The present experiment exploits the interference between the Deeply Virtual Compton Scattering (DVCS) and the Bethe-Heitler processes to extract the imaginary part of DVCS amplitudes on the neutron and on the deuteron from the helicity-dependent D$({\vec e},e'γ)X$ cross section measured at $Q^2$=1.9 GeV$^2$ and $x_B$=0.36. We extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to $E_q$, the least constrained GPD. A model dependent constraint on the contribution of the up and down quarks to the nucleon spin is deduced.
Deeply virtual Compton scattering at Jefferson Laboratory
广义部分子分布（GPD）已经成为描述强子的基本成分夸克和胶子的通用工具。质子或中子（$N$）上的深度虚康普顿散射（DVCS），$e N \rightarrow e' N' γ$，是根据GPD更直接可解释的过程。DVCS和Bethe-Heitler的振幅，即入射或散射电子发射光子的过程，可以通过横截面测量或利用它们的干涉来获得，这会引起自旋不对称。自旋不对称性、横截面和横截面差异可以与每种夸克味的四种领先扭转GPD（${H}$、${E}$、${\tilde{H}}$、${\tilde{E}}$）的不同组合联系起来，这取决于可观察的目标和目标的类型。本文概述了杰斐逊实验室在A厅和B厅进行的DVCS的最新实验结果。几个实验已经完成了提取DVCS观测大的运动学区域。具有重叠运动学区域的多个测量允许执行康普顿形状因子的准模型独立提取，康普顿形状因子是GPD积分，揭示核子的3D图像。
Measuring DVCS on a neutron target is a necessary step to deepen our understanding of the structure of the nucleon in terms of Generalized Parton Distributions (GPDs). The combination of neutron and proton targets allows to perform a flavor decomposition of the GPDs. Moreover, neutron-DVCS plays a complementary role to DVCS on a transversely polarized proton target in the determination of the GPD $E$, the least known and constrained GPD that enters Ji's angular momentum sum rule. A measurement of the beam-charge asymmetry (BCA) in the $e^{\pm} d\to e^{\pm}nγ(p)$ reaction can significantly impa…
The Generalized Parton Distributions (GPDs) constitute an appropriate framework for a universal description of the partonic structure of the nucleon. Double Deeply Virtual Compton Scattering (DDVCS) process provides the only experimental way to measure independently the dependences of the GPDs on the average and transferred momentum. This proceeding discusses the feasibility of a DDVCS experiment in the context of JLab 12 GeV, model-predicted pseudo-data, and extraction of the relevant GPDs information based on a fitter algorithm.
We use a generalization of the Rosenbluth separation method for a model independent simultaneous extraction of the Compton Form Factors ${\cal H}$ and ${\cal E}$, from virtual Compton scattering data on an unpolarized target. A precise evaluation of ${\cal H}$ and ${\cal E}$, enabled by the proposed method, is the first step towards pinning down the the distribution of angular momentum inside the proton.
Double Deeply Virtual Compton Scattering (DDVCS) is the only experimental channel for the determination of the dependence of the Generalized Parton Distributions (GPDs) on both the average and the transferred momentum independently. The physics observables of the electron induced di-muon production reaction $\vv{e}^{\pm}p \to e^{\pm}pμ^+μ^-$ off unpolarized hydrogen are discussed. Their measurement with the high luminosity and large acceptance SoLID spectrometer at the Thomas Jefferson National Accelerator Facility, using polarized and unpolarized positron and electron beams at 11 GeV is inves…
我们研究了最近的深虚康普顿散射（DVCS）的数据在一个双重参数化的广义部分子分布（GPD）。这种参数化允许量化的最大数量的信息，可以从DVCS数据中提取，在一个“精粹”的功能。我们提出了一个“零步”模型，后者完全基于前向夸克密度，提供了一个参数的DVCS振幅的虚部的预测。结果表明，$e p \to e p γ$光束螺旋度不对称的体效应可以在这样的模型内理解。
We study the deeply virtual Compton scattering process with both twist-two and twist-three Compton form factors and present our cross-sections formulas with all polarization configurations. While the twist-three contributions are generally assumed to be negligible in the literature due to the kinematical suppression, we compare them with the twist-two ones at typical JLab 6 GeV and 12 GeV kinematics as well as EIC kinematics and show their kinematical suppression explicitly, justifying the leading-twist approximation made in the literature. In addition, we also estimate the twist-three Compton…
We study the deeply virtual Compton scattering cross-section in twist-two generalized parton distribution (GPD) approximation, and show that different choices of light-cone vectors and gauges for the final photon polarization will lead to different higher-order kinematical corrections to the cross-section formula. The choice of light-cone vectors affects kinematic corrections at the twist-three level, accounting for the differences between the cross-section formulas in the literature. On the other hand, kinematical corrections from higher-twist GPDs should eliminate the light-cone dependence a…
Diffractive deeply virtual Compton scattering (DiDVCS) is the process $γ^*(- Q^2) + N \rightarrow ρ^0 + γ^* (Q'^2)+ N'$, where N is a nucleon or light nucleus, in the kinematical regime of large rapidity gap between the $ρ^0$ and the final photon-nucleus system, and in the generalized Bjorken regime where both photon virtualities $Q^2$ 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 li…
Study of the structure and dynamics of the nucleon has been deeply renewed with the advent of a parameterization of the partonic structure of the nucleon in terms of the Generalized Parton Distributions (GPDs). Encoding the correlations between the elementary constituents of the nucleon, GPDs allow a 3-dimensional imaging of the nucleon from the dynamical link between the transverse position and the longitudinal momentum of partons. Double Deeply Virtual Compton Scattering (DDVCS) corresponds to the scattering from the nucleon of a virtual photon that finally generates a lepton pair, where the…
Coherent deeply virtual Compton scattering on helium-4 beyond leading power
IRFU, CEA, Université Paris-Saclay（法国原子能与替代能源署萨克雷大学）；Center for Frontiers in Nuclear Science, Stony Brook University（石溪大学核科学前沿中心）；Centre de Physique Théorique, CNRS, École polytechnique, I.P. Paris（巴黎理工学院巴黎综合理工学院理论物理中心法国国家科学研究中心）；National Centre for Nuclear Research (NCBJ)（波兰国家核研究中心）
The sub-leading power of the scattering amplitude for deeply-virtual Compton scattering (DVCS) off the nucleon contains leading-twist and twist-3 generalized parton distributions (GPDs). We point out that in DVCS, at twist-3 accuracy, one cannot address any individual twist-3 GPD. This complication appears on top of the deconvolution issues familiar from the twist-2 DVCS amplitude. Accessible are exclusively linear combinations involving both vector and axial-vector twist-3 GPDs. This implies, in particular, that the (kinetic) orbital angular momentum of quarks can hardly be constrained by twi…
Generalised parton distributions are instrumental to study both the three-dimensional structure and the energy-momentum tensor of the nucleon, and motivate numerous experimental programmes involving hard exclusive measurements. Based on a next-to-leading order analysis and a careful study of evolution effects, we exhibit non-trivial generalised parton distributions with arbitrarily small imprints on deeply virtual Compton scattering observables. This means that in practice the reconstruction of generalised parton distributions from measurements, known as the deconvolution problem, does not pos…
This paper reports the measurement of polarized and unpolarized cross sections for the ep -> e'p' reaction, which is comprised of Deeply Virtual Compton Scattering (DVCS) and Bethe-Heitler (BH) processes, at an electron beam energy of 5.88 GeV at the Thomas Jefferson National Accelerator Facility using the Large Acceptance Spectrometer CLAS. The unpolarized cross sections and polarized cross section differences have been measured over broad kinematics, 0.10 < x_B < 0.58, 1.0 < Q^2 < 4.8 GeV^2, and 0.09 < -t < 2.00 GeV^2. The results are found to be consistent with previous CLAS data, and these…
