Nucleon Knockout Research Articles

Survey of [formula omitted] asymmetries in semi-exclusive electron scattering on 4He and 12C

Single spin azimuthal asymmetries A L T ′ were measured at Jefferson Lab using 2.2 and 4.4 GeV longitudinally polarised electrons incident on 4He and 12C targets in the CLAS detector. A L T ′ is related to the imaginary part of the longitudinal-transverse interference and in quasifree nucleon knockout it provides an unambiguous signature for final state interactions (FSI). Experimental values of A L T ′ were found to be below 5%, typically | A L T ′ | ⩽ 3 % for data with good statistical precision. Optical model in eikonal approximation (OMEA) and relativistic multiple-scattering Glauber approximation (RMSGA) calculations are shown to be consistent with the measured asymmetries.

Identifying neutrinos and antineutrinos in neutral-current scattering reactions.

We study neutrino-induced nucleon knockout from nuclei. Expressions for the induced polarization are derived within the framework of the independent-nucleon model and the nonrelativistic plane-wave approximation. Large dissimilarities in the nucleon polarization asymmetries are observed between neutrino- and antineutrino-induced processes. These asymmetries represent a potential way to distinguish between neutrinos and antineutrinos in neutral-current neutrino scattering on nuclei. We discuss astrophysical applications of these polarization asymmetries. Our findings are illustrated for neutrino scattering on 16O and 208P b.

Coulomb distortion effects for electron or positron induced (e,e′) reactions in the quasielastic region

In response to recent experimental studies we investigate Coulomb distortion effects on $(e,e')$ reactions from medium and heavy nuclei for the case of electrons and positrons. We extend our previously reported full DWBA treatment of Coulomb distortions to the case of positrons for the $^{208}Pb(e,e')$ reaction in the quasielastic region for a particular nuclear model. In addition, we use previously reported successful approaches to treating Coulomb corrections in an approximate way to calculate the Coulomb distortion effects for $(e,e')$ reactions for both electrons and positrons for the case of a simple nuclear model for quasielastic knock-out of nucleons. With these results in hand we develop a simple {\em ad-hoc} approximation for use in analyzing experiments, and discuss methods of extracting the ``longitudinal structure function" which enters into evaluation of the Coulomb sum rule. These techniques are generally valid for lepton induced reactions on nuclei with momentum transfers greater than approximately 300 $MeV/c$.

Fermi hypernetted-chain calculation of the half-diagonal two-body density matrix of model nuclear matter

The half-diagonal two-body density matrix ρ2h(r1,r2,r1′) is a key quantity in theoretical descriptions of nucleon knockout reactions at intermediate energies, due to its central role in treatments of the propagation and final-state interactions of ejected nucleons in the nuclear medium. This quantity is calculated for a simple model of the ground state of infinite symmetrical nuclear matter, based on a Jastrow-correlated wave function and a Fermi hypernetted-chain analysis. The dependence of ρ2h on the variables r11′=|r1−r1′|, r12=|r1−r2|, and the angle θ between r1−r1′ and r1−r2 is investigated in some detail. Significant departures from ideal Fermi gas behavior in certain domains reflect the importance of short-range correlations and their interplay with statistical correlations. The quality of approximations to ρ2h proposed by Gersch et al., Rinat, and Silver is assessed by comparison with the Fermi hypernetted-chain results.

Nuclear structure experiments with unstable fragment beams at GSI

Nuclear structure results from recent experiments at GSI using secondary beams of neutron-rich nuclei are presented. Single nucleon knockout reactions from two-neutron halo nuclei and Coulomb dissociation of unstable neutron rich oxygen isotopes are addressed in particular.

Separated spectral functions for the quasifree12C(e,e′p)reaction

A separation of the longitudinal and transverse 12 C(e, e8p) cross sections in the quasifree region has been performed in parallel kinematics at Q 2 of 0.64 and 1.8 GeV 2 for initial proton momentum ,80 MeV. The separated transverse and longitudinal spectral functions at Q 2 50.64 GeV 2 show significant differences for missing energy between 25 and 60 MeV indicating a breakdown in the single nucleon knockout picture. The transverse spectral functions exhibit definite momentum transfer dependence.

Supermultiplet symmetry and near-threshold levels in systems featuring two or three extremely light clusters

On the basis of the supermultiplet potential model for the interaction of extremely light clusters (formalism of Young diagrams), it is explained why the d+t system does not feature a (1/2)+(L=0) level allied to (3/2)+(L=0) thermonuclear resonance. By using known data on the quasielastic knockout of nucleons from the inner1s shell of light nuclei, a systematics of near-threshold states in the t+d+d, t+t+d, and t+t+t systems is constructed in terms of excited Young diagrams like {f}={322}. The emergence of similar states in going over from the d+d and t+t systems (which feature no such states) to the α+d+d, α+t+d, and α+t+t systems (where they appear owing to the binding effect of the alpha particle) is discussed. The three-cluster states being considered may be important not only for nuclear physics proper but also for nuclear astrophysics.

Orthogonality effects in relativistic models of nucleon knockout reactions

We study the effect of wave function orthogonality in the relativistic treatment of the nucleon removal reactions γ,p and e,e ′p . The continuum wave function describing the outgoing nucleon is made orthogonal to the relevant bound states using the Gram–Schmidt procedure. This procedure has the advantage of preserving the asymptotic character of the continuum wave function and hence the elastic observables are unaffected. The orthogonality effects are found to be negligible for e,e ′p reactions for missing momenta up to 700 MeV/c . This holds true for both parallel and perpendicular kinematics. By contrast the orthogonalization of the wave functions appears to have a more pronounced effect in the case of γ,p reactions. We find that the orthogonality effect can be significant in this case particularly for large angles. Polarization of the outgoing protons and photon asymmetry show more sensitivity than the cross sections. If the orthogonality condition is imposed solely on this one hole state the effects are usually smaller.

In-medium NN interactions and nucleon and meson masses studied with nucleon knockout reactions

In-medium NN interactions and nucleon and meson masses studied with nucleon knockout reactions

Channel coupling inA(e→,e′N→)Breactions

The sensitivity of momentum distributions, recoil polarization observables, and response functions for nucleon knockout by polarized electrons to channel coupling in final-state interactions is investigated using a model in which both the distorting and the coupling potentials are constructed by folding density-dependent effective interactions with nuclear transition densities. Calculations for $^{16}$O are presented for 200 and 433 MeV ejectile energies, corresponding to proposed experiments at MAMI and TJNAF, and for $^{12}$C at 70 and 270 MeV, corresponding to experiments at NIKHEF and MIT-Bates. The relative importance of charge exchange decreases as the ejectile energy increases, but remains significant for 200 MeV. Both proton and neutron knockout cross sections for large recoil momenta, $p_m > 300$ MeV/c, are substantially affected by inelastic couplings even at 433 MeV. Significant effects on the cross section for neutron knockout are also predicted at smaller recoil momenta, especially for low energies. Polarization transfer for proton knockout is insensitive to channel coupling, even for fairly low ejectile energies, but polarization transfer for neutron knockout retains nonnegligible sensitivity to channel coupling for energies up to about 200 MeV. The present results suggest that possible medium modifications of neutron and proton electromagnetic form factors for $Q^2 \gtrsim 0.5 (GeV/c)^2$ can be studied using recoil polarization with relatively little sensitivity due to final state interactions.

Fragmentation of secondary beams leading to unbound nuclear subsystems of exotic nuclei

Results from recent experiments at GSI using energetic secondary beams of neutron-rich halo nuclei are summarized. Nuclear structure information is deduced from nucleon knockout reactions and inelastic excitations into the continuum.

A new phenomenon in heavy ion inelastic scattering: the towing mode

The inelastic scattering of Ar-40 on Ni-58 has been studied at 44 MeV per nucleon incident energy in coincidence with light particle emission. Besides the well known mechanisms of inelastic excitation, pick up break up and nucleon knock out, a new phenomenon has been observed, giving rise to fast forward moving particles with specific angular correlations. This newly observed mechanism seems to be a generic phenomenon present for various projectile-target combinations and incident energies. Its contribution to the inelastic spectrum has been extracted and a tentative interpretation is given. (C) 1998 Elsevier Science B.V. All rights reserved.

Spin-dependent scattering of deeply bound nucleons

Exclusive measurements of the analyzing power and two spin-transfer observables for nucleon knockout from an ${}^{16}\mathrm{O}$ target are presented, at kinematic conditions chosen to emphasize interactions in the nuclear interior. The analyzing power data are substantially reduced in comparison with values calculated in the distorted wave impulse approximation (DWIA) using the free nucleon-nucleon interaction, particularly for knockout of the deeply bound ${1s}_{1/2}$ nucleons. Inclusion of density dependence for the interaction in the calculations improves the agreement with the data, but does not provide a satisfactory description for nucleon knockout from ${}^{16}\mathrm{O}.$ Spin-orbit distortions are shown to strongly affect the DWIA predictions of the ${1s}_{1/2}$ analyzing powers over most of the experimental kinematic range, but notably not near the points of negligible recoil momentum. Hence these data offer constraints on the optical potentials and independently on the two-body effective interaction.

Spin Observables in Coincidence Electron Scattering from Nuclei: I. Reduced Response Functions

A theoretical description of nucleon knockout reactions initiated by polarized electron scattering from polarized nuclei is presented. Explicit expressions for the complete set of reduced response functions (independent of the polarization angle) that can be experimentally obtained assuming plane waves for the electron are given in a general multipole expansion. The formalism is applied to the particular case of closed-shell-minus-one nuclei using two models for the ejected nucleon, including the final-state interaction phenomenologically with a complex optical potential and in the factorized plane-wave impulse approximation. Relativistic effects in the kinematics and in the electromagnetic current are incorporated throughout—specifically a new expansion of the electromagnetic current in powers only of the struck nucleon momentum is employed. Results are presented for the nucleus39K.

Spectroscopic factors for nucleon knock-out fromO16at small missing energy

Spectroscopic factors for one-nucleon knock-out from $^{16}\mathrm{O}$ are calculated for states with low excitation energy in $^{15}\mathrm{N}$ with the Bonn-C potential. A method is proposed to deal with both short- and long-range correlations consistently. For this purpose a Green's function formalism is used and the self-energy in the Dyson equation is approximated as the sum of an energy-dependent Hartree-Fock (HF) term and dispersion and correlation terms of higher order in the G-matrix interaction. This G matrix is obtained by solving the Bethe-Goldstone equation with a Pauli operator which excludes just the model space treated in the subsequent calculation of the self-energy. The energy dependence of the HF energies induces an additional reduction of the spectroscopic factors for quasiparticle states close to the Fermi level by about 10%. Experimental data may signal the need of some further improvement in the treatment of intermediate- and long-range correlations. \textcopyright{} 1996 The American Physical Society.

Description of inclusive scattering of few-GeV electrons from finite A nuclei and nuclear matter

We study $eA$ scattering in a model where the total nuclear structure function is a convolution of components $W_N$ for an isolated nucleon and $W_{\NA}$ for a nucleus composed of point-nucleons. $W_N$ is represented by its elastic and lowest-energy inelastic parts, while $W_{\NA}$ is computed from its asymptotic limit, supplemented by final state interactions due to binary collisions between the knocked-out and core nucleons. $(q/M)W_{\NA}$ per nucleon appears to be practically independent of mass number $A$ and momentum transfer $q$. Consequently, predicted inclusive cross sections per nucleon for given kinematic conditions hardly depend on the target and the same can be extracted from the data. Exceptions are low energy loss regions, where relatively small cross sections are shown to be sensitive to details of the single nucleon momentum distribution and to FSI, involving multiple collisions. The agreement with the data is good.

Effect of NN correlations on predictions of nuclear transparencies for protons, knocked out in high Q2 ( e, e′ p) reactions

We study the transparency T of nuclei for nucleons knocked out in high-energy semi-inclusive ( e, e′ p) reactions, using an improved theoretical input, discussed by Nikolaev et al. We establish that neglectof NN correlations between the knocked-out and core nucleons reduces nuclear transparencies by ≈ 15% for light, to ≈ 10% for heavy nuclei. About the same is predicted for transparencies, integrated over the transverse or longitudinal momentum of the outgoing proton. Hadron dynamics predicts a roughly constant T beyond Q 2 ≈ 2 GeV 2, whereas for all targets the largest measured data point Q 2 = 6.7 GeV 2 appears to lie above that plateau. Large error bars on those data points preclude a conclusion regarding the onset of colour transparency.

Momentum and energy distributions of nucleons in finite nuclei due to short-range correlations.

The influence of short-range correlations on the momentum and energy distribution of nucleons in nuclei is evaluated assuming a realistic meson-exchange potential for the nucleon-nucleon interaction. Using the Green-function approach the calculations are performed directly for the finite nucleus $^{16}$O avoiding the local density approximation and its reference to studies of infinite nuclear matter. The nucleon-nucleon correlations induced by the short-range and tensor components of the interaction yield an enhancement of the momentum distribution at high momenta as compared to the Hartree-Fock description. These high-momentum components should be observed mainly in nucleon knockout reactions like $(e,e'p)$ leaving the final nucleus in a state of high excitation energy. Our analysis also demonstrates that non-negligible contributions to the momentum distribution should be found in partial waves which are unoccupied in the simple shell-model. The treatment of correlations beyond the Brueckner-Hartree-Fock approximation also yields an improvement for the calculated ground-state properties.

Spin Dependent Momentum Distributions in Deformed Nuclei

We study the properties of the spin-dependent one-body density in momentum space for odd- A polarized deformed nuclei within the mean field approximation. We derive analytic expressions connecting intrinsic and laboratory momentum distributions. The latter are related to observable transition densities in p-space that can be proved in one nucleon knockout reactions from polarized targets. It is shown that most of the information contained in the intrinsic spin-dependent momentum distribution is lost when the nucleus is not polarized. Results are presented and discussed for two prolate nuclei, 21Ne and 25Mg, and for one oblate nucleus, 37Ar. The effects of deformation are highlighted by comparison to the case of odd- A nuclei in the spherical model.

Short range correlations and spectral functions for nuclear matter and finite nuclei

The influence of short-range correlations on the momentum and energy distribution of nucleons in nuclear matter and finite nuclei is investigated using realistic nucleon-nucleon interactions. The calculations are performed in the framework of the Green-function theory. The spectral functions are calculated from the momentum and energy dependent self-energy by solving the Dyson equation. The nucleon-nucleon correlations induced by the short-range and tensor components of the interaction yield an enhancement of the momentum distribution at high momenta as compared to the Hartree-Fock description. These high-momentum components should be observed mainly in nucleon knockout reactions like ( e, e′ p) leaving the final nucleus in a state of high excitation energy.