Deeply Virtual Compton Scattering Cross Section Research Articles

Deeply Virtual Compton Scattering Cross Section at High Bjorken xB

We report high-precision measurements of the deeply virtual Compton scattering (DVCS) cross section at high values of the Bjorken variable x_{B}. DVCS is sensitive to the generalized parton distributions of the nucleon, which provide a three-dimensional description of its internal constituents. Using the exact analytic expression of the DVCS cross section for all possible polarization states of the initial and final electron and nucleon, and final state photon, we present the first experimental extraction of all four helicity-conserving Compton form factors (CFFs) of the nucleon as a function of x_{B}, while systematically including helicity flip amplitudes. In particular, the high accuracy of the present data demonstrates sensitivity to some very poorly known CFFs.

Coherent deeply virtual Compton scattering off3He and neutron generalized parton distributions

It has been recently proposed to study coherent deeply virtual Compton scattering (DVCS) off 3He nuclei to access neutron generalized parton distributions (GPDs). In particular, it has been shown that, in Impulse Approximation (IA) and at low momentum transfer, the sum of the quark helicity conserving GPDs of 3He, H and E, is dominated by the neutron contribution. This peculiar result makes the 3He target very promising to access the neutron information. We present here the IA calculation of the spin dependent GPD tilde-H of 3He. Also for this quantity the neutron contribution is found to be the dominant one, at low momentum transfer. The known forward limit of the IA calculation of tilde-H, yielding the polarized parton distributions of 3He, is correctly recovered. The extraction of the neutron information could be anyway non trivial, so that a procedure, able to take into account the nuclear effects encoded in the IA analysis, is proposed. These calculations, essential for the evaluation of the coherent DVCS cross section asymmetries, which depend on the GPDs H, E and tilde-H, represent a crucial step for planning possible experiments at Jefferson Lab.

Deeply virtual Compton scattering from gauge/gravity duality

We use gauge/gravity duality to study deeply virtual Compton scattering (DVCS) in the limit of high center of mass energy at fixed momentum transfer, corresponding to the limit of low Bjorken x, where the process is dominated by the exchange of the pomeron. Using conformal Regge theory we review the form of the amplitude for pomeron exchange, both at strong and weak 't Hooft coupling. At strong coupling, the pomeron is described as the graviton Regge trajectory in AdS space, with a hard wall to mimic confinement effects. This model agrees with HERA data in a large kinematical range. The behavior of the DVCS cross section for very high energies, inside saturation, can be explained by a simple AdS black disk model. In a restricted kinematical window, this model agrees with HERA data as well.

Leading twist nuclear shadowing, nuclear generalized parton distributions, and nuclear deeply virtual Compton scattering at smallx

We generalize the leading twist theory of nuclear shadowing and calculate quark and gluon generalized parton distributions (GPDs) of spinless nuclei. We predict very large nuclear shadowing for nuclear GPDs. In the limit of the purely transverse momentum transfer, our nuclear GPDs become impact-parameter-dependent nuclear parton distribution functions (PDFs). Nuclear shadowing induces nontrivial correlations between the impact parameter $b$ and the light-cone fraction $x$. We make predictions for the deeply virtual Compton scattering (DVCS) amplitude and the DVCS cross section on $^{208}\mathrm{Pb}$ at high energies. We calculate the cross section of the Bethe-Heitler (BH) process and address the issue of the extraction of the DVCS signal from the $eA\ensuremath{\rightarrow}e\ensuremath{\gamma}A$ cross section. We find that the $eA\ensuremath{\rightarrow}e\ensuremath{\gamma}A$ differential cross section is dominated by DVCS at the momentum transfer $t$ near the minima of the nuclear form factor. We also find that nuclear shadowing leads to dramatic oscillations of the DVCS beam-spin asymmetry, ${A}_{\mathit{LU}}$, as a function of $t$. The position of the points where ${A}_{\mathit{LU}}$ changes sign is directly related to the magnitude of nuclear shadowing.

On the mistake in the implementation of the minimal model of the dual parametrization and resulting inability to describe the high-energy deeply virtual Compton data

We correct the mistaken claim made by Guzey and Polyakov [Eur. Phys. J. C 46, 151 (2006) ] and Guzey and Teckentrup [Phys. Rev. D 74, 054027 (2006)] that the minimal model of the dual parametrization of nucleon generalized parton distributions gives a good, essentially model-independent description of high-energy data on deeply virtual Compton scattering (DVCS). In the implementation of the dual parametrization by Guzey-Polyakov and Guzey-Teckentrup (see above), the numerical prefactor of two in front of the DVCS amplitude was missing. We show that the corrected minimal model of the dual parametrization significantly overestimates the Hadron Electron Ring Accelerator data (H1 and ZEUS experiments) on the DVCS cross section.

Generalized parton distributions and deeply virtual Compton scattering in color glass condensate model

Within the framework of the Color Glass Condensate model, we evaluate quark and gluon Generalized Parton Distributions (GPDs) and the cross section of Deeply Virtual Compton Scattering (DVCS) in the small-$x_{B}$ region. We demonstrate that the DVCS cross section becomes independent of energy in the limit of very small $x_{B}$, which clearly indicates saturation of the DVCS cross section. Our predictions for the GPDs and the DVCS cross section at high-energies can be tested at the future Electron-Ion Collider and in ultra-peripheral nucleus-nucleus collisions at the LHC.

Deeply Virtual Compton Scattering on nucleons and nuclei in the Generalized Vector Meson Dominance model

We consider Deeply Virtual Compton Scattering (DVCS) on nucleons and nuclei in the framework of the Generalized Vector Meson Dominance (GVMD) model. We demonstrate that the GVMD model provides a good description of the HERA data on the dependence of the proton DVCS cross-section on Q2, W (at Q 2 = 4 GeV^2) and t . At Q 2 = 8 GeV^2, the soft W -behavior of the GVMD model somewhat underestimates the W -dependence of the DVCS cross-section due to the hard contribution not present in the GVMD model. We estimate 1/Q 2 power-suppressed corrections to the DVCS amplitude and the DVCS cross-section and find them large. We also make predictions for the nuclear DVCS amplitude and cross-section in the kinematics of the future Electron-Ion Collider. We predict significant nuclear shadowing, which matches well predictions of the leading-twist nuclear shadowing in DIS on nuclei.

Deeply virtual compton scattering in color dipole formalism

In this contribution we summarize recent investigations on the deeply virtual Compton Scattering (DVCS) within the color dipole approach. The color dipole cross section is implemented through the phenomenological saturation model. The role played by its QCD evolution and skewedness effects in the DVCS cross section are discussed. The results are compared with the recent H1 and ZEUS Collaborations data. The skewing factor, defined as the ratio of the imaginary parts of the amplitudes ImA(g*p ® g* p)/ImA(g*p ® gp) can be extracted from the data using recent DVCS and the inclusive inelastic cross section measurements at DESY-HERA. We report on this experimental extraction and compare the results to the theoretical predictions for NLO QCD and the color dipole approach.

DVCS on nuclei: observability and consequences

In this paper, we discuss the feasibility of measuring deeply virtual Compton scattering (DVCS) on nuclei in a collider setting, as for example, the planned high-luminosity Electron-Ion-Collider (EIC). We demonstrate that employing our recent model for nuclear generalized parton distributions (nGPDs), the one-photon unpolarized DVCS cross section as well as the azimuthal- and spin asymmetry are of the same size as in the proton case. This will allow for an experimental extraction of nuclear GPDs with high precision shedding new light on nuclear shadowing at small $x_{bj}$ and the interplay of shadowing and nuclear enhancement at $x_{bj}~0.1$.

A detailed next-to-leading order QCD analysis of deeply virtual Compton scattering observables

We present a detailed next-to-leading order (NLO) leading twist QCD analysis of deeply virtual Compton scattering (DVCS) observables, for several different input scenarios, in the \(\overline{\mbox{MS}}\) scheme. We discuss the size of the NLO effects and the behavior of the observables in skewedness \(\zeta\), momentum transfer,t, and photon virtuality, \(q^2=-Q^2\). We present results on the amplitude level for unpolarized and longitudinally polarized lepton probes, and unpolarized and longitudinally polarized proton targets. We make predictions for various asymmetries and for the DVCS cross section and compare with the available data.

Colour dipoles and virtual Compton scattering

An analysis of Deeply Virtual Compton Scattering (DVCS) is made within the colour dipole model. We compare and contrast two models for the dipole cross-section which have been successful in describing structure function data. Both models agree with the available cross section data on DVCS from HERA. We give predictions for various azimuthal angle asymmetries in HERA kinematics and for the DVCS cross section in the THERA region.

Off-forward quark distributions of the nucleon in the large-Nclimit

We study the off-forward quark distributions (OFQD's) in the chiral quark-soliton model of the nucleon. This model is based on the large-N_c picture of the nucleon as a soliton of the effective chiral lagrangian and allows to calculate the leading twist quark- and antiquark distributions at a low normalization point. We demonstrate the consistency of the approach by checking various sum rules for the OFQD's. We present numerical estimates of the isosinglet distribution H(x,\xi,\Delta^2). In contrast to other approaches we find a strong qualitative dependence on the longitudinal momentum transfer, \xi. In particular, H(x,\xi,\Delta^2) as a function of x exhibits fast crossovers at |x| = \xi/2. Such behaviour could lead to a considerable enhancement of the deeply-virtual Compton scattering cross section.