Wim Cosyn

Stylized features of single-nucleon momentum distributions

Nuclear short-range correlations (SRC) typically manifest themselves in the tail parts of the single-nucleon momentum distributions. We propose an approximate practical method for computing those SRC contributions to the high-momentum parts. The framework adopted in this work is applicable throughout the nuclear mass table and corrects mean-field models for central, spin–isospin and tensor correlations by shifting the complexity induced by the SRC from the wave functions to the operators. It is argued that the expansion of these modified operators can be truncated to a low order. The proposed model can generate the SRC-related high-momentum tail of the single-nucleon momentum distribution. These are dominated by correlation operators acting on mean-field pairs with vanishing relative radial and angular-momentum quantum numbers. The proposed method explains the dominant role of proton–neutron pairs in generating the SRC and accounts for the magnitude and mass dependence of SRC as probed in inclusive electron scattering. It also provides predictions for the ratio of the amount of correlated proton–proton to proton–neutron pairs which are in line with the observations. In asymmetric nuclei, the correlations make the average kinetic energy for the minority nucleons larger than for the majority nucleons.

Final-state interactions in semi-inclusive deep inelastic scattering off the Deuteron

Semi-inclusive deep inelastic scattering off the deuteron with production of a slow nucleon in recoil kinematics is studied in the virtual nucleon approximation, in which the final-state interaction (FSI) is calculated within generalized eikonal approximation. The cross section is derived in a factorized approach, with a factor describing the virtual photon interaction with the off-shell nucleon and a distorted spectral function accounting for the final-state interactions. One of the main goals of the study is to understand how much the general features of the diffractive high-energy soft rescattering accounts for the observed features of FSI in deep inelastic scattering (DIS). Comparison with the Jefferson Lab data shows good agreement in the covered range of kinematics. Most importantly, our calculation correctly reproduces the rise of the FSI in the forward direction of the slow nucleon production angle. By fitting our calculation to the data we extracted the W and Q(2) dependencies of the total cross section and slope factor of the interaction of DIS products, X, off the spectator nucleon. This analysis shows the XN-scattering cross section rising with W and decreasing with an increase of Q(2). Finally, our analysis points at a largely suppressed off-shell part of the rescattering amplitude.

Color transparency and short-range correlations in exclusive pion photo- and electroproduction from nuclei

A relativistic and quantum mechanical framework to compute nuclear transparencies for pion photo- and electroproduction reactions is presented. Final-state interactions for the ejected pions and nucleons are implemented in a relativistic eikonal approach. At sufficiently large ejectile energies, a relativistic Glauber model can be adopted. At lower energies, the framework possesses the flexibility to use relativistic optical potentials. The proposed model can account for the color-transparency (CT) phenomenon and short-range correlations (SRC) in the nucleus. Results are presented for kinematics corresponding to completed and planned experiments at Jefferson Lab. The influence of CT and SRC on the nuclear transparency is studied. Both the SRC and CT mechanisms increase the nuclear transparency. The two mechanisms can be clearly separated, though, as they exhibit a completely different dependence on the hard-scale parameter. The nucleon and pion transparencies as computed in the relativistic Glauber approach are compared with optical-potential and semiclassical calculations. The similarities in the trends and magnitudes of the computed nuclear transparencies indicate that they are not subject to strong model dependences.

Counting the number of correlated pairs in a nucleus

We suggest that the number of correlated nucleon pairs in an arbitrary nucleus can be estimated by counting the number of proton-neutron, proton-proton, and neutron-neutron pairs residing in a relative S state. We present numerical calculations of those amounts for the nuclei {sup 4}He, {sup 9}Be, {sup 12}C, {sup 27}Al, {sup 40}Ca, {sup 48}Ca, {sup 56}Fe, {sup 63}Cu, {sup 108}Ag, and {sup 197}Au. The results are used to predict the values of the ratios of the per-nucleon electron-nucleus inelastic scattering cross section to the deuteron in the kinematic regime where correlations dominate.

Neutron spin structure with polarized deuterons and spectator proton tagging at EIC

The neutrons deep-inelastic structure functions provide essential information for the flavor separation of the nucleon parton densities, the nucleon spin decomposition, and precision studies of QCD phenomena in the flavor-singlet and nonsinglet sectors. Thus, traditional inclusive measurements on nuclear targets are limited by dilution from scattering on protons, Fermi motion and binding effects, final-state interactions, and nuclear shadowing at x << 0.1. An Electron-Ion Collider (EIC) would enable next-generation measurements of neutron structure with polarized deuteron beams and detection of forward-moving spectator protons over a wide range of recoil momenta (0 < pR << several 100 MeV in the nucleus rest frame). The free neutron structure functions could be obtained by extrapolating the measured recoil momentum distributions to the on-shell point. The method eliminates nuclear modifications and can be applied to polarized scattering, as well as to semi-inclusive and exclusive final states. We review the prospects for neutron structure measurements with spectator tagging at EIC, the status of R&D efforts, and the accelerator and detector requirements.

Final-state interactions in inclusive deep-inelastic scattering from the deuteron

We explore the role of final-state interactions (FSIs) in inclusive deep-inelastic scattering from the deuteron. Relating the inclusive cross section to the deuteron forward virtual Compton scattering amplitude, a general formula for the FSI contribution is derived in the generalized eikonal approximation, utilizing the diffractive nature of the effective hadron-nucleon interaction. The calculation uses a factorized model with a basis of three resonances with mass W<2 GeV and a continuum contribution for larger W as the relevant set of effective hadron states entering the final-state interaction amplitude. The results show sizeable on-shell FSI contributions for Bjorken x≳0.6 and Q2≲10 GeV^2, increasing in magnitude for lower Q2, but vanishing in the high-Q2 limit because of phase-space constraints. The off-shell rescattering contributes at x≳0.8 and is taken as an uncertainty on the on-shell result.

Density dependence of quasifree single-nucleon knockout reactions

We address the issue whether quasifree single-nucleon knockout measurements carry sufficient information about the nuclear interior. To this end, we present comparisons of the reaction probability densities for A(e, e′p) and A(p, 2p) in quasifree kinematics for the target nuclei He, C, Fe, and Pb. We adopt a comprehensive framework based on the impulse approximation and on a relativized extension of Glauber multiple-scattering reaction theory in which the medium effects related to short-range correlations (SRC) are implemented. It is demonstrated that SRC weaken the effect of attenuation. For light target nuclei, both the quasifree (p, 2p) and (e, e′p) can probe average densities of the same order as nuclear saturation density ρ0. For heavy nuclei like Pb, the probed average densities are smaller than 0.1ρ0 and the (e, e ′p) reaction is far more efficient in probing the bulk regions than (p, 2p).

Quasielastic contribution to antineutrino-nucleus scattering

We report on a calculation of cross sections for charged-current quasielastic antineutrino scattering off 12C in the energy range of interest for the MiniBooNE experiment. We adopt the impulse approximation (IA) and use the nonrelativistic continuum random phase approximation (CRPA) to model the nuclear dynamics. An effective nucleon-nucleon interaction of the Skyrme type is used. We compare our results with the recent MiniBooNE antineutrino cross-section data and confront them with alternate calculations. The CRPA predictions reproduce the gross features of the shape of the measured double-differential cross sections. The CRPA cross sections are typically larger than those of other reported IA calculations but tend to underestimate the magnitude of the MiniBooNE data. We observe that an enhancement of the nucleon axial mass in CRPA calculations is an effective way of improving on the description of the shape and magnitude of the double-differential cross sections. The rescaling of MA is illustrated to affect the shape of the double-differential cross sections differently than multinucleon effects beyond the IA.

Factorization of exclusive electron-induced two-nucleon knockout

We investigate the factorization properties of the exclusive electroinduced two-nucleon knockout reaction A(e,e′pN). A factorized expression for the cross section is derived and the conditions for factorization are studied. The A(e,e′pN) cross section is shown to be proportional to the conditional center-of-mass (c.m.) momentum distribution for close-proximity pairs in a state with zero relative orbital momentum and zero radial quantum number. The width of this conditional c.m. momentum distribution is larger than the one corresponding with the full c.m. momentum distribution. It is shown that the final-state interactions (FSIs) only moderately affect the shape of the factorization function for the A(e,e′pN) cross sections. Another prediction of the proposed factorization is that the mass dependence of the A(e,e′pp) [A(e,e′pn)] cross sections is much softer than Z(Z−1)/2 [NZ].

Nuclear transparencies from photoinduced pion production

The strong interaction exhibits a strong scale dependence. At low energies, hadrons are undoubtedly the adequate degrees of freedom. The properties of nuclei can be fairly well understood in a picture in which nucleons exchange mesons. At high energies, that particular role of fermions interacting through force-carrying bosons is played by quarks and gluons. The transition energy region is a topic of current intensive research at, for example, the Thomas Jefferson National Accelerator Facility (Jefferson Lab), where hadronic matter can be studied with intense beams of real and virtual photons possessing the proper wavelengths in the femtometer and subfemtometer range. An observable commonly used to pin down the underlying dynamics of hadronic matter is the nuclear transparency to the transmission of hadrons. The nuclear transparency for a certain reaction process is defined as the ratio of the cross section per target nucleon to the one for a free nucleon. Accordingly, the transparency is a measure of the effect of the medium on the passage of energetic hadrons. It provides an excellent tool for searching for deviations from predictions of models based on traditional nuclear physics. One such phenomenon is color transparency (CT). Color transparency predicts the reduction of final state interactions (FSI) of hadrons propagating through nuclear matter in processes at high momentum transfer. Experiments have been carried out to measure nuclear transparencies in search of CT