Parametrization Of Generalized Parton Distributions Research Articles

Hard exclusive π0 muoproduction at COMPASS

Hard Exclusive Meson Production (HEMP) and Deeply Virtual Compton Scattering (DVCS) are very promising reactions to access Generalized Parton Distributions (GPDs). Such exclusive measurements were performed at COMPASS in 2016 and 2017 at the M2 beamline of the CERN SPS using the 160 GeV muon beam scattering off a 2.5 m long liquid hydrogen target surrounded by a barrel-shaped time-of-flight system to detect the recoiling target proton. The scattered muons and the produced real photons were detected by the COMPASS spectrometer, which was supplemented by an additional electromagnetic calorimeter for the detection of large-angle photons. Exclusive π0 production is the main source of background for the DVCS measurement, while it provides complementary information for the parametrization of GPDs. We will report on preliminary results of the exclusive π0 production cross section and its dependence on the squared four-momentum transfer |t| and on the azimuthal angle ϕ between the scattering plane and the π0 production plane. The COMPASS data will provide further input to constrain GPDs, in particular chiral-odd (“transversity”) GPDs.

Longitudinal target-spin asymmetries for deeply virtual compton scattering.

A measurement of the electroproduction of photons off protons in the deeply inelastic regime was performed at Jefferson Lab using a nearly 6GeV electron beam, a longitudinally polarized proton target, and the CEBAF Large Acceptance Spectrometer. Target-spin asymmetries for ep→e^{'}p^{'}γ events, which arise from the interference of the deeply virtual Compton scattering and the Bethe-Heitler processes, were extracted over the widest kinematics in Q^{2}, x_{B}, t, and ϕ, for 166 four-dimensional bins. In the framework of generalized parton distributions, at leading twist the t dependence of these asymmetries provides insight into the spatial distribution of the axial charge of the proton, which appears to be concentrated in its center. These results also bring important and necessary constraints for the existing parametrizations of chiral-even generalized parton distributions.

Transversity in exclusive vector-meson leptoproduction

The role of transversity or helicity-flip generalized parton distributions (GPDs) in leptoproduction of vector mesons is investigated within the framework of the handbag approach. The transversity GPDs in combination with twist-3 meson wave functions, occur in the amplitudes for transitions from a transversely polarized virtual photon to a longitudinal polarized vector meson. The importance of the transversity GPDs can be examined in some of the spin density matrix elements and in transverse target spin asymmetries. Using suitable parametrizations of both helicity-flip and non-flip GPDs, which are essentially taken from our previous papers, we estimate these observables and compare the results with available data.

Flavor structure of generalized parton distributions from neutrino experiments

The analysis of deeply virtual meson production is extended to neutrino-production of the pseudo-Goldstone mesons $(\ensuremath{\pi},K,\ensuremath{\eta})$ on nucleons, with the flavor content of the recoil baryon either remaining intact, or changing to a hyperon from the $SU(3)$ octet. We rely on the $SU(3)$ relations and express all the cross sections in terms of the proton generalized parton distributions (GPDs). The corresponding amplitudes are calculated at the leading twist level and in the leading order in ${\ensuremath{\alpha}}_{s}$, using a phenomenological model of GPDs. We provide a computational code, which can be used for evaluation of the cross sections employing various GPD parametrizations. We conclude that these processes can be studied in the experiment Minerva at Fermilab, which could supplement the measurements at JLAB helping to extract the GPD flavor structure from data.

Deeply virtual Compton scattering and generalized parton distributions at CLAS⋆

The exclusive electroproduction of real photons and mesons at high momentum transfer allows us to access the Generalized Parton Distributions (GPDs). The formalism of the GPDs provides a unified description of the hadronic structure in terms of quark and gluonic degrees of freedom. In particular, the Deeply Virtual Compton Scattering (DVCS), ep → e′p′γ , is one of the key reactions to determine the GPDs experimentally, as it is the simplest process that can be described in terms of GPDs. A dedicated experiment to study DVCS has been carried out in Hall B at Jefferson Lab. Beam-spin asymmetries, resulting from the interference of the Bethe-Heitler process and DVCS have been extracted over the widest kinematic range ever accessed for this reaction ( 1.2 < Q2 < 3.7 (GeV/c2, 0.09 < - t < 1.3 (GeV/c2, 0.13 < xB < 0.46 . In this paper, the results obtained experimentally are shown and compared to GPD parametrizations.

Tomography for amplitudes of hard exclusive processes

We discuss which part of information about hadron structure encoded in the Generalized Parton Distributions (GPDs) [part of total GPD image] can be restored from the known amplitude of a hard exclusive process. The physics content of this partial image is analyzed. Among other things, we show that this partial image contains direct information about how the target hadron responses to the (string) quark–antiquark operator of arbitrary spin J. Explicit equations relating physics content of the partial image of GPDs directly to the data are derived. Also some new results concerning the dual parametrization of GPDs are presented.

Nucleon form factors from generalized parton distributions

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.

Deeply exclusive processes and generalized parton distributions

We discuss how generalized parton distributions (GPDs) enter into hard exclusive processes, and focuss on the links between 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 the GPDs at small momentum transfer –t is extended to the large-t region and it is found to catch the basic features of proton and neutron electromagnetic form factor data. This parametrization allows to estimate the quark contribution to the nucleon spin. It is furthermore discussed how these GPDs at large-t enter into two-photon exchange processes and resolve the discrepancy between Rosenbluth and polarization experiments of elastic electron nucleon scattering. PACS: 12.38.Bx Perturbative calculations – 13.60.Fz Elastic and Compton scattering

Generalized parton distributions, nucleon form factors, and large‐t processes*

AbstractThe connection of generalized parton distributions (GPDs) to processes at large momentum transfer (high‐t), and the links between GPDs and elastic nucleon form factors are discussed here. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for the GPDs at small‐t, which is surprisingly observed from the chiral quark soliton model results, is extended to the large‐t region and it is found to catch the basic features of proton and neutron electromagnetic form factor data. It is furthermore discussed how these GPDs at large‐t enter into two‐photon exchange processes and resolve the discrepancy between Rosenbluth and polarization experiments of elastic electron nucleon scattering.

Overview on all reactions linked to GPDs

A short overview is given on how generalized parton distributions (GPDs) enter in a variety of hard exclusive processes such as deeply virtual Compton scattering (DVCS) and hard meson electroproduction reactions on the nucleon. We firstly discuss the links between GPDs and elastic nucleon form factors which represent powerful constraints on parametrizations of GPDs. Subsequently, we show some key observables which are sensitive to the various hadron structure aspects of the GPDs, and which are at present under experimental investigation at different facilities (HERMES, H1/ZEUS, JLab and Compass), or will be addressed by experiments in the near future.