Search

Single-beam, single-target, and double spin asymmetries for hard exclusive electroproduction of a photon on the proton ep → epγ are presented. The data were taken at Jefferson Lab using the CEBAF large acceptance spectrometer and a longitudinally polarized 14NH3 target. The three asymmetries were measured in 165 four-dimensional kinematic bins, covering the widest kinematic range ever explored simultaneously for beam and target-polarization observables in the valence quark region. The kinematic dependences of the obtained asymmetries are discussed and compared to the predictions of models of generalized parton distributions. The measurement of three DVCS spin observables at the same kinematic points allows a quasi-model-independent extraction of the imaginary parts of the H and H Compton form factors, which give insight into the electric and axial charge distributions of valence quarks in the proton. [ABSTRACT FROM AUTHOR]

We derive two relations involving spin polarizabilities of a spin-1/2 particle and consider their empirical implications for the proton. Using the empirical values of the proton anomalous magnetic moment, electric and magnetic charge radii, moments of the spin structure functions g1, g2, and of two spin polarizabilities, the present relations constrain the low-momentum behavior of generalized polarizabilities appearing in virtual Compton scattering. In the case of the proton, the dispersive model evaluations of the spin and generalized polarizabilities appear to be consistent with these relations. The ongoing measurements of different electromagnetic observables at the MAMI, Jefferson Lab, and HIγS facilities may be able to put these relations to a test, or use them to unravel the low-energy spin structure of the nucleón. [ABSTRACT FROM AUTHOR]

Using the helicity amplitudes formalism, we study deeply virtual exclusive electron photoproduction off an unpolarized nucleon target, ep→e′p′γ, through a range of kinematics both in the fixed target setting with initial electron energies of 6, 11, and 24 GeV and for an electron ion collider. We reformulate the cross section bringing to the forefront the defining features of the ep→e′p′γ process, where the observables are expressed as bilinear products of the independent helicity amplitudes which completely describe it in terms of the electric, magnetic, and axial currents of the nucleon. These contributions are checked against the Fourier harmonics-based formalism which has provided so far the underlying mathematical framework to study deeply virtual Compton scattering (DVCS) and related experiments. Using theoretical model calculations of the twist-two generalized parton distributions, H, E, ˜H, and ˜E, we uncover large discrepancies between the harmonic series and our proposed framework. Most importantly, these numerical differences appear in the intermediate Q² range which represents a sweet spot for extracting generalized parton distributions from data. We provide a framework that is ideal, on one side, to study and compare the different conventions that can be used to describe the leading order contribution to DVCS in QCD, while on the other it facilitates a quantitative extraction of physically meaningful information from experiment through traceable and controllable approximations in the intermediate Q² region. [ABSTRACT FROM AUTHOR]

We have examined the existence of the N*(1685) resonance, which is recently listed by the Particle Data Group as a one-star nucleon resonance, by using a covariant isobar model for kaon photoproduction and assuming that the resonance has Jp=1/2+, in accordance with our previous finding. After the inclusion of this resonance, the changes in the total χ² show two clear minima at MN*=1650 and 1696 MeV, which correspond to two different resonance states. The former corresponds to the narrow nucleon resonance found in our previous investigation, whereas the latter corresponds to a new resonance found as we increase the resonance width. From the latter we derive the mass and width relation of the N*(1685) resonance. We observe that the properties of both the N*(1685) and N*(1710)P11 resonances are strongly correlated. Although the best fit of the present work yields MN*=1696 MeV and ΓN*=76 MeV, the apparently small N*(1710)P11 coupling constant to the K+Λ channel found in previous investigations suggests that ΓN*≲35 MeV, which, according to the mass and width relation, corresponds to MN*≲1680 MeV. [ABSTRACT FROM AUTHOR]

The newly developed approach to model nucleon generalized parton distributions (GPDs) H and E is based on two types of their representation in terms of double distributions. Within this approach, we reconsider the derivation of GPD sum rules that allow one to use border functions H(x,x) and E(x,x) instead of full GPDs H(x,ξ) and E(x,ξ) in the integrals producing Compton form factors of deeply virtual Compton scattering. Using factorized double distribution Ansatz to model GPDs, we discuss the relation between the border functions and underlying parton densities. We find that a substantial contribution to the H(x,x) border function comes from the extra term required by new double distribution representations and related to E(x,ξ) GPD. [ABSTRACT FROM AUTHOR]

We calculate the leading-order QED radiative corrections to the process e-p → e-pl-l+ in the soft-photon approximation, in two different energy regimes which are of relevance to extract nucleon structure information. In the low-energy region, this process is studied to better constrain the hadronic corrections to precision muonic hydrogen spectroscopy. In the high-energy region, the beam-spin asymmetry for double-virtual Compton scattering allows us to directly access the generalized parton distributions. We find that the soft-photon radiative corrections have a large impact on the cross sections and are therefore of paramount importance to extract the nucleon structure information from this process. For the forward-backward asymmetry, the radiative corrections are found to affect the asymmetry only around or below the 1% level, whereas the beam-spin asymmetry is not affected at all in the soft-photon approximation, which makes them gold-plated observables to extract nucleon structure information in both the low- and high-energy regimes. [ABSTRACT FROM AUTHOR]

Generalized parton distributions are instrumental to study both the three-dimensional structure and the energy-momentum tensor of the nucleon, and motivate numerous experimental programs involving hard exclusive measurements. Based on a next-to-leading order analysis and a careful study of evolution effects, we exhibit nontrivial generalized parton distributions with arbitrarily small imprints on deeply virtual Compton scattering observables. This means that in practice the reconstruction of generalized parton distributions from measurements, known as the deconvolution problem, does not possess a unique solution for this channel. In this paper we discuss the consequences on the extraction of generalized parton distributions from data and advocate for a multichannel analysis. [ABSTRACT FROM AUTHOR]

The forward Compton amplitude describes the process of virtual photon scattering from a hadron and provides an essential ingredient for the understanding of hadron structure. As a physical amplitude, the Compton tensor naturally includes all target mass corrections and higher twist effects at a fixed virtuality, Q². By making use of the second-order Feynman-Hellmann theorem, the nucleon Compton tensor is calculated in lattice QCD at an unphysical quark mass across a range of photon momenta 3≲Q²≲7  GeV². This allows for the Q² dependence of the low moments of the nucleon structure functions to be studied in a lattice calculation for the first time. The results demonstrate that a systematic investigation of power corrections and the approach to parton asymptotics is now within reach. [ABSTRACT FROM AUTHOR]

In this work, we present a new set of unpolarized (H) and polarized (H) generalized parton distributions (GPDs) that have been determined using a simultaneous χ² analysis of the nucleon axial form factor (AFF) and wide-angle Compton scattering (WACS) experimental data at the next-to-leading order (NLO) accuracy in QCD. We explore various Ansatzes presented in the literature for GPDs, which use forward parton distributions as input, and choose the ones most suited to our analysis. The experimental data included in our analysis cover a wide range of the squared transverse momentum, which is 0.025<-t<6.46 GeV². We show that the WACS data affect significantly the large -t behavior of H. The polarized GPDs obtained from the simultaneous analysis of AFF and WACS data differ considerably from the corresponding ones obtained by analyzing AFF and WACS separately, and have less uncertainties. We show that the theoretical predictions obtained using our GPDs are in good agreement with the analyzed AFF and WACS data for the entire range of -t studied. Finally, we obtain the impact parameter dependent parton distributions, both in an unpolarized and in a transversely polarized proton, and present them as tomography plots. [ABSTRACT FROM AUTHOR]

Diffractive deeply virtual Compton scattering (DiDVCS) is the process γ*(-Q²)+N→ρ0+γ*(Q'²)+N′, where N is a nucleon or light nucleus, in the kinematical regime of large rapidity gap between the ρ0 and the final photon-nucleus system, and in the generalized Bjorken regime where both photon virtualities Q² and Q'² are large. We show that this process has the unique virtue of combining the large diffractive cross sections at high energy with the tomographic ability of deeply virtual Compton scattering to scrutinize the quark and gluon content of nucleons and light nuclei. Its study at an electron-ion collider will enlighten the internal structure of hadrons. [ABSTRACT FROM AUTHOR]

A new generation of experiments is expected to shed light on the elusive parton structure of the bound proton. One of the most promising directions is incoherent deeply virtual Compton scattering, which can provide a tomographic view of the bound proton. The first measurement has been recently performed, using 4He targets at Jefferson Lab. In the work presented here, a rigorous impulse approximation analysis of this process is proposed. As ingredients, state-of-the-art models of the nuclear spectral function and of the parton structure of the struck proton, together with novel scattering amplitudes expressions for a bound moving nucleon, have been used. The overall agreement obtained with the data, good in particular at high values of the photon virtuality, demonstrates the solidity of the framework, which is also suitable for further improvements. It is found that possible big differences between results for the bound proton and those for the free one could be related to kinematical nuclear effects and not to modifications of the parton structure. The analysis demonstrates that the comparison of the results of this approach, based on a conventional description, with future precise data, has the potential to expose exotic quark and gluon effects in nuclei. [ABSTRACT FROM AUTHOR]

We provide the general expression of the cross section for exclusive deeply virtual photon electroproduction from a spin-1/2 target using current parametrizations of the off-forward correlation function in a nucleon for different beam and target polarization configurations up to twist-three accuracy. All contributions to the cross section including deeply virtual Compton scattering, the Bethe-Heitler process, and their interference, are described within a helicity-amplitude-based framework which is also relativistically covariant and readily applicable to both the laboratory frame and in a collider kinematic setting. Our formalism renders a clear physical interpretation of the various components of the cross section by making a connection with the known characteristic structure of the electron scattering coincidence reactions. In particular, we focus on the total angular momentum, Jz, and on the orbital angular momentum, Lz. On one side, we uncover an avenue to a precise extraction of Jz, given by the combination of generalized parton distributions, H+E, through a generalization of the Rosenbluth separation method used in elastic electron proton scattering. On the other side, we single out for the first time, the twist-three angular modulations of the cross section that are sensitive to Lz. The proposed generalized Rosenbluth technique adds constraints and can be extended to additional observables relevant to the mapping of the three-dimensional structure of the nucleon. [ABSTRACT FROM AUTHOR]

The paper presents an extension and a refinement of our previous work on the extraction of the doubly virtual forward Compton scattering amplitude on the lattice by using the background field technique [1]. The zero frequency limit for the periodic background field is discussed, in which the well-known result is reproduced. Further, an upper limit for the magnitude of the external field is established for which the perturbative treatment is still possible. Finally, the framework is set for the evaluation of the finite-volume corrections allowing for the analysis of upcoming lattice results. [ABSTRACT FROM AUTHOR]

In recent work, Hill and Paz apply the operator product expansion to forward doubly virtual Compton scattering. The resulting large-Q² form of the amplitude W1(0,Q²) is compatible with the one we obtain by extrapolation of low-Q² results from a chiral effective field theory, providing support for our approach. That paper also presents a result for the two-photon contribution to the Lamb shift in muonic hydrogen that has a much larger uncertainty than in previous work. We show that this is an overestimate arising from the inclusion of the proton pole term in the subtracted dispersion relation for W1. [ABSTRACT FROM AUTHOR]

We derive two new sum rules for the unpolarized doubly virtual Compton scattering process on a nucleon, which establish novel low-Q2 relations involving the nucleon's generalized polarizabilities and moments of the nucleon's unpolarized structure functions F1(x,Q2) and F2(x,Q2). These relations facilitate the determination of some structure constants which can only be accessed in off-forward doubly virtual Compton scattering, not experimentally accessible at present. We perform an empirical determination for the proton and compare our results with a next-to-leading-order chiral perturbation theory prediction. We also show how these relations may be useful for a model-independent determination of the low-Q2 subtraction function in the Compton amplitude, which enters the two-photon-exchange contribution to the Lamb shift of (muonic) hydrogen. An explicit calculation of the Δ(1232)-resonance contribution to the muonic-hydrogen 2P-2S Lamb shift yields -1±1 μeV, confirming the previously conjectured smallness of this effect. [ABSTRACT FROM AUTHOR]

The energy shift of a nucleon in a static periodic magnetic field is evaluated at second order in the external field strength in perturbation theory. It is shown that the measurement of this energy shift on the lattice allows one to determine the unknown subtraction function in the forward doubly virtual Compton scattering amplitude. The limits of applicability of the obtained formula for the energy shift are discussed. [ABSTRACT FROM AUTHOR]