Search

Radiative corrections to elastic scattering represent an important part of the interpretation of electron-induced nuclear reactions at small energy transfers, where they make for a dominant part of background. Here we present and validate a new event generator for mimicking QED radiative processes in electron-carbon scattering that exactly calculates the coherent sum of the Bethe-Heitler amplitudes for the leading diagrams and can be reliably employed for a more robust determination of inelastic cross-sections.

The Virtual Compton scattering (VCS) process at low energies explores the electromagnetic structure of the proton in terms of generalized polarizabilities (GPs). In the one-photon exchange approximation, VCS can be accessed with exclusive photon production reactions with electron or positron beams. The extraction of the GPs from VCS with electron beam has seen substantial progress over the past two decades. Nonetheless, a consistent picture of the GPs from low to higher scales demands further investigations. Complementary measurements with positron beams offer an unique possibility, and we present an impact study of such experimental program., Comment: 9 pages, 7 figures, invited article for the EPJA Topical Issue about "Positron beams and physics at Jefferson Lab"

Positron beams, both polarized and unpolarized, are identified as essential ingredients for the experimental programs at the next generation of lepton accelerators. In the context of the hadronic physics program at Jefferson Lab (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of nucleons and nuclei, in both the elastic and deep-inelastic regimes. For instance, elastic scattering of polarized and unpolarized electrons and positrons from the nucleon enables a model independent determination of its electromagnetic form factors. Also, the deeply-virtual scattering of polarized and unpolarized electrons and positrons allows unambiguous separation of the different contributions to the cross section of the lepto-production of photons and of lepton-pairs, enabling an accurate determination of the nucleons and nuclei generalized parton distributions, and providing an access to the gravitational form factors. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model of particle physics through the search of a dark photon, the precise measurement of electroweak couplings, and the investigation of charged lepton flavor violation. This document discusses the perspectives of an experimental program with high duty-cycle positron beams at JLab., Comment: 18 pages, 7 figures This version superseeds the previous version which scientific content was decomposed into several more elaborated articles. All of these articles will be collected in the EPJ A Topical Issue about "Positron beam and physics at Jefferson Lab (e+@Jlab)"

We review the present status of the Standard Model calculation of the anomalous magnetic moment of the muon. This is performed in a perturbative expansion in the fine-structure constant $\alpha$ and is broken down into pure QED, electroweak, and hadronic contributions. The pure QED contribution is by far the largest and has been evaluated up to and including $\mathcal{O}(\alpha^5)$ with negligible numerical uncertainty. The electroweak contribution is suppressed by $(m_\mu/M_W)^2$ and only shows up at the level of the seventh significant digit. It has been evaluated up to two loops and is known to better than one percent. Hadronic contributions are the most difficult to calculate and are responsible for almost all of the theoretical uncertainty. The leading hadronic contribution appears at $\mathcal{O}(\alpha^2)$ and is due to hadronic vacuum polarization, whereas at $\mathcal{O}(\alpha^3)$ the hadronic light-by-light scattering contribution appears. Given the low characteristic scale of this observable, these contributions have to be calculated with nonperturbative methods, in particular, dispersion relations and the lattice approach to QCD. The largest part of this review is dedicated to a detailed account of recent efforts to improve the calculation of these two contributions with either a data-driven, dispersive approach, or a first-principle, lattice-QCD approach. The final result reads $a_\mu^\text{SM}=116\,591\,810(43)\times 10^{-11}$ and is smaller than the Brookhaven measurement by 3.7$\sigma$. The experimental uncertainty will soon be reduced by up to a factor four by the new experiment currently running at Fermilab, and also by the future J-PARC experiment. This and the prospects to further reduce the theoretical uncertainty in the near future-which are also discussed here-make this quantity one of the most promising places to look for evidence of new physics., Comment: 196 pages, 103 figures, version published in Phys. Rept., bib files for the citation references are available from: https://muon-gm2-theory.illinois.edu

The first CLAS12 experiments will provide high-precision data on inclusive electron scattering observables at a photon virtuality $Q^2$ ranging from 0.05 GeV$^2$ to 12 GeV$^2$ and center-of-mass energies $W$ up to 4 GeV. In view of this endeavour, we present the modeling of the resonant contributions to the inclusive electron scattering observables. As input, we use the existing CLAS electrocoupling results obtained from exclusive meson electroproduction data off protons, and evaluate for the first time the resonant contributions based on the experimental results on the nucleon resonance electroexcitation. The uncertainties are given by the data and duly propagated through a Monte Carlo approach. In this way, we obtain estimates for the resonant contributions, important for insight into the nucleon parton distributions in the resonance region and for the studies of quark-hadron duality.

We review various applications of dispersion relations (DRs) to the electromagnetic structure of hadrons. We discuss the way DRs allow one to extract information on hadron structure constants by connecting information from complementary scattering processes. We consider the real and virtual Compton scattering processes off the proton, and summarize recent advances in the DR analysis of experimental data to extract the proton polarizabilities, in comparison with alternative studies based on chiral effective field theories. We discuss a multipole analysis of real Compton scattering data, along with a DR fit of the energy-dependent dynamical polarizabilities. Furthermore, we review new sum rules for the double-virtual Compton scattering process off the proton, which allow for model independent relations between polarizabilities in real and virtual Compton scattering, and moments of nucleon structure functions. The information on the double-virtual Compton scattering is used to predict and constrain the polarizability corrections to muonic hydrogen spectroscopy., Comment: preprint version of a review to appear in Ann. Rev. Nucl. Part. Sci. 68 (2018)

We study the production of a tensor glueball in the reaction $\gamma\gamma\rightarrow G_2\pi^0$. We compute the cross section at higher momentum transfer using the collinear factorisation approach. We find that for a value of the tensor gluon coupling of $f^T_g\sim 100$~MeV, the cross section can be measured in the near future by the Belle~II experiment., Comment: 7 pages, 5 figures

Transverse momentum distributions and generalized parton distributions provide a comprehensive framework for the three-dimensional imaging of the nucleon and the nucleus experimentally using deeply virtual semi-exclusive and exclusive processes. The advent of combined high luminosity facilities and large acceptance detector capabilities enables experimental investigation of the partonic structure of hadrons with time-like virtual probes, in complement to the rich on-going space-like virtual probe program. The merits and benefits of the dilepton production channel for nuclear structure studies are discussed within the context of the International Workshop on Nucleon and Nuclear Structure through Dilepton Production taking place at the European Center for Theoretical Studies in Nuclear Physics and Related Areas (ECT$^{\star}$) of Trento. Particularly, the double deeply virtual Compton scattering, the time-like Compton scattering, the deeply virtual meson production, and the Drell-Yan processes are reviewed and a strategy for high impact experimental measurements is proposed., Comment: 21 pages, 23 figures, Conference proceedings

We evaluate the two-photon exchange correction to the elastic electron-proton scattering cross section within a dispersive framework. Besides the elastic contribution, we account for all $\pi N$ intermediate state contributions using the phenomenological MAID fit as an input. We develop a novel method for the analytical continuation of the two-photon exchange amplitudes into the unphysical region and generalize our previous work to the momentum transfer region $ 0.064~\mathrm{GeV}^2 \lesssim Q^2 \lesssim 1~\mathrm{GeV}^2$. We compare our results with recent OLYMPUS, CLAS and VEPP-3 data as well as with empirical fits and estimates in the forward angular region., Comment: 43 pages, 21 figures, version published in Phys. Rev. D 96, 096001 (2017)

We present a discussion of radiative decays $\chi_{cJ}\rightarrow\gamma \rho (\omega, \phi)$. The decay amplitudes are computed within the QCD factorisation framework. NRQCD has been used in the heavy meson sector as well as a collinear expansion in order to describe the overlap with light mesons in the final state. The colour-singlet contributions to all helicity amplitudes have been computed using the light-cone distribution amplitudes of twist-2 and twist-3. All obtained expressions are well defined at least in the leading-order approximation. The colour-octet contributions have also been studied in the Coulomb limit in order to exhibit their scaling behaviour. In order to understand the relevance of the different contributions we perform numerical estimates using the colour-singlet contributions. For the $\chi_{c1}\rightarrow \gamma V_\perp$ decays, to vector mesons with transverse polarisation, we find that the colour-singlet contribution potentially allows for a reliable description. On the other hand, for the $\chi_{c1}\rightarrow \gamma V_\Vert$ decays, to vector mesons with longitudinal polarisation, our finding indicate that the colour-octet mechanism may be important for a good description. We expect that more accurate measurements of the decay $\chi_{c2}\rightarrow\gamma V_{\perp,\Vert} $ can help to better understand the mechanism of radiative decays., Comment: 25 pages, 8 figures, new version: text is improved and typos corrected

We evaluate the pion-nucleon intermediate-state contribution to the two-photon exchange (TPE) correction in the elastic electron-nucleon scattering within a dispersive framework. We calculate the contribution from all $\pi N$ partial waves using the MAID parametrization. We provide the corresponding TPE correction to the unpolarized $e p$ scattering cross section in the region of low momentum transfer $ Q^2 \lesssim 0.064~\mathrm{GeV}^2$, where no analytical continuation into the unphysical region of the TPE scattering amplitudes is required. We compare our result in the forward angular region with an alternative TPE calculation, in terms of structure functions, and find a good agreement, indicating a small contribution at low $Q^2$ due to discontinuities beyond $ \pi N$. We also compare our results with empirical fits., Comment: 34 pages, 14 figures, version published in Phys. Rev. D 95, 096001 (2017)

We evaluate the two-photon exchange (TPE) correction to the muon-proton elastic scattering at small momentum transfer. Besides the elastic (nucleon) intermediate state contribution, which is calculated exactly, we account for the inelastic intermediate states by expressing the TPE process approximately through the forward doubly virtual Compton scattering. The input in our evaluation is given by the unpolarized proton structure functions and by one subtraction function. For the latter, we provide an explicit evaluation based on a Regge fit of high-energy proton structure function data. It is found that, for the kinematics of the forthcoming muon-proton elastic scattering data of the MUSE experiment, the elastic TPE contribution dominates, and the size of the inelastic TPE contributions is within the anticipated error of the forthcoming data., Comment: 33 pages, 15 figures, version published in Eur. Phys. J. C 76, no. 3, 125 (2016)

We present a calculation of the width for $\chi_{cJ}\rightarrow e^{+}e^{-}$ decay. The amplitude of the process is computed within the NRQCD framework. The leading-order contribution is described by two terms associated with the two different integration domains in the electromagnetic loop describing two-photon annihilation of the heavy quark-antiquark pair. The corresponding operators are defined in the framework of NRQCD. The matrix element of one of these operators describes a configuration with an ultrasoft photon and can be associated with the higher Fock state of the heavy meson. In order to compute this contribution we use the heavy hadron chiral perturbation theory. We obtain that this contribution is numerically dominant. The obtained estimates for the decay widths of the $\chi_{c1}$ and $\chi_{c2}$ states are $0.09$ eV and $0.06$ eV, respectively., Comment: revised version, couple mistakes in analytical expressions are fixed, numerical results for chi_{c2} are slightly modified

We evaluate the two-photon exchange (TPE) correction to the unpolarized elastic electron-proton scattering at small momentum transfer $ Q^2 $. We account for the inelastic intermediate states approximating the double virtual Compton scattering by the unpolarized forward virtual Compton scattering. The unpolarized proton structure functions are used as input for the numerical evaluation of the inelastic contribution. Our calculation reproduces the leading terms in the $ Q^2 $ expansion of the TPE correction and goes beyond this approximation by keeping the full $ Q^2 $ dependence of the proton structure functions. In the range of small momentum transfer, our result is in good agreement with the empirical TPE fit to existing data., Comment: 37 pages, 11 figures, version published in Phys. Rev. D 93, 013023 (2016)

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 $-t,-u>2.5$~GeV$^{2}$ 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., Comment: 15 pages, 5 figures

We apply a subtracted dispersion relation formalism with the aim to improve predictions for the two-photon exchange corrections to elastic electron-proton scattering observables at finite momentum transfers. We study the formalism on the elastic contribution, and make a detailed comparison with existing data for unpolarized cross sections as well as polarization transfer observables., Comment: 49 pages, 26 figures, version published in Eur. Phys. J. A 51, no. 2, 24 (2015)

We present a dispersive representation of the D-term form factor for hard exclusive reactions, using unsubtracted $t$-channel dispersion relations. The $t$-channel unitarity relation is saturated with the contribution of two-pion intermediate states, using the two-pion distributions amplitude for the $\gamma^*\gamma\rightarrow \pi\pi$ subprocess and reconstructing the $\pi\pi\rightarrow N\bar N$ subprocess from available information on pion-nucleon partial-wave helicity amplitudes. Results for the D-term form factor as function of $t$ as well as at $t=0$ are discussed in comparison with available model predictions and phenomenological parametrizations., Comment: 8 pages, 3 figures; improved presentation of the results; version to appear in Phys. Lett. B

We extend the general formalism of two-photon exchange to elastic lepton-nucleon scattering by accounting for all lepton mass terms. We then perform a numerical estimate of the muon-proton scattering at low momentum transfer in view of the future MUSE experiment. For this purpose, we estimate the two-photon exchange corrections to muon-proton scattering observables by considering the contribution of the proton intermediate state, which is expected to dominate at very low momentum transfers. We find that the two-photon exchange effect to the unpolarized muon- proton scattering cross section in the MUSE kinematical region is of the order of 0.5%., Comment: 13 pages, 7 figures, version published in Phys. Rev. D 90, 013006 (2014)

We derive the complete factorization formula for the leading power contribution in wide angle Compton scattering. It consists of the soft- and hard-spectator contributions. The hard-spectator contribution is well known and defined in the form of the convolution of a hard kernel with the nucleon distribution amplitudes. The soft-spectator contribution describes the scattering which involves the soft modes. We use the soft collinear effective theory in order to define this term in a field theoretical approach. Using the SCET framework we provide the proof of the factorization formula. We also compute the next-to-leading QCD corrections to the hard coefficient function of the soft spectator contribution and perform a phenomenological analysis of existing experimental data within the developed formalism., Comment: 28 pages, 10 figures

Electron scattering fixed target experiments are a versatile tool to explore various physics phenomena. Recently these experiments came into focus to search for $U(1)$ extensions of the Standard Model of particle physics at low energies. These extensions are motivated from anomalies in astrophysical observations as well as from deviations from Standard Model predictions, such as the discrepancy between the experimental and theoretical determination of the anomalous magnetic moment of the muon. They also arise naturally when the Standard Model is embedded into a more general theory. In the considered $U(1)$ extensions a new, light messenger particle $\gamma^\prime$, the hidden photon, couples to the hidden sector as well as to the electromagnetic current of the Standard Model by kinetic mixing, which allows for a search for this particle e.g. in the invariant mass distribution of the process $e (A,\,Z)\rightarrow e (A,\,Z) l^+ l^-$. In this process the hidden photon is emitted by bremsstrahlung and decays into a pair of Standard Model leptons. In this work we study the applicability of the Weizs\"acker-Williams approximation to calculate the signal cross section of the process, which is widely used to design such experimental setups. Furthermore, based on a previous work, we investigate the discovery potential of future experimental setups at the Jefferson Lab. For that purpose we simulate the relevant cross sections for the signal and the QED background in the actual kinematical setups and obtain projected exclusion limits., Comment: 16 pages, 12 figures

The mini-proceedings of the Workshop on "Constraining the hadronic contributions to the muon anomalous magnetic moment" which included the "13th meeting of the Radio MonteCarLow WG" and the "Satellite meeting R-Measurements at BES-III" held in Trento from April 10th to 12th, 2013, are presented. This collaboration meeting aims to bring together the experimental e+e- collider communities from BaBar, Belle, BESIII, CMD2, KLOE, and SND, with theorists working in the fields of meson transitions form factors, hadronic contributions to (g-2)_\mu and effective fine structure constant, and development of Monte Carlo generator and Radiative Corrections for precision e+e- and tau physics., Comment: 45 pages, 17 contributions. Editors: P. Masjuan and G. Venanzoni

We apply a sum rule for the forward light-by-light scattering process within the context of the $\phi^4$ quantum field theory. As a consequence of the sum rule a stringent causality criterion is presented and the resulting constraints are studied within a particular resummation of graphs. Such resummation is demonstrated to be consistent with the sum rule to all orders of perturbation theory. We furthermore show the appearance of particular non-perturbative solutions within such approximation to be a necessary requirement of the sum rule. For a range of values of the coupling constant, these solutions manifest themselves as a physical bound state and a $K$-matrix pole. For another domain however, they appear as tachyon solutions, showing the inconsistency of the approximation in this region., Comment: 12 pages, 7 figures

Motivated by anomalies in cosmic ray observations and by attempts to solve questions of the Standad Model of particle physics like the (g-2)_mu discrepancy, U(1) extensions of the Standard Model have been proposed in recent years. Such U(1) extensions allow for the interaction of Dark Matter by exchange of a photon-like massive force carrier \gamma' not included in the Standard Model. In order to search for \gamma' bosons various experimental programs have been started. One approach is the dedicated search at fixed-target experiments at modest energies as performed at MAMI or at the Jefferson Lab. In these experiments the process e(A,Z)->e(A,Z)l^+l^- is investigated and a search for a very narrow resonance in the invariant mass distribution of the l^+l^- pair is performed. In this work we analyze this process in terms of signal and background in order to describe existing data obtained by the A1 experiment at MAMI with the aim to give accurate predictions for exclusion limits in the \gamma' parameter space. We present a detailed theoretical analysis of the cross sections entering in the description of such processes., Comment: 18 pages, 12 figures

We calculate the two-photon exchange (TPE) corrections in the region where the kinematical variables describing the elastic $ep$ scattering are moderately large momentum scales relative to the soft hadronic scale. For such kinematics we use the QCD factorization approach formulated in the framework of the soft-collinear effective theory (SCET). Such technique allows us to develop a description for the soft-spectator scattering contribution which is found to be important in the region of moderately large scales. Together with the hard-spectator contribution we present the complete factorization formulas for the TPE amplitudes at the leading power and leading logarithmic accuracy. The momentum region where both photons are hard is described by only one new nonperturbative SCET form factor. It turns out that the same form factor also arises for wide-angle Compton scattering which is also described in the framework of the SCET approach. This allows us to estimate the soft-spectator contribution associated with the hard photons in a model independent way. The main unknown in our description of the TPE contribution is related with the configuration where one photon is soft. The nonperturbative dynamics in this case is described by two unknown SCET amplitudes. We use a simple model in order to estimate their contribution. The formalism is then applied to a phenomenological analysis of existing data for the reduced cross section as well as for the transverse and longitudinal polarization observables., Comment: 59 pages, 22 figures, text is improved, several typos are corrected, new appendix is added

We study the annihilation process $p\bar{p} \to \pi^0 e^+ e^-$ 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, $\bar{p} p \to \pi^0 \gamma$, where data in the energy range of 2.911 GeV$\leq\sqrt{s} \leq 3.686$ 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., Comment: 10 pages, 8 figures, published version

With anomalies found in cosmic ray observations and unsolved questions of the Standard Model of particle physics like the discrepancy in the muon's anomalous magnetic moment, the idea of an U(1) extension of the Standard Model arose. This extension allows for the interaction of Dark Matter by exchange of a photon-like massive force carrier $A'$ not included in the Standard Model. We discuss the possibility to constrain the dark photon parameter space by using data taken from rare kaon decays. Therefore we analyze the decay $K\rightarrow \mu \nu A'$ as a signal process and calculate the expected Standard Model background. Using these results we calculate new limits on the $A'$ parameter space, providing motivations for new rare kaon decay experiments to extend the existing $A'$ bounds., Comment: 5 pages, 4 figures

This colloquium addresses the issue of the shape of hadrons and in particular that of the proton. The concept of shape in the microcosm is critically examined. Special attention is devoted to properly define the meaning of shape for bound-state systems of near massless quarks. The ideas that lead to the expectation of non-sphericity in the shape of hadrons, the calculations that predict it, and the experimental information obtained from recent high-precision measurements are examined. Particular emphasis is given to the study of the electromagnetic transition between the nucleon and its first excited state, the $\Delta$(1232)-resonance. The experimental evidence is is critically examined and compared with lattice calculations, as well as with effective-field theories and phenomenological models., Comment: Accepted for publication in Rev. of Mod. Phys. 22 pages, 17 figs

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, S11, and D13 nucleon resonances., Comment: 30 pages, 19 figures, 9 tables

An expansion of the electromagnetic form factors of the nucleon and Delta(1232) in small momentum transfer and pion mass is performed in a manifestly-covariant EFT framework consistent with chiral symmetry and analyticity. We present the expressions for the nucleon and Delta(1232) electromagnetic form factors, charge radii, and electromagnetic moments in the framework of SU(2) baryon chiral perturbation theory, with nucleon and Delta(1232)-isobar degrees of freedom, to next-to-leading order. Motivated by the results for the proton electric radius obtained from the muonic-hydrogen atom and electron-scattering process, we extract values for the second derivative of the electric form factor which is a genuine prediction of the p^3 BChPT. The chiral behavior of radii and moments is studied and compared to that obtained in the heavy-baryon framework and lattice QCD. The chiral behavior of Delta(1232)-isobar properties exhibits cusps and singularities at the threshold of Delta->pi N decay, and their physical significance is discussed., Comment: 42 pages, 10 figures

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., Comment: 17 pages, 7 figures, 1 Table; proceedings of "Many-Body Structure of Strongly Interacting Systems", Mainz, Germany, Feb. 23-25 2011 (to appear in EPJ Special Topics)

We discuss the chiral behavior of nucleon and Delta(1232) electromagnetic properties within the framework of a SU(2) covariant baryon chiral perturbation theory. Our one-loop calculation is complete to the order p^3 and p^4/Delta with Delta as the Delta(1232)-nucleon energy gap. We show that the magnetic moment of a resonance can be defined through the linear energy shift only when an additional relation between the involved masses and the applied magnetic field strength is fulfilled. Singularities and cusps in the pion mass dependence of the Delta(1232) electromagnetic moments reflect a non-fulfillment. We show results for the pion mass dependence of the nucleon iso-vector electromagnetic quantities and present results for finite volume effects on the iso-vector anomalous magnetic moment., Comment: 4 pages, 3 figures, prepared for Proceedings of the International Conference on the Structure of Baryons (BARYONS'10), Osaka, Japan, Dec. 7-11, 2010

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 the application of a small external magnetic field changes the particle's energy in a non-analytic way. Conventional electromagnetic moments are only well-defined when the background field B satisfies |eB|/(2M_*|M_*-M-m|) where M_* is the mass of the resonance and M, m the masses of the decay products. An application of this situation is the chiral extrapolation of 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., Comment: 4 pages, 3 figures, prepared for Proceedings of the 12th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU10), Williamsburg VA, USA, May 31-June 4, 2010

We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasi-elastic scattering on the deuteron, at backward angles (lab scattering angle of 108 degrees) for Q2 = 0.22 GeV^2/c^2 and 0.63 GeV^2/c^2 at beam energies of 362 MeV and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (piN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasi-static deuterium approximation, and is also in agreement with 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 multidimensional pictures of the hadron and to develop a semi-classical intuition of the quark dynamics. We comment briefly the results obtained from the Form Factors of the nucleon. A generalization to a target with arbitrary spin led to a set of preferred values for the electromagnetic coupling characterizing structureless particles. Generalized polarizabilities can also be interpreted in that frame as the distortion of the charge densities due to an external electromagnetic field. Finally, we present preliminary results for the Generalized Transverse-Momentum dependent Distributions which encode in principle the most complete information about quark distributions., Comment: 8 pages, 2 figures, 1 table; contribution to the proceedings of "The 4th Workshop on Exclusive Reactions at High Momentum Transfer", Jefferson Lab., May 18-21, 2010

We extend the recently developed formalism to extract light-front quark charge densities from nucleon form factor data to the deformations of these quark charge densities when applying an external electric field. We show that the resulting induced polarizations can be extracted from proton generalized polarizabilities. The available data for the generalized electric polarizabilitiy of the proton yield a pronounced structure in its induced polarization at large transverse distances, which will be pinned down by forthcoming high precision virtual Compton scattering experiments., Comment: 4 pages, 4 figures, reference added, minor typos corrected

We discuss the techniques to extract the electromagnetic Delta form factors in Lattice QCD. We evaluate these form factors using dynamical fermions with smallest pion mass of about 350 MeV. We pay particular attention to the extraction of the electric quadrupole form factor that signals a deformation of the Delta. The magnetic moment of the $\Delta$ is extrapolated using a chiral effective field theory. Using the form factors we evaluate the transverse density distributions in the infinite momentum frame showing deformation in the Delta., Comment: 9 pages, 6 figures, Invited Talk at the 6th International Workshop on Chiral Dynamics, CD09, July 6-10, 2009, Bern, Switzerland

The unitary isobar model MAID2007 has been used to analyze the recent data of pion electroproduction. The model contains all four-star resonances in the region below W=2 GeV and both single-Q^2 and Q^2 dependent transition form factors could be obtained for the Delta, Roper, D13(1520), S11(1535), S31(1620), S11(1650), D15(1675), F15(1680) and P13(1720). From the complete world data base, including also pi- data on the neutron, also Q^2 dependent neutron form factors are obtained. For all transition form factors we also give convenient numerical parameterizations that can be used in other reactions. Furthermore, we show how the transition form factors can be used to obtain empirical transverse charge densities and our first results are given for the Roper, the S11 and D13 resonances., Comment: 8 pages, 11 figures, 4 tables, Proc. of NSTAR2009, Beijing

Using recent experimental data, we analyze the electromagnetic transition from the nucleon to the P11(1440) resonance. From the resulting empirical transition form factors, we map out the quark transverse charge densities which induce the N to P11(1440) transition. It is found that the transition from the proton to its first radially excited state is dominated by up quarks in a central region of around 0.5 fm and by down quarks in an outer band which extends up to about 1 fm., Comment: 11 pages, 5 figures

Using only the current empirical information on the nucleon electromagnetic form factors we map out the transverse charge density in proton and neutron as viewed from a light front moving towards a transversely polarized nucleon. These charge densities are characterized by a dipole pattern, in addition to the monopole field corresponding with the unpolarized density. Furthermore, we use the latest empirical information on the $N \to \Delta$ transition form factors to map out the transition charge density which induces the $N \to \Delta$ excitation. This transition charge density in a transversely polarized $N$ and $\Delta$ contains both monopole, dipole and quadrupole patterns, the latter corresponding with a deformation of the hadron's charge distribution., Comment: 4 pages, 3 figures

Polarized Compton scattering off the proton is studied within the framework of subtracted dispersion relations for photon energies up to 300 MeV. As a guideline for forthcoming experiments, we focus the attention on the role of the proton's spin polarizabilities and investigate the most favorable conditions to extract them with a minimum of model dependence. We conclude that a complete separation of the four spin polarizabilities is possible, at photon energies between threshold and the $\Delta(1232)$ region, provided one can achieve polarization measurements with an accuracy of a few percent., Comment: 26 pages, 7 figures

There has been much activity in the measurement of the elastic electromagnetic proton and neutron form factors in the last decade, and the quality of the data has been greatly improved by performing double polarization experiments, in comparison with previous unpolarized data. Here we review the experimental data base in view of the new results for the proton, and neutron, obtained at MIT-Bates, MAMI, and JLab. The rapid evolution of phenomenological models triggered by these high-precision experiments will be discussed, including the recent progress in the determination of the valence quark generalized parton distributions of the nucleon, as well as the steady rate of improvements made in the lattice QCD calculations., Comment: 71 pages, 43 figures (published version with typos corrected)

We estimate the reaction mechanisms for the photoproduction of the Theta^+(1540) resonance on the nucleon, through K and K^* Regge exchanges. We compare the size of the cross sections for the gamma n -> K^- Theta^+ and gamma p -> K^0 bar Theta^+ reactions, and investigate their sensitivity to the spin-parity assignments J^P = 1/2^\pm, 3/2^\pm for the Theta^+ resonance. The model allows to estimate the cross sections corresponding with a given upper bound on the width of the Theta^+. Within this model, the cross sections on the neutron are found to be around a factor 5 larger than the ones on the proton, due to the presence of charged K exchange for the reaction on a neutron target. Furthermore, the photon asymmetry is found to display a pronounced sensitivity to the parity of the Theta^+, making it a very promising observable to help determining the quantum numbers of the Theta^+ resonance., Comment: 29 pages, 15 figures. A few corrections in introduction and acknowledgment, typo corrections and added reference

We elaborate on a recently proposed extension of the Gerasimov-Drell-Hearn (GDH) sum rule which is achieved by taking derivatives with respect to the anomalous magnetic moment. The new sum rule features a {\it linear} relation between the anomalous magnetic moment and the dispersion integral over a cross-section quantity. We find some analogy of the linearized form of the GDH sum rule with the `sideways dispersion relations'. As an example, we apply the linear sum rule to reproduce the famous Schwinger's correction to the magnetic moment in QED from a tree-level cross-section calculation and outline the procedure for computing the two-loop correction from a one-loop cross-section calculation. The polarizabilities of the electron in QED are considered as well by using the other forward-Compton-scattering sum rules. We also employ the sum rules to study the magnetic moment and polarizabilities of the nucleon in a relativistic chiral EFT framework. In particular we investigate the chiral extrapolation of these quantities., Comment: 24 pages, 7 figures; several additions, published version

We discuss the target and beam normal spin asymmetries in elastic electron-nucleon scattering which depend on the imaginary part of two-photon exchange processes between electron and nucleon. In particular, we estimate these transverse spin asymmetries for beam energies below 2 GeV, where thetwo-photon exchange process is dominated from the resonance contribution to the doubly virtual Compton scattering tensor on the nucleon., Comment: 4 pages, 7 figures, presented at International Workshop on Parity Violation and Hadronic Structure (PAVI04), Grenoble, France, 8-11 Jun 2004. Eur. Phys. J. A, to be published

We discuss the links between Generalized Parton Distributions (GPDs) and elastic nucleon form factors. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for GPDs at small momentum transfer, is extended to the large momentum transfer region and it is found to describe the basic features of proton and neutron electromagnetic form factor data. This parametrization is used to estimate the quark contribution to the nucleon spin., Comment: 18 pages, 5 figures, replaced with published version

Radiative pion photoproduction in the $\Delta(1232)$ resonance region is studied with the aim to access the $\Delta^+(1232)$ magnetic dipole moment. We present a unitary model of the $\gamma p \to \gamma \pi N$ ($\pi N = \pi^0 p, \pi^+ n$) reactions, where the $\pi N$ rescattering is included in an on-shell approximation. In this model, the low energy theorem which couples the $\gamma p \to \gamma \pi N$ process in the limit of a soft final photon to the $\gamma p \to \pi N$ process is exactly satisfied. We study the sensitivity of the $\gamma p \to \gamma \pi^0 p$ process at higher values of the final photon energy to the $\Delta^+(1232)$ magnetic dipole moment. We compare our results with existing data and give predictions for forthcoming measurements of angular and energy distributions. It is found that the photon asymmetry and a helicity cross section are particularly sensitive to the $\Delta^+$ magnetic dipole moment., Comment: 23 pages, 18 figures

We discuss the target and beam normal spin asymmetries in elastic electron-nucleon scattering which depend on the imaginary part of two-photon exchange processes between electron and nucleon. We express this imaginary part as a phase space integral over the doubly virtual Compton scattering tensor on the nucleon. We use unitarity to model the doubly virtual Compton scattering tensor in the resonance region in terms of $\gamma^* N \to \pi N$ electroabsorption amplitudes. Taking those amplitudes from a phenomenological analysis of pion electroproduction observables, we present results for beam and target normal single spin asymmetries for elastic electron-nucleon scattering for beam energies below 1 GeV and in the 1-3 GeV region, where several experiments are performed or are in progress., Comment: 36 pages, 16 figures

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 spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions., Comment: 17 pages, 5 figures

We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer through the scattering off a parton in the proton. We relate the process on the nucleon to the generalized parton distributions which also enter in other wide angle scattering processes. We find that when taking the polarization transfer determinations of the form factors as input, adding in the 2 photon correction, does reproduce the Rosenbluth data., Comment: 4 pages, 4 figures

In view of the numerous experimental results recently released, we provide in this letter an update on the performance of our simple Regge model for strangeness electroproduction on the nucleon. Without refitting any parameters, a decent description of all measured observables and channels is achieved. We also give predictions for spin transfer observables, recently measured at Jefferson Lab which have high sensitivity to discriminate between different theoretical approaches., Comment: 5 pages, 5 figures