Virtual Compton Research Articles

Deeply virtual compton scattering at future electron-ion colliders

The study of hadronic structure has been carried out for many years. Generalized parton distribution functions (GPDs) provide broad information on the internal structure of hadrons. Combining GPDs and high-energy scattering experiments, we expect yielding three-dimensional physical quantities from experiments. The Deeply Virtual Compton Scattering (DVCS) process is a powerful tool for studying GPDs. It is one of the important experiments of Electron Ion Collider (EIC) and Electron ion collider at China (EicC) in the future. In the initial stage, the proposed EicC will have polarized electrons with energies of 3∼5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3 \\sim 5$$\\end{document} GeV colliding with polarized protons with energies of 12∼25\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$12 \\sim 25$$\\end{document} GeV, with luminosity up to 1∼2×1033\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1 \\sim 2 \ imes 10^{33}$$\\end{document} cm-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-2}$$\\end{document} s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document}. EIC, which will be constructed in the coming years, will cover center-of-mass energies ranging from 30 to 50 GeV, with a luminosity of about 1033∼1034\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{33} \\sim 10^{34}$$\\end{document} cm-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-2}$$\\end{document} s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document}. In this work, we present a detailed simulation of DVCS to study the feasibility of experiments at EicC and EIC. Referring the method used by HERMES Collaboration, and comparing the model calculations with pseudo data of asymmetries attributed to the DVCS, we obtained a model-dependent constraint on the total angular momentum of up and down quarks in the proton.

Geometric scaling applied to lepton–ion collisions

AbstractIn this article, we discuss a scaling function for the cross section in deeply virtual compton scattering (DVCS) and exclusive vector meson production in both γ*p and γ*A processes. Our description relies on the geometric scaling phenomenon in which cross sections are a function of a single variable , where Q2 is the photon virtuality and represents the saturation scale that drives the energy dependence and nuclear effects. We make a prediction to be tested in future electron–ion colliders.

Border and skewness functions from a leading order fit to DVCS data

We propose new parameterizations for the border and skewness functions appearing in the description of 3D nucleon structure in the language of generalized parton distributions (GPDs). These parameterizations are constructed in a way to fulfill the basic properties of GPDs, like their reduction to parton density functions and elastic form factors. They also rely on the power behavior of GPDs in the x rightarrow 1 limit and the propounded analyticity property of Mellin moments of GPDs. We evaluate compton form factors (CFFs), the sub-amplitudes of the deeply virtual compton scattering (DVCS) process, at the leading order and leading twist accuracy. We constrain the restricted number of free parameters of these new parameterizations in a global CFF analysis of almost all existing proton DVCS measurements. The fit is performed within the PARTONS framework, being the modern tool for generic GPD studies. A distinctive feature of this CFF fit is the careful propagation of uncertainties based on the replica method. The fit results genuinely permit nucleon tomography and may give some insight into the distribution of forces acting on partons.

A glimpse of gluons through deeply virtual compton scattering on the proton

The internal structure of nucleons (protons and neutrons) remains one of the greatest outstanding problems in modern nuclear physics. By scattering high-energy electrons off a proton we are able to resolve its fundamental constituents and probe their momenta and positions. Here we investigate the dynamics of quarks and gluons inside nucleons using deeply virtual Compton scattering (DVCS)—a highly virtual photon scatters off the proton, which subsequently radiates a photon. DVCS interferes with the Bethe-Heitler (BH) process, where the photon is emitted by the electron rather than the proton. We report herein the full determination of the BH-DVCS interference by exploiting the distinct energy dependences of the DVCS and BH amplitudes. In the regime where the scattering is expected to occur off a single quark, measurements show an intriguing sensitivity to gluons, the carriers of the strong interaction.

Overview of the COMPASS Experimental Programme on TMDs and GPDs

The investigation of the recently introduced transverse momentum dependent (TMD) parton distribution functions (PDFs) and generalized parton distributions (GPDs) is an important part of the programme of the COMPASS experiment at CERN. A variety of Semi-inclusive deeply inelastic scattering (SIDIS) measurements on transversely polarized proton and deuteron targets from the 2002–2010 data have allowed to obtain a wealth of results on the transversity distribution and on the TMD PDFs. Further SIDIS measurements are planned in 2016 and 2017 on a pure hydrogen target. In a complementary approach, the first ever Drell–Yan measurement on a transversely polarized proton target is the objective of the 2015 run, still ongoing at the time of this conference, to address the fundamental issue of the change of sign of the Sivers function when measured in the Drell–Yan process as compared to that measured in SIDIS. GPDs are also being investigated. Measurements of the exclusive muon-production of vector mesons, in particular the \(\rho ^0\) and the \(\omega \), on transversely polarized proton targets have provided information on the GPD E. Preliminary results from a test run on deeply virtual compton scattering of 160 GeV muons on a liquid hydrogen target are also given, as well as plans for a two-year long run in 2016 and 2017 with a suitably modified spectrometer.

Nuclear effects and neutron structure in deeply virtual Compton scattering off 3He

The study of nuclear generalized parton distributions (GPDs) could be a crucial achievement of hadronic physics since they open new ways to obtain new information on the structure of bound nucleons, in particular, to access the neutron GPDs. Here, the results of calculations of 3He GPDs in Impulse Approximation (IA) are presented. The calculation of the sum of GPDs H + E, and , with the correct limits, will be shown. These quantities, at low momentum transfer, are largely dominated by the neutron contribution so that 3He is an ideal target for these kind of studies. Nevertheless the extraction of neutron information from future 3He data could be non trivial. A procedure, which takes into account nuclear effects encoded in IA, is presented. The calculation of H, E and allows also to evaluate the cross section asymmetries for deeply virtual compton scattering (DVCS) at Jefferson Lab kinematics. Thanks to these observations, DVCS off 3He could be an ideal process to access the neutron information in the next future.

Deeply virtual compton scattering from the neutron with CLAS and CLAS12

Generalised Parton Distributions (GPDs) offer an insight into the three-dimensional structure of the nucleon and its internal dynamics, relating the longitudinal momentum of quarks to their transverse position. A very effective means of accessing GPDs is via measurements of cross-sections and polarisation-asymmetries in Deeply Virtual Compton Scattering (DVCS). In particular, the beam-spin asymmetry (BSA) in DVCS from the neutron is especially sensitive to angular momentum of the up- and down-quarks, and its measurement therefore has potential to shed important light on the puzzle of nucleon spin. We present a preliminary extraction of BSA from a recent experiment using a 6 GeV electron beam and the CLAS detector at Jefferson Laboratory and introduce the Central Neutron Detector to be integrated with CLAS12 for the exclusive measurement of neutron DVCS at 11 GeV, made possible by the Jefferson Lab upgrade.

Deeply virtual compton scattering at HERMES

Generalized Parton Distributions (GPDs) provide a way to access total angular momenta of partons and give a multidimensional picture of the nucleon structure. Deeply Virtual Compton Scattering (DVCS) is the most direct exclusive process to study GPDs. Different azimuthal cross-section asymmetries with respect to beam helicity, beam charge, and target polarization have been measured in the HERMES experiment. A recoil detector was installed at HERMES to directly detect the recoil proton.

Generalized parton distributions from deep virtual compton scattering at CLAS

We have analyzed the beam spin asymmetry and the longitudinally polarized target spin asymmetry of the Deep Virtual Compton Scattering process, recently measured by the Jefferson Lab CLAS collaboration. Our aim is to extract information about the Generalized Parton Distributions of the proton. By fitting these data, in a largely model-independent procedure, we are able to extract numerical values for the two Compton Form Factors HIm and H˜Im with uncertainties, in average, of the order of 30%.

Increase with energy of parton transverse momenta in the fragmentation region in DIS and related phenomena

The dipole and the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) approximations are combined with the ${k}_{t}$ factorization theorem to demonstrate the fundamental property of perturbative QCD: the smaller the size of the colorless quark-gluon configurations in the fragmentation region, the more rapid is the increase of its interaction with the target as a function of energy. First, we consider two closely related properties of the wave function of the projectile: (i) the transverse momenta of the quark(antiquark) within the $q\overline{q}$ pair, produced in the fragmentation region by the strongly virtual photon, increase with the decrease of $x$ for fixed ${Q}^{2}$, (ii) the relative contribution of perturbative QCD to the structure functions increases as compared to the soft QCD contribution at central impact parameters due to a rapid increase with energy of the cross section of interaction of small dipoles. Practical consequences of these effects are presented for the cases of the cross sections of deep inelastic scattering (DIS) and double virtual compton scattering (DVCS). We predict that the ratio of DVCS to DIS amplitudes should very slowly approach one from above at very large collision energies. Second, we study a closely related phenomenon of the increase of the transverse momenta with the energy of the characteristic transverse momenta of the gluon/quark configurations responsible for the transition to the black disk regime. We discuss the impact of this phenomenona on the slowing of the dependence on the initial energy of the coherence length. We demonstrate that a rapid projectile has the biconcave shape, which is different from the expectations of the pre-QCD parton model where a fast hadron has a pancake shape. We show that the increase of the transverse momenta leads to a new expression for the total cross section of a DIS at very large energies, relevant to LHeC and LHC. We discuss the impact of the discovered phenomena on the hard processes in $pp$ collisions, and on the dominance of different phases of chiral and conformal symmetries in the central and peripheral $pp$, $pA$, and $AA$ collisions.

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.

Exclusive processes in diffraction at HERA

Recent results from the H1 and ZEUS experiments on exclusive processes in diffractive ep collisions at HERA are reviewed. The measurements discussed here include dijets and charm in photoproduction as well as in deep inelastic scattering. Such processes are sensitive to the partonic structure of the diffractive exchange, particularly to the gluon content, and represent complementary tests for the validity of QCD factorization in diffraction. The measurements are compared to theoretical models, including NLO predictions, using the most recent diffractive parton densities as extracted from inclusive measurements. Recent results on exclusive vector meson production and deeply virtual compton scattering are also presented.

Feasibility study of deeply virtual compton scattering using COMPASS at CERN

This paper presents the reactions which can be performed at COMPASS to study the Generalized Parton Distributions (GPDs). The high energy muon beam at CERN allows to measure Hard Exclusive Meson Production or Deeply Virtual Compton Scattering (DVCS) in the Bjorken regime in a large range of Q 2 and x Bj (1.5 ≤ Q 2≤ 7.5 GeV2 and 0.03 ≤ x Bj ≤ 0.25). Exploratory measurements dedicated to ρ ° or π° production can be investigated with the present setup. DVCS measurement require an upgrade of the COMPASS setup

Separation of soft and hard physics in deeply virtual compton scattering

A model for deeply virtual Compton scattering, based on analytical light-cone hadron wave functions is presented and studied at energies currently accessible at Jefferson Laboratory, DESY, and beyond. It is shown that perpendicular vector components play an important role at Q^2 < 10 GeV^2 and that the meson-exchange diagrams are important at all energies. This could significantly impact the physical interpretation of the underlying hadron amplitudes.

Deeply virtual compton scattering using COMPASS at CERN

In this paper, we present the recent studies our group has conducted in order to show the feasibility of a Deeply Virtual Compton Scattering experiment using COMPASS and its high energy muon beam at CERN. The measurement of the cross section and the beam charge asymmetry in the kinematical domain: 0.03 < χ bj < 0.25 and 1.5 < Q 2 < 7.5 GeV 2 will provide a check of the factorization and will put strong constraints on the models. Experimental studies show that detection of the DVCS exclusive reaction is feasible with some upgrade of the COMPASS apparatus.

Deeply virtual compton scattering at HERA

Deeply virtual Compton scattering has recently been studied by three HERA experiments, H1, ZEUS and HERMES, covering a broad range of kinematic regimes. We present cross section measurments of the two collider experiments in the kinematic region 2< Q 2<100 GeV 2 and 30< W<140 GeV, and compare them to QCD-based calculations. HERMES measurements of azimuthal asymmetries and their kinematical dependences are presented for Q 2>1 GeV 2 and 2< W<7 GeV.

First measurement ofπ−e→π−eγpion virtual compton scattering

Pion virtual compton scattering (VCS) via the reaction π−e→π−eγ was observed in the Fermilab E781 SELEX experiment. SELEX used a 600GeV/c π− beam incident on target atomic electrons, detecting the incident π− and the final state π−, electron and γ. Theoretical predictions based on chiral perturbation theory are incorporated into a Monte Carlo simulation of the experiment and are compared to the data. The number of reconstructed events (=9) and their distribution with respect to the kinematic variables (for the kinematic region studied) are in reasonable accord with the predictions. The corresponding π− VCS experimental cross section is σ=38.8±13 nb, in agreement with the theoretical expectation of σ=34.7 nb.Received 9 September 2001DOI:https://doi.org/10.1103/PhysRevC.66.034613©2002 American Physical Society

The virtual compton scattering at TJNAF

Through the Virtual Compton Scattering it is possible to cause electromagnetic deformations of the proton and hence to probe its internal structure. The VCS experiment E93050 performed in Hall A at TJNAF is now giving its first results: cross sections have been extracted below pion threshold and in the resonance region, providing an evidence of the generalised polarisabilities of the proton and giving new stringent inputs to theoretical models.

Deeply virtual compton scattering at HERMES: present and future

The beam-helicity asymmetry in hard electroproduction of photons has been measured for the first time by the HERMES experiment using the HERA 27.6 GeV longitudinally polarized positron beam and an unpolarized hydrogen target. The observed asymmetry is attributed to the interference of the Bethe-Heitler (BH) and Deeply Virtual Compton Scattering (DVCS) processes. Prospects for future measurements of DVCS at HERMES are studied using simple models for parameterizations of generalized parton distributions (GPDs).

Rho zero electroproduction and the hadronic contribution to deeply virtual compton scattering

A two-gluon-exchange model incorporating perturbative and non-perturbative effects is presented for rho electroproduction which provides an excellent description of all current data. This is then used to calculate the contribution from the rho to deeply virtual Compton scattering via the vector-meson-dominance transition rho -> gamma. This is found to be sufficiently large to provide a significant contribution through interference with the perturbative QCD term.