Deeply Virtual Compton Scattering off Helium-4 (original) (raw)
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Coherent deeply virtual Compton scattering off 3 He and neutron generalized parton distributions
EPJ Web of Conferences, 2014
It has been recently proposed to study coherent deeply virtual Compton scattering (DVCS) off 3 He 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 3 He, H and E, is dominated by the neutron contribution. This peculiar result makes the 3 He target very promising to access the neutron information. We present here the IA calculation of the spin dependent GPDH of 3 He. 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 ofH , yielding the polarized parton distributions of 3 He, 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 andH , represent a crucial step for planning possible experiments at Jefferson Lab.
Deeply Virtual Compton Scattering measurement off bound protons
2018
We report the first beam spin asymmetry (BSA) measurement of incoherent deeply virtual Compton scattering (DVCS) off ^4He. The data were accumulated using a 6 GeV longitudinally polarized electron beam incident on a pressurized ^4He gaseous target placed within the CLAS spectrometer in Hall-B at the Thomas Jefferson National Accelerator Facility. The azimuthal angle (ϕ) dependence of the BSA was studied in a wide range of virtual photon and scattered proton kinematics. The Q^2, x_B, and t dependencies of the BSA on the bound proton are compared with those on the free proton. In the whole kinematical region of our measurements, the BSA on the bound proton is smaller by 20% to 40%, indicating possible medium modification of its partonic structure. This measurement opens a new avenue for studying the structure of bound nucleons and for exploring the origin of the EMC effect using the framework of the generalized parton distributions.
2019
We provide the general expression of the cross section for exclusive deeply virtual photon electroproduction from a spin 1/2 target using current parameterizations 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, J_z, and on the orbital angular momentum, L_z. On one side, we uncover an avenue to a precise extracti...
Scaling Tests of the Cross Section for Deeply Virtual Compton Scattering
Physical Review Letters, 2006
We present the first measurements of ep → epγ cross section in the deep virtual Compton scattering (DVCS) regime and the valence quark region (xBj = 0.36). From JLab E00-110, we extract the imaginary part of the Bethe-Heitler (BH)-DVCS interference terms, to order twist-3 for Q 2 = 1.5, 1.9, and 2.3 GeV 2 , and the real part of the BH-DVCS interference terms at Q 2 = 2.3 GeV 2 . We present the first model-independent measurement of linear combinations of generalized parton distributions (GPDs) and GPD integrals up to twist-3 approximation. The validity of this approximation is strongly supported by the absence of Q 2 -variation of the extracted terms -thereby constraining the size of higher twist contributions to our observables.
Deeply Virtual Compton Scattering at HERA and perspectives at CERN
Proceedings of the XVII International Workshop on Deep-Inelastic Scattering and Related Topics, 2009
Standard parton distribution functions contain neither information on the correlations between partons nor on their transverse motion, then a vital knowledge about the three dimensional structure of the nucleon is lost. Hard exclusive processes, in particular DVCS, are essential reactions to go beyond this standard picture. In the following, we examine the most recent data in view of the dipole model predictions and their implication on the quarks/gluons imaging (tomography) of the nucleon.
Deeply virtual Compton scattering off 4He
We propose to measure the Beam Spin Asymmetry (BSA) in Deeply Virtual Compton Scattering (DVCS) off 4 He. The measurements will use a 6 GeV polarized electron beam, a 4 He pressurized gas target, the CLAS and the BoNuS RTPC detectors. The major goal of this proposal is to perform the first fully quantitative investigation of the DVCS reaction off a nuclear target in the coherent and incoherent channels. The spin zero of the 4 He target allows for a simple parametrization of its partonic structure characterized at leading twist by one chirally-even Generalized Parton Distribution H A. This experiment proposes a model-independent extraction of the real and imaginary parts of the Compton form factor H A (x B , t) from the BSA measured in the 4 He(e, e ′ γ 4 He) coherent channel. The x B and t evolution of these quantities will be studied, allowing us to explore the spatial distribution of the strong interactions between partons in a nucleus. The BSA of the 4 He(e, e ′ γp) incoherent channel will be simultaneously measured. The comparison of both measurements with the CLAS free proton data will allow us to address novel nuclear effects such as the role of the transverse degrees of freedom in the nuclear structure and to study their contributions to the EMC effect. This proposal requests 45 days of highly polarized electron beam that can run concurrently with the approved experiment E07-009.
Measurement of deeply virtual Compton scattering at HERA
The European Physical Journal C, 2005
In this thesis the results of the first measurement of Deeply Virtual Compton Scattering in electron-proton collisions at HERA are presented. The analysed data were taken with the H1 detector during the 1997 data taking period corresponding to an integrated luminosity of 8 pb −1. The differential cross sections dσ dQ 2 and dσ dW of the reaction ep → eγp have been measured in the kinematic domain 2 < Q 2 < 20 GeV 2 , 30 < W < 120 GeV and |t| < 1 GeV 2. After subtracting the contribution of the Bethe-Heitler process the photon proton cross section for the DVCS process σ γ * p−→γp has been determined. The results are compared to QCD based calculations which are able to describe the measurement. Kurzfassung Die Ergebnisse der ersten Messung tiefvirtueller Comptonstreuung in Elektron-Proton Kollisionen bei HERA werden vorgestellt. Die analysierten Daten wurden mit dem H1 Detektor während der Datennahmeperiode 1997 aufgezeichnet und entsprechen einer integrierten Luminosität von 8 pb −1. Die differentiellen Wirkungsquerschnitte dσ dQ 2 und dσ dW für die Reaktion ep → eγp wurden im kinematischen Bereich 2 < Q 2 < 20 GeV 2 , 30 < W < 120 GeV und |t| < 1 GeV 2 gemessen. Nach Abzug des Beitrags des Bethe-Heitler Prozesses, wurde der Photon Proton Wirkungsquerschnitt σ γ * p−→γp bestimmt. Die Ergebnisse werden mit QCD basierten Berechnungen verglichen, die in der Lage sind die Messung zu beschreiben. Resumé La présente thèse de doctorat porte sur l'analyse des données accumulées par l'expérience H1 auprès du collisionneurélectron-proton HERA. Ce travail présente, pour la première fois, la mesure de la section efficace de la diffusion Compton profondément virtuel. Cette mesure utilise lesévènements collectés durant la prise de données de 1997, accumulant une luminosité de 8 pb −1. La section efficace de la réaction ep → eγp est présentée differentiellement en dσ dQ 2 et dσ dW , dans la domaine 2 < Q 2 < 20 GeV 2 , 30 < W < 120 GeV et |t| < 1 GeV 2. Aprés la soustration du contribution Bethe-Heitler la section efficace photon-proton σ γ * p−→γp est extrahit. Les résultats obtenus sont en bon accords avec certaines prédictions basées sur les calculs de chromodynamique quantique.
Nuclear medium modifications of hadrons from generalized parton distributions
Physical Review C, 2005
We study the structure of generalized parton distributions in spin 0 nuclei within a microscopic approach for nuclear dynamics. GPDs can be used on one side as tools to unravel the deep inelastic transverse structure of nuclei in terms of both transverse spatial and transverse momentum degrees of freedom. On the other, one can obtain information on GPDs themselves by observing how they become modified in the nuclear environment. We derive the structure of the nuclear deeply virtual Compton scattering tensor and generalized parton distributions at leading order in Q in a fieldtheoretical framework. The nuclear generalized parton distributions are calculated using a two step process -the convolution approach -where the scattering process happens from a quark inside a nucleon, itself inside a nucleus, disregarding final state interactions with both the nuclear and nucleon debris. We point out that details of the nuclear long range interactions such as twobody currents, can be disregarded compared to the deep inelastic induced modifications of the bound GPDs. We show how the pattern of nuclear modifications predicted, and in particular the deviations of off-shell effects from the longitudinal convolution provide clear signals to be sought in experimental measurements. Finally, we find interesting relationships by studying Mellin moments in nuclei: in particular we predict the A-dependence for the D-term of GPDs within a microscopic approach, and the behavior with t of the total momentum carried by quarks in a nucleus. The latter provides an important element for the evaluation of nuclear hadronization phenomena which are vital for interpreting current and future data at RHIC, HERMES and Jefferson Lab.
Deeply Virtual Compton Scattering off 4
2007
We propose to measure the Beam Spin Asymmetry (BSA) in Deeply Virtual Compton Scattering (DVCS) off 4He. The measurements will use a 6 GeV polarized electron beam, a 4He pressurized gas target, the CLAS and the BoNuS RTPC detectors. The major goal of this proposal is to perform the first fully quantitative investigation of the DVCS reaction off a nuclear target in the coherent and incoherent channels. The spin zero of the 4He target allows for a simple parametrization of its partonic structure characterized at leading twist by one chirally-even Generalized Parton Distribution HA. This experiment proposes a model-independent extraction of the real and imaginary parts of the Compton form factor HA(xB , t) from the BSA measured in the 4He(~e, eγ 4He) coherent channel. The xB and t evolution of these quantities will be studied, allowing us to explore the spatial distribution of the strong interactions between partons in a nucleus. The BSA of the 4He(~e, eγp) incoherent channel will be s...