arXiv subjects

Marc Vanderhaeghen

Publications and source records attributed to Marc Vanderhaeghen.

At least 109 records · Page 6Linked to original sources

arXiv2009

Partonic calculation of $γZ$ Exchange Corrections to Parity-Violating Elastic Electron-Proton Scattering

We calculate $γZ$ exchange corrections to the parity-violating asymmetry of the elastic electron-proton scattering in a parton model using the formalism of generalized parton distributions(GPDs). We also examine the validity of the zero-momentum-transfer approximation adopted in the literatures and find that it overestimates the $γZ$ exchange effect significantly at the forward scattering angles.

nucl-th↗

arXiv2009

Quark transverse charge densities in the $Δ(1232)$ from lattice QCD

We extend the formalism relating electromagnetic form factors to transverse quark charge densities in the light-front frame to the case of a spin-3/2 baryon and calculate these transverse densities for the $Δ(1232)$ isobar using lattice QCD. The transverse charge densities for a transversely polarized spin-3/2 particle are characterized by monopole, dipole, quadrupole, and octupole patterns representing the structure beyond that of a pure point-like spin-3/2 particle. We present lattice QCD results for the $Δ$-isobar electromagnetic form factors for pion masses down to approximatively 350 MeV for three cases: quenched QCD, two-degenerate flavors of dynamical Wilson quarks, and three flavors of quarks using a mixed action that combines domain wall valence quarks and dynamical staggered sea quarks. We extract transverse quark charge densities from these lattice results and find that the $Δ$ is prolately deformed, as indicated by the fact that the quadrupole moment $G_{E2}(0$) is larger than the value -3 characterizing a point particle and the fact that the transverse charge density in a $Δ^+$ of maximal transverse spin projection is elongated along the axis of the spin.

hep-ph↗

arXiv2008

Electromagnetic properties of the Delta(1232) and decuplet baryons in the self-consistent SU(3) chiral quark-soliton model

We examine the electromagnetic properties of the Delta(1232) resonance within the self-consistent chiral quark-soliton model. In particular we present the Delta form factors of the vector-current GE0, GE2 and GM1 for a momentum-transfer range of $Q^{2} \leq 1GeV^{2}$. We apply the symmetry-conserving quantization of the soliton and take 1/N_c rotational corrections into account. Values for the magnetic moments of all decuplet baryons as well as for the N-Delta transition are given. Special interest is also given to the electric quadrupole moment of the Delta.

hep-ph↗

arXiv2008

Empirical transverse charge densities in the deuteron

Using the recent empirical information on the deuteron electromagnetic form factors we map out the transverse charge density in the deuteron as viewed from a light front moving towards the deuteron. The charge densities for a transversely polarized deuteron are characterized by monopole, dipole and quadrupole patterns.

hep-ph↗

arXiv2008

Taming Deeply Virtual Compton Scattering

We study recent Deeply Virtual Compton scattering (DVCS) data within a dual parameterization of the Generalized Parton Distributions (GPDs). This parameterization allows to quantify the maximum amount of information, that can be extracted from DVCS data, in a ``quintessence'' function. We present a ``zero step'' model for the latter solely based on the forward quark density, providing a parameter free prediction for the imaginary part of the DVCS amplitude. It is shown that the bulk effect of the $e p \to e p γ$ beam helicity asymmetry can be understood within such a model.

hep-ph↗

arXiv2007

Overview of nucleon structure studies

A brief overview of the recent activity in the measurement of the elastic electromagnetic proton and neutron form factors is presented. It is discussed how the quality of the data has been greatly improved by performing double polarization experiments, and the role of two-photon exchange processes will be highlighted. The spatial information on the quark charge distribibutions in the nucleon resulting from the form factors measurements will be discussed, as well as the steady rate of improvements made in the lattice QCD calculations. It is discussed how generalized parton distributions have emerged as a unifying theme in hadron physics linking the spatial densities extracted from form factors with the quark momentum distribution information residing in quark structure functions. The recent progress in the electromagnetic excitation of the $Δ(1232)$ resonance will also briefly be discussed.

nucl-th↗

arXiv2007

Large-$N_c$ relations for the electromagnetic N to Delta(1232) transition

We examine the large-$N_c$ relations which express the electromagnetic $N$-to-$Δ$ transition quantities in terms of the electromagnetic properties of the nucleon. These relations are based on the known large-$N_c$ relation between the $N\to Δ$ electric quadrupole moment and the neutron charge radius, and a newly derived large-$N_c$ relation between the electric quadrupole ($E2$) and Coulomb quadrupole ($C2$) transitions. Extending these relations to finite, but small, momentum transfer we find that the description of the electromagnetic $N\toΔ$ ratios ($R_{EM}$ and $R_{SM}$) in terms of the nucleon form factors predicts a structure which may be ascribed to the effect of the ``pion cloud''. These relations also provide useful constraints for the $N \to Δ$ generalized parton distributions.

hep-ph↗

arXiv2007

Chiral effective-field theory in the Delta(1232) region: II. radiative pion photoproduction

We present a theoretical study of the radiative pion photoproduction on the nucleon ($γN \to πN γ'$) in the $\De$-resonance region, with the aim to determine the magnetic dipole moment (MDM) of the $Δ^+(1232)$. The study is done within the framework of chiral effective-field theory where the expansion is performed (to next-to-leading order) in the $δ$ power-counting scheme which is an extension of chiral perturbation theory to the $Δ$-resonance energy region. We present the results for the absorptive part of the $Δ$ MDM, as well as perform a sensitivity study of the dependence of $γN \to πN γ'$ observables on the real part of the $Δ$ MDM. We find that an asymmetry for circular polarization of the photon beam may provide a model-independent way to measure the $Δ$ MDM.

hep-ph↗

arXiv2007

Two-Photon Physics in Hadronic Processes

Two-photon exchange contributions to elastic electron-scattering are reviewed. The apparent discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments is discussed, as well as the understanding of this puzzle in terms of two-photon exchange corrections. Calculations of such corrections both within partonic and hadronic frameworks are reviewed. In view of recent spin-dependent electron scattering data, the relation of the two-photon exchange process to the hyperfine splitting in hydrogen is critically examined. The imaginary part of the two-photon exchange amplitude as can be accessed from the beam normal spin asymmetry in elastic electron-nucleon scattering is reviewed. Further extensions and open issues in this field are outlined.

hep-ph↗

arXiv2006

The gamma-N -> Delta transition in chiral effective-field theory

We describe the pion electroproduction processes in the $Δ$(1232)-resonance region within the framework of chiral effective-field theory. By studying the observables of pion electroproduction in a next-to-leading order calculation we are able to make predictions and draw conclusions on the properties of the $N\to Δ$ electromagnetic form factors.

hep-ph↗

arXiv2006

New large-$N_c$ relations among the nucleon and nucleon-to-Delta GPDs

We establish relations which express the generalized parton distributions (GPDs) describing the $N\to Δ$ transition in terms of the nucleon GPDs. These relations are based on the known large-$N_c$ relation between the $N\to Δ$ electric quadrupole moment and the neutron charge radius, and a newly derived large-$N_c$ relation between the electric quadrupole ($E2$) and Coulomb quadrupole ($C2$) transitions. Namely, in the large-$N_c$ limit we find $C2=E2$. The resulting relations among the nucleon and $N\toΔ$ GPDs provide predictions for the $N\toΔ$ electromagnetic form factors which are found to be in very good agreement with experiment for moderate momentum transfers.

hep-ph↗

arXiv2006

New Bounds on Isotropic Lorentz Violation

Violations of Lorentz invariance that appear via operators of dimension four or less are completely parameterized in the Standard Model Extension (SME). In the pure photonic sector of the SME, there are nineteen dimensionless, Lorentz-violating parameters. Eighteen of these have experimental upper bounds ranging between 10^{-11} and 10^{-32}; the remaining parameter, k_tr, is isotropic and has a much weaker bound of order 10^{-4}. In this Brief Report, we point out that k_tr gives a significant contribution to the anomalous magnetic moment of the electron and find a new upper bound of order 10^{-8}. With reasonable assumptions, we further show that this bound may be improved to 10^{-14} by considering the renormalization of other Lorentz-violating parameters that are more tightly constrained. Using similar renormalization arguments, we also estimate bounds on Lorentz violating parameters in the pure gluonic sector of QCD.

hep-ph↗

arXiv2006

Electromagnetic excitation of the Delta(1232)-resonance

We review the description of the lowest-energy nucleon excitation -- the $Δ$(1232)-resonance. Much of the recent experimental effort has been focused on the precision measurements of the nucleon-to-$Δ$ transition by means of electromagnetic probes. We confront the results of these measuremnts with the state-of-the-art calculations based on chiral effective-field theories (EFT), lattice QCD, large-$N_c$ relations, perturbative QCD, and QCD-inspired models. We also discuss the link of the nucleon-to-$Δ$ form factors to generalized parton distributions (GPDs). Some of the theoretical approaches are reviewed in detail, in particular, recent dynamical and unitary-isobar models of pion electroproduction, which are extensively used in the interpretation of experiments. Also, the novel extension of chiral EFTs to the energy domain of the $Δ$-resonance is described in detail. The two-photon exchange effects in the electroexcitation of the $\De$-resonance are addressed here as well.

hep-ph↗

arXiv2006

Two-photon physics

It is reviewed how Compton scattering sum rules relate low-energy nucleon structure quantities to the nucleon excitation spectrum. In particular, the GDH sum rule and recently proposed extensions of it will be discussed. These extensions are sometimes more calculationally robust, which may be an advantage when estimating the chiral extrapolations of lattice QCD results, such as for anomalous magnetic moments. Subsequently, new developments in our description of the nucleon excitation spectrum will be discussed, in particular a recently developed chiral effective field theory framework for the $Δ(1232)$-resonance region. Within this framework, we discuss results on $N$ and $Δ$ masses, the $γN Δ$ transition and the $Δ$ magnetic dipole moment.

hep-ph↗

arXiv2006

Chiral Effective-Field Theory in the Delta(1232) Region: Pion Electroproduction on the Nucleon

We develop an extension of chiral perturbation theory to the $Δ$(1232)-resonance energy region and apply it to investigate the pion electroproduction off the nucleon ($e^- N \to e^- Nπ$). We present a complete calculation of this process, in the $Δ$-resonance region, up to next-to-leading order in the {\it $δ$-expansion}. At this order, the only free parameters are the three low-energy constants corresponding to the magnetic (M1), electric (E2), and Coulomb (C2) $γN \to Δ$ transition strength. After fitting these parameters to a few well-known data, our calculation provides a prediction for observables and multipole amplitudes of pion electroproduction. These results compare favorably with the phenomenological multipole solutions and recent experimental results from MIT-Bates and MAMI. Our prediction for the pion-mass dependence of the $γNΔ$ form factors offers an explanation for the discrepancy between the recent lattice-QCD results and the experimental value for the "C2/M1 ratio" at low $Q^2$.

hep-ph↗

arXiv2005

The nucleon and Delta-resonance masses in relativistic chiral effective-field theory

We study the chiral behavior of the nucleon and $Δ$-isobar masses within a manifestly covariant chiral effective-field theory, consistent with the analyticity principle. We compute the $πN$ and $πΔ$ one-loop contributions to the mass and field-renormalization constant, and find that they can be described in terms of universal relativistic loop functions, multiplied by appropriate spin, isospin and coupling constants. We show that these relativistic one-loop corrections, when properly renormalized, obey the chiral power-counting and vanish in the chiral limit. The results including only the $πN$-loop corrections compare favorably with the lattice QCD data for the pion-mass dependence of the nucleon and $Δ$ masses, while inclusion of the $πΔ$ loops tends to spoil this agreement.

hep-ph↗

arXiv2005

Electromagnetic Nucleon-to-Delta Transition in Chiral Effective-Field Theory

We perform a relativistic chiral effective-field theory calculation of the pion electroproduction off the nucleon ($e^- N \to e^- N π$) in the Delta(1232)-resonance region. After fixing the three low-energy constants, corresponding to the magnetic (M1), electric (E2), and Coulomb (C2) $γNΔ$ couplings, our calculation provides a prediction for the momentum-transfer and pion-mass dependence of the $γNΔ$ form factors. The prediction for the pion-mass dependence resolves the discrepancy between the recent lattice QCD results and the experimental value for the "C2/M1 ratio" at low $Q^2$.

hep-ph↗

arXiv2005

Two-photon-exchange effects in the electro-excitation of the $Δ$ resonance

We evaluate the two-photon exchange contribution to the $e N \to e Δ(1232) \to e πN$ process at large momentum transfer with the aim of a precision study of the ratios of electric quadrupole (E2) and Coulomb quadrupole (C2) to the magnetic dipole (M1) $γ^* N Δ$ transitions. We relate the two-photon exchange amplitude to the $N \to Δ$ generalized parton distributions and obtain a quantitative estimate of the two-photon effects. The two-photon exchange corrections to the C2/M1 ratio depend strongly on whether this quantity is obtained from an interference cross section or from the Rosenbluth-type cross sections, in similarity with the elastic, $e N \to e N$, process.

hep-ph↗