Theoretical Description of Deeply Virtual Compton Scattering off 3He

Recently, coherent deeply virtual Compton scattering (DVCS) off ³He nuclei has been proposed to access the neutron generalized parton distributions (GPDs). In impulse approximation (IA) studies, it has been shown, in particular, that the sum of the two leading twist, quark helicity conserving GPDs of ³He, H and E, at low momentum transfer, is dominated by the neutron contribution, so that ³He is very promising for the extraction of the neutron information. Nevertheless, such an extraction could be not trivial. A technique, able to take into account the nuclear effects included in the IA analysis in the extraction procedure, has been therefore developed. In this work, the IA calculation of the spin dependent GPD \({\tilde H}\) of ³He is presented for the first time. This quantity is found to be largely dominated, at low momentum transfer, by the neutron contribution, which could be extracted using arguments similar to the ones previously proposed for the other GPDs. The known forward limit of the IA calculation of \({\tilde H}\) , yielding the polarized parton distributions of ³He, is correctly recovered. The knowledge of the GPDs H, E and \({\tilde H}\) of ³He will allow now the evaluation of the cross section asymmetries which are relevant for coherent DVCS off ³He at Jefferson Lab kinematics, an important step towards the planning of possible experiments.     

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.     

Recent HERMES Results on DVCS from Proton and Nuclear Targets

Spin structure is one of the fundamental subjects in the study of nucleon structure.Recently it is found that Generalized Parton Distributions(GPDs) are related to the total angular momentum carried by partons,which offers a possible solution to the spin puzzle in the first time.We get access to certain GPDs by looking at the azimuthal angle asymmetries attributed to the interference between Deeply Virtual Compton Scattering (DVCS) and Bethe-Heilter processes in HERMES experiment.By measuring the asymmetry with respect to transverse target polarization from proton target,a model-dependent constraint on J_u vs J_d is obtained.Another worldwide unique channel is nuclear DVCS.The preliminary results on asymmetries with respect to beam spin and beam charge are reported.     

Neutron scattering from superfluid 4He at very large momentum transfer

Measurements of inelastic neutron scattering from liquid ⁴He were performed at very large momentum transfer (about 150 Å⁻¹) using an eV spectrometer with a pulsed spallation neutron source. A difference in the scattered neutron spectrum measured at 1.2 and 2.5 K was observed for the first time within this momentum-transfer range.     

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. PACS  12.38.Mh; 13.60.Fz; 13.85.Fb; 24.85.+p; 25.20.Dc     

Deeply virtual compton scattering off the neutron

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(e,e'gamma)X cross section measured at Q2=1.9 GeV2 and xB=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.     

Generalized parton distributions from meson leptoproduction

Generalized parton distributions (GPDs) extracted from exclusive meson leptoproduction within the handbag approach are briefly reviewed. Only the GPD E is discussed in some detail. Applications of these GPDs to virtual Compton scattering (DVCS) and to Ji’s sum rule are also presented.     

Scaling tests of the cross section for deeply virtual Compton scattering

We present the first measurements of the e[over -->]p-->epgamma cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q(2) dependence (from 1.5 to 2.3 GeV(2)) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q(2). The helicity-independent cross section is also measured at Q(2)=2.3 GeV(2). We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.     

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 not only on nuclear shadowing at small x_Bj but also on the interplay of shadowing and nuclear enhancement at .Received: 8 September 2003, Revised: 24 November 2003, Published online: 15 January 2004     

Hadron optics: Diffraction patterns in deeply virtual Compton scattering

We show that the Fourier transform of the deeply virtual Compton scattering (DVCS) amplitude with respect to the skewness variable ζ at fixed invariant momentum transfer squared t   provides a unique way to visualize the structure of the target hadron in the boost-invariant longitudinal coordinate space. The results are analogous to the diffractive scattering of a wave in optics. As a specific example, we utilize the quantum fluctuations of a fermion state at one loop in QED to obtain the behavior of the DVCS amplitude for electron–photon scattering. We then simulate the wavefunctions for a hadron by differentiating the above LFWFs with respect to M²$M^2$ and study the corresponding DVCS amplitudes in light-front longitudinal space. In both cases we observe that the diffractive patterns in the longitudinal variable conjugate to ζ sharpen and the positions of the first minima move in with increasing momentum transfer. For fixed t, higher minima appear at positions which are integral multiples of the lowest minimum. Both these observations strongly support the analogy with diffraction in optics.     

Highlights from the magnetism reflectometer at the SNS

The magnetism reflectometer at the SNS has passed the phase of commissioning and is in operation for users. The high power of the neutron source demands that special attention be paid to the optimization of the background in order to be able to measure the two-dimensional maps of reflected and scattered intensities with polarized neutrons in a broad range of momentum transfer. It implies an effective separation of the magnetic and non-magnetic reflectivity, off-specular scattering and the grazing incidence SANS in a high range of momentum transfer. Therefore the polarizing and the analyzing efficiencies are of particular importance on this time-of-flight instrument. At the beginning of July 2008 the world's first ³He neutron analyzer with on-line pump-up polarization was successfully installed and the tests with a magnetic multilayer film showing a strong off-specular spin-flip scattering were made. The performance of the instrument is under constant improvement in order to make it an effective and optimal instrument for the applications in nanosciences.     

Angular correlations in peripheral fragmentation of halo nuclei

The angular correlation between momenta of ⁵He and its decay products has been calculated for the peripheral fragmentation of ⁶He using a two-step model in a sudden approximation: (1) One neutron knock-out leading to the unbound nucleus ⁵He. (2) Subsequent decay of ⁵He undisturbed by the target in an α-particle and a neutron. Angular correlations measured in a recent experiment dealing with the peripheral fragmentation of 240 MeV/u ⁶He on a carbon target can be well described by a dominant ⁵He 3/2^? ground state configuration plus a small admixture of ≈ 7% of the first excited 1/2^? state. The influence of an s-state admixture as well as a contribution of the momentum transfer to the correlation function are investigated. The results obtained for ⁶He are then used as a touch stone for a discussion of the methods to investigate peripheral fragmentation of other Borromean halo nuclei like ¹¹Li and ¹⁴Be. It is shown that the evidence on the possible existence of low-lying s-states in ¹⁰Li and ¹³Be can be derived from the corresponding angular correlations.     

Nuclear DVCS at small <Emphasis Type="Italic">x</Emphasis> using color-dipole phenomenology

Using the high-energy color-dipole formalism, we study the coherent and incoherent nuclear DVCS process, γ ^* A→γ  X, in the small-x regime. We consider simple models for the elementary dipole–hadron scattering amplitude that capture the main features of the dependence on atomic number A, on energy and on momentum transfer t. Using the amplitudes obtained we make predictions for the nuclear DVCS cross section at the photon level in collider kinematics.     

COMPASS-II

On December 1st 2010 the proposal of the COMPASS-II Experiment [1] has been approved by the CERN Research Board. After almost ten years of important results achieved by the COMPASS Collaboration in both nucleon spin physics, with the use of muon beam, and hadron spectroscopy, using hadron beams, this second phase offers now a unique chance to address in the very near future newly opened QCD-related challenges, at very moderate upgrade cost, thanks to the versatility of the COMPASS apparatus [2]. This implies mainly study of chiral perturbation theory (ChPT), by measuring the pion polarizability through Primakoff reaction; generalized parton distributions (GPDs), by measuring exclusive deeply virtual compton scattering (DVCS) and hard exclusive meson production (DVMP); transverse momentum dependent parton distributions (TMDs) in single-polarised pion-induced Drell-Yan muon production and in SIDIS on a liquid hydrogen target (in parallel to DVCS). An overview of the COMPASS-II proposal is presented here, with a main focus on the new upcoming investigation of the nucleon structure via the Drell-Yan and DVCS processes.     

Helicity-dependent generalized parton distributions for nonzero skewness

We investigate the helicity-dependent generalized parton distributions (GPDs) in momentum as well as transverse position (impact) spaces for the u and d quarks in a proton when the momentum transfer in both the transverse and the longitudinal directions are nonzero. The GPDs are evaluated using the light-front wave functions of a quark–diquark model for nucleon where the wave functions are constructed by the soft-wall AdS/QCD correspondence. We also express the GPDs in the boost-invariant longitudinal position space.     

A fitter code for Deep Virtual Compton Scattering and Generalized Parton Distributions

We have developed a fitting code based on the leading-twist handbag Deep Virtual Compton Scattering (DVCS) amplitude in order to extract Generalized Parton Distribution (GPD) information from DVCS observables in the valence region. In a first stage, with simulations and pseudo-data, we show that the full GPD information can be recovered from experimental data if enough observables are measured. If only some of these observables are measured, valuable information can still be extracted, with certain observables being particularly sensitive to certain GPDs. In a second stage, we make a practical application of this code to the recent DVCS Jefferson Lab Hall-A data from which we can extract numerical constraints for the two H GPD Compton form factors. An erratum to this article can be found at     

The 33 and 43 MeV (3He, xnγ) exclusive reactions on targets from Zr to Pb

Exclusive neutron spectra for neutron multiplicities x = 3, 4 and 5 have been obtained using the 33 and 43 MeV (³He, xnγ) reactions on targets of Zr, Pd, Sn, Nd, Pt, Au, and Pb. The exclusive spectral shapes vary systematically with the energy available for neutron decay in the initial or precompound nucleus. The (³He, xnγ) reaction differs from the (αxnγ) reaction at the same beam energy per nucleon in its dependence on available energy, target, and incident angular momentum.     

Target mass correction on the 3He polarized structure function

Target mass correction (TMC) is employed to amend the polarized helium structure functions, ³He . The structure function can be obtained via the convolution of the light cone momentum distribution with the polarized structure of the proton and neutron. The calculation of the polarized structure function of the nucleon is based on the constituent quark model. The analytical result for ³He polarized structure function at low values of Q² is not in good agreement with the available experimental data. The reliability of calculations is increased using TMC effect. New comparison confirms a better agreement with the experimental data.     

Scattering of neutrons on a Bose condensate

The spectrum of noncondensate excitations in neutron scattering on bosons is obtained in the framework of the Bogoliubov models both for liquid ⁴He and a dilute gas. The problem is solved using a path-integral representation of the partition function of the system. We describe the influence of scattering of neutrons on a Bose condensate in a stationary (time-independent) picture in the Gibbs equilibrium ensemble. This influence is a stationary boson response, and it depends on the initial neutron momentum k, transfer momentum p, and the neutron-boson interaction λ, which is related to the scattering length. The contribution of the neutrons to the initial Bogoliubov spectrum is found to be important for “quasi-elastic” scattering on the noncondensate, while the contribution of deep inelastic scattering is small; no contribution from elastic scattering on the Bose condensate is found. In the case of liquid Helium, the response is unlikely to be observable for all values p. On the other hand, for a gas one may expect a visible effect, in particular for a small momentum transfer p and a small density of the Bose condensate ϱ.