Search

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 present a physically motivated parametrization of the chiral-odd generalized parton distributions. The parametrization is an extension of our previous one in the chiral-even sector which was based on the Reggeized diquark model. While for chiral-even generalized distributions a quantitative fit with uncertainty estimation can be performed using deep inelastic scattering data, nucleon electromagnetic, axial and pseudoscalar form factors measurements, and all available deeply virtual Compton scattering data, the chiral-odd sector is far less constrained. While awaiting the analysis of measurements on pseudoscalar mesons exclusive electroproduction which are key for the extraction of chiral-odd GPDs, we worked out a connection between the chiral-even and chiral-odd reduced helicity amplitudes using parity transformations. The connection works for quark-parton models including both scalar and axial vector diquark models, and spectator models in general. This relation allows us to estimate the size of the various chiral-odd contributions and it opens the way for future quantitative fits. [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]

We derive new Lorentz invariance and equation of motion relations between twist-three generalized parton distributions (GPDs) and moments in the parton transverse momentum, kT, of twist-two generalized transverse momentum-dependent distributions (GTMDs), as a function of the parton longitudinal momentum fraction x. Although GTMDs in principle define the observables for partonic orbital motion, experiments that can unambiguously detect them appear remote at present. The relations presented here provide a solution to this impasse in that, e.g., the orbital angular momentum density is connected to directly measurable twist-three GPDs. Out of 16 possible equation of motion relations that can be written in the T-even sector, we focus on three helicity configurations that can be detected analyzing specific spin asymmetries: two correspond to longitudinal proton polarization and are associated with quark orbital angular momentum and spin-orbit correlations; the third, obtained for transverse proton polarization, is a generalization of the relation obeyed by the g2 structure function. We also exhibit an additional relation connecting the off-forward extension of the Sivers function to an off-forward Qiu-Sterman term. [ABSTRACT FROM AUTHOR]

We present a machine learning based approach to the cross section and asymmetries for deeply virtual Compton scattering from an unpolarized proton target using both an unpolarized and polarized electron beam. Machine learning methods are needed to study and eventually interpret the outcome of deeply virtual exclusive experiments since these reactions are characterized by a complex final state with a larger number of kinematic variables and observables, exponentially increasing the difficulty of quantitative analyses. Our deep neural network (FemtoNet) uncovers emergent features in the data and learns an accurate approximation of the cross section that outperforms standard baselines. FemtoNet reveals that the predictions in the unpolarized case systematically show a smaller relative median error than the polarized that can be ascribed to the presence of the Bethe-Heitler process. It also suggests that the t dependence can be more easily extrapolated than for the other variables, namely the skewness, ξ and four-momentum transfer, Q². Our approach is fully scalable and will be capable of handling larger datasets as they are released from future experiments. [ABSTRACT FROM AUTHOR]

The quark orbital angular momentum component of proton spin, Lq, can be defined in QCD as the integral of a Wigner phase space distribution weighting the cross product of the quark's transverse position and momentum. It can also be independently defined from the operator product expansion for the off-forward Compton amplitude in terms of a twist-three generalized parton distribution. We provide an explicit link between the two definitions, connecting them through their dependence on partonic intrinsic transverse momentum. Connecting the definitions provides the key for correlating direct experimental determinations of Lq and evaluations through lattice QCD calculations. The direct observation of quark orbital angular momentum does not require transverse spin polarization but can occur using longitudinally polarized targets. [ABSTRACT FROM AUTHOR]