We study the exclusive photoproduction of a lepton pair off the proton with the aim of studying the proton quark structure via the Generalized Parton Distributions (GPD) formalism. After deriving the amplitudes of the processes contributing to the gamma P --> P'e(+)e(-), the timelike Compton scattering and the Bethe-Heitler process, we calculate all unpolarized, single- and double- spin beam-target observables in the valence region in terms of GPDs.

We study the wide angle Compton scattering process on a proton within the soft-collinear factorization (SCET) framework. The main purpose of this work is to estimate the effect due to certain power suppressed corrections. We consider all possible kinematical power corrections and also include the subleading amplitudes describing the scattering with nucleon helicity flip. Under certain assumptions we present a leading-order factorization formula for these amplitudes which includes the hard- and soft-spectator contributions. We apply the formalism and perform a phenomenological analysis of the cross section and asymmetries in the wide angle Compton scattering on a proton. We assume that in the relevant kinematical region where GeV the dominant contribution is provided by the soft-spectator mechanism. The hard coefficient functions of the corresponding SCET operators are taken in the leading-order approximation. The analysis of existing cross section data shows that the contribution of the helicity-flip amplitudes to this observable is quite small and comparable with other expected theoretical uncertainties. We also show predictions for double polarization observables for which experimental information exists.

Ledwig, T.
Martin-Camalich, J.
Pascalutsa, V.
Vanderhaeghen, M.

We address electromagnetic properties of quasi-stable baryons in the context of chiral extrapolations of lattice QCD results. For particles near their decay threshold we show that an application of a small external magnetic field changes the particle's energy in a non-analytic way. The conventional electromagnetic moments are only well-defined when the background field B satisfies |eB|/2M(*) << |M-* - M - m| whereM(*) is themass of the resonance andM, m the masses of the decay products. A direct application of this situation is the chiral extrapolation of the Delta(1232)-isobar electromagnetic properties. We discuss such an extrapolation of the Delta(1232)-isobar magnetic dipole, electric quadrupole and magnetic octupole moments by a covariant chiral effective field theory.

We discuss the recent interpretation of quark-distribution functions in the plane transverse to the light-cone direction. Such a mapping is model independent and allows one to build up multidimensional pictures of the hadron and to develop a semi-classical interpretation of the quark dynamics. We comment briefly the results obtained from the form factors of the nucleon. We show that a generalization to a target with arbitrary spin leads to a set of preferred values for the electromagnetic coupling characterizing structureless particles. Finally, we present the Wigner distribution for an unpolarized quark in an unpolarized proton and we propose an interpretation of the observed distortion as due to the orbital angular momentum of the quark.

Tiator, L.
Drechsel, D.
Kamalov, S. S.
Vanderhaeghen, M.

Recent progress on the extraction of electromagnetic properties of nucleon resonance excitation through pion photo- and electroproduction is reviewed. Cross section data measured at MAMI, ELSA, and CEBAF are analyzed and compared to the analysis of other groups. On this basis, we derive longitudinal and transverse transition form factors for most of the four-star nucleon resonances. Furthermore, we discuss how the transition form factors can be used to obtain empirical transverse charge densities. Contour plots of the thus derived densities are shown for the Delta, Roper, S (11), and D (13) nucleon resonances.

The beam normal spin asymmetry in elastic electron-nucleon scattering is discussed. This beam normal spin asymmetry depends on the imaginary part of two-photon exchange processes between electron and nucleon, and measures the non-forward structure functions of the nucleon. After briefly reviewing the theoretical formalism, we discuss calculations in the threshold region, in the resonance region, as well as in the diffractive region, corresponding with high energy and forward angles.

We review recent developments in the theoretical investigation of the nucleon polarizabilities. We first report on the static polarizabilities as measured in real Compton scattering, comparing and interpreting the results from various theoretical approaches. In a second step, we extend the discussion to the generalized polarizabilities which can be accessed in virtual Compton scattering, showing how the information encoded in these quantities can provide a spatial interpretation of the induced polarization densities in the nucleon.

Guttmann, J.
Kivel, N.
Meziane, M.
Vanderhaeghen, M.

Using the available cross-section and polarization data for elastic electron-proton scattering, we provide an extraction of the two-photon exchange amplitudes at a common value of four-momentum transfer, around Q(2) = 2.5 GeV(2). This analysis also predicts the e(+)p/e(-)p elastic-scattering cross-section ratio, which will be measured by forthcoming experiments.

Janssens, P.
Van Hoorebeke, L.
Fonvieille, H.
D'Hose, N.
Bertin, P. Y.
Bensafa, I.
Degrande, N.
Distler, M.
Di Salvo, R.
Doria, L.
Friedrich, J. M.
Friedrich, J.
Hyde-Wright, Ch.
Jaminion, S.
Kerhoas, S.
Laveissiere, G.
Lhuillier, D.
Marchand, D.
Merkel, H.
Roche, J.
Tamas, G.
Vanderhaeghen, M.
Van de Vyver, R.
Van de Wiele, J.
Walcher, Th.

This paper describes the Monte Carlo simulation developed specifically for the Virtual Compton Scattering (VCS) experiments below pion threshold that have been performed at MAMI and JLab. This simulation generates events according to the (Bethe-Heitler + Born) cross-section behaviour and takes into account all relevant resolution-deteriorating effects. It determines the "effective" solid angle for the various experimental settings which are used for the precise determination of the photon electroproduction absolute cross-section. (c) 2006 Elsevier B.V. All rights reserved.

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 Delta(1232)-resonance region. Within this framework, we discuss results on N and Delta masses, the gamma N Delta transition and the Delta magnetic dipole moment.

We discuss the two-photon exchange contribution to observables which involve lepton helicity flip in elastic lepton-nucleon scattering. This contribution is accessed through the single spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal single spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions

We study the annihilation process p (p) over bar -> pi(0)e(+)e(-) within a Regge framework, as a means to provide constraints on timelike nucleon form factors. We present results for the e(+)e(-) angular distributions and the differential cross sections in kinematics which will be accessible by PANDA@FAIR. To check the consistency of the model, we first test the approach on the process of real photon production, (p) over barp -> pi(0)gamma, where data in the energy range of 2.911 GeV <= root s <= 3.686 GeV exist. We find that a Regge pole model is able to well reproduce the available data. The analysis is then extended to a timelike virtual photon in the final state. (C) 2013 Elsevier B.V. All rights reserved.

Pasquini, B.
Gorchtein, M.
Drechsel, D.
Metz, A.
Vanderhaeghen, M.

We present in detail a dispersion relation formalism for virtual Compton scattering (VCS) off the proton from threshold into the Delta(1232)-resonance region. Such a formalism can be used as a tool to extract the generalized polarizabilities of the proton from both unpolarized and polarized VCS observables over a larger energy range. We present calculations for existing and forthcoming VCS experiments and demonstrate that the VCS observables in the energy region between pion production threshold and the Delta(1232)-resonance show an enhanced sensitivity to the generalized polarizabilities

Teodorescu, L.
Gueye, P.
Angelescu, T.
Baker, O.K.
Ent, R.
Guidal, M.
Laget, J.M.
Mitchell, J.
Niculescu, G.
Vanderhaeghen, M.
Williams, R.

The result of a feasibility study to measure the Lambda polarization in associated K +-Lambda electro-production is presented. This measurement was performed in the experimental Hall C at Jefferson Lab. The scattered electron was detected in the HMS spectrometer, and the electro-produced kaon and the proton from the Lambdararrppi - decay were both detected in the SOS spectrometer. This quantity is very sensitive to the elementary p(e,e'K)Lambda process and gives information on resonance production, and Regge exchange, among others. The result presented was measured at Q 2=1.50 (GeV/c) 2 and costhetas KgammaCM=14deg. The limits of the Lambda polarization, with respect to the p gammatimesp K axis, were found to be -0.21 and +0.89 with a confidence level of 68%. The result is compared to theoretical predictions based on an effective hadronic field Lagrangian model and a Regge framework model

A model for high energy pion and kaon photoproduction (E gammagsim4 GeV) is presented which interpolates between the low and higher momentum transfer regions. At low momentum transfer (which is described in terms of Regge trajectory exchanges), the model is able to give an economical description of unpolarized and polarized data for both the charged and neutral pion photoproduction reactions as well as for K +Lambda and K +Sigma photoproduction data, available at high energy. At higher momentum transfer, an extrapolation based on saturating Regge trajectories, and which satisfies scaling laws at asymptotically large transfers, is found able to describe the few existing gammaprarrnpi + data. The model can be used as a guidance to interpret forthcoming data at larger momentum transfers