCD-based approaches to the
theoretical interpretation of these fundamental quantities. This program will
afford access to the dynamics of the non-perturbative strong interaction
responsible for resonance formation, and will be crucial in understanding the
nature of confinement and dynamical chiral symmetry breaking in baryons, and
how excited nucleons emerge from QCD.
The structure of the nucleon and the first radial excitation of the nucleon, the Roper, $N(1440)$, is studied within the formalism of Light-Front holography. The nucleon elastic form factors and $\gamma^\ast N \to N(1440)$ transition form factors are calculated under the assumption of the dominance of the valence quark degrees of freedom. Contrary to the previous studies, the bare parameters of the model associated with the valence quark are fixed by the empirical data for large momentum transfer ($Q^2$) assuming that the corrections to the three-quark picture (meson cloud contributions) are suppressed. The $\gamma^\ast N \to N(1440)$ transition form factors are then calculated without any adjustable parameters. Our estimates are compared with results from models based on valence quarks and others. The model compares well with the $\gamma^\ast N \to N(1440)$ transition form factor data, suggesting that meson cloud effects are not large, except in the region $Q^2< 1.5$ GeV$^2$. In particular, the meson cloud contributions for the Pauli form factor are small.
We review some recent results on hadrons using holographic models for strong intereractions inspired in the AdS/CFT correspondence, also known as AdS/QCD models.
