Electron-induced Proton Knockout Research Articles

Search for a bound di-neutron by comparing 3He(e,e'p)d and 3H(e,e'p)X measurements

We report on a search for a bound di-neutron by comparing electron-induced proton-knockout (e,e′p) measurements from Helium-3 (3He) and Tritium (3H). The measurements were performed at Jefferson Lab Hall A with a 4.326 GeV electron beam, and kinematics of large momentum transfer (〈Q2〉≈1.9 (GeV/c)2) and xB>1, to minimize contributions from non quasi-elastic (QE) reaction mechanisms. Analyzing the measured 3He missing mass (Mmiss) and missing energy (Emiss) distributions, we can distinguish the two-body break-up reaction, in which the residual proton-neutron system remains bound as a deuteron. In the 3H mirror case, under the exact same kinematic conditions, we do not identify a signature for a bound di-neutron with similar binding energy to that of the deuteron. We calculate exclusion limits as a function of the di-neutron binding energy and find that, for binding equivalent to the deuteron, the two-body break-up cross section on 3H is less than 0.9% of that on 3He in the measured kinematics at the 95% confidence level. This limit implies that the di-neutron content of the tritium spectral function is less than 1.5%. With a dedicated measurement using similar high resolution spectrometers, but lower beam energy and vacuum coupling, significantly better energy missing energy resolution could be achieved, extending the sensitivity of the method to search for a di-neutron with far smaller binding energy.

Electron-induced proton knockout from neutron rich nuclei

We study the evolution of the (e,e'p) cross section on nuclei with increasing asymmetry between the number of neutrons and protons. The calculations are done within the framework of the nonrelativistic and relativistic distorted-wave impulse approximation. In the nonrelativistic model phenomenological Woods-Saxon and Hartree-Fock wave functions are used for the proton bound-state wave functions, in the relativistic model the wave functions are solutions of Dirac-Hartree equations. The models are first tested against experimental data on 40 Ca and 48Ca nuclei, and then they are applied to a set of spherical calcium isotopes.

Exploring Correlations in Exotic Nuclei

The difference in neutron total cross section between 40 Ca and 48 Ca is sensitive to the n/p asymmetry dependence of the surface imaginary potential in a dispersive optical model analysis, from which spectroscopic information can be extracted. The measured cross sections imply that the strength of neutron correlations changes little as neutron number increases in neutron-rich isotopes. In contrast, for the light-nucleus-induced knockout of the valence neutron in 36 Ca, the small experimental knockout cross section (as compared to an eikonal reaction theory) implies a very small spectroscopic factor (and a strong trend in correlations with asymmetry). If this type of knockout analysis is correct, it implies that standard shellmodel calculations miss spectroscopic strength as the Fermi energy nears the continuum. If this is not the case, it may be that there is a problem with the application of the eikonal theory to very deeply bound nucleons at these intermediate energies. The term correlations refers to interactions between nucleons that go beyond the mean field considered in an independent-particle-model (IPM) approach. The effect of these correlations is that in real nuclei, the spectroscopic strength of a singleparticle orbital is fragmented over energy. The spectroscopic factor (SF) quantifies the strength found at a discrete energy, and the sum of spectroscopic strength over all energies below the Fermi energy EF gives the occupation of the orbital. Correlations reduce the occupation of a single-particle state relative to its IPM value, since some of the fragmented spectroscopic strength is shifted above EF . The nuclear shell model (SM) can account for some of these correlations by including mixing between single-particle states (within a finite basis set). But due to the finite model space, the presence of the hard-core of the N-N interaction, and the tensor interaction (which further reduce the occupancies by shifting strength to high-momentum states), standard SM calculations still overestimate the occupancies of bound states. The reduction in occupancy due to correlations is not a direct experimental observable, but it will affect particle removal cross sections. By comparing experimental cross sections to calculated single-particle cross sections, one can learn about the strength of correlations. Electron-induced proton knockout (e,e’p) experiments have indicated a roughly constant 30% reduction in spectroscopic strength for valence protons in beta-stable nuclei. For unstable nuclei, light-nucleus-induced knockout experiments in inverse kinematics suggest that the affect of correlations is weaker for weakly-bound nucleons and much stronger for very deeply-bound nucleons (e.g.

Formula omitted] reaction at large missing energy

We investigate the origin of the strength at large missing energies in electron-induced proton knockout reactions. For that purpose the reaction O16(e,e′p) was studied at a central value ω=215 MeV of the energy transfer, and two values of the momentum transfer: q=300,400 MeV/c, corresponding to the “dip region”. Differential cross sections were determined in a large range of missing energy (Em=0–140 MeV) and proton emission angle (γpq=0–110°), and compared to predictions of a model that includes nucleon–nucleon short-range correlations and two-body currents. It is observed that, in the kinematic domain covered by this experiment, the largest contribution to the cross section stems from two-body currents, while short-range correlations contribute a significant fraction.

Spectroscopic information from different theoretical descriptions of (un)polarized (e, e′p) reactions

We analyze the unpolarized and polarized electron-induced proton knockout reactions on 16O in different kinematical settings using two theoretical approaches. The first one is based on a relativistic mean-field distorted-wave description of the bound and scattering states of the proton, including a fully relativistic electromagnetic current operator. The second approach adopts the same current operator, but describes the proton properties on the basis of microscopic calculations of the self-energy in 16O below the Fermi energy and final-state damping in nuclear matter above the Fermi energy, using the same realistic short-range and tensor correlations. Good agreement with all unpolarized data is obtained at low and high Q2 by using the same spectroscopic factors fixed by the low-Q2 analysis. A reasonable agreement is achieved for polarization observables.

Electron helicity-dependence in (e,e′p) reactions with polarized nuclei and the fifth response function

The cross section for electron-induced proton knockout reactions with polarized beam and target is computed in a DWIA model. The analysis is focused on the electron helicity-dependent (HD) piece of the cross section. For the case of in-plane emission, a new symmetry is found in the HD cross section, which takes opposite values for specific opposing pairs of nuclear orientations, explaining why the fifth response (involving polarized electrons, but unpolarized nuclei) is zero for this choice of kinematics. Even when this symmetry breaks down in the case of nucleon emission out of the scattering plane, it is shown that it is still present in the separate response functions. A physical explanation of these results is presented using a semiclassical model of the reaction based on the PWIA and on the concept of a nucleon orbit with its implied expectation values of position and spin.

Quasielastic Scattering from Relativistic Bound Nucleons: Transverse-Longitudinal Response

Predictions for electron induced proton knockout from ${p}_{1/2}$ and ${p}_{3/2}$ shells in ${}^{16}\mathrm{O}$ are presented using various approximations for the relativistic nucleonic current. Results for differential cross section, transverse-longitudinal response ( ${R}_{\mathrm{TL}}$), and left-right asymmetry ${A}_{\mathrm{TL}}$ are compared at $|{Q}^{2}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.8(\mathrm{GeV}/c{)}^{2}$. We show that there are important dynamical and kinematical relativistic effects which can be tested by experiment.

Radial Dependence of the Nucleon Effective Mass in10B

The dynamic properties of the atomic nucleus depend strongly on correlations between the nucleons. We present a combined analysis of inelastic electron-scattering data and electron-induced proton knockout measurements in an effort to obtain phenomenological information on nucleon-nucleon correlations. Our results indicate that the ratio of radial wave functions extracted from precise ${}^{10}\mathrm{B}({e,e}^{\ensuremath{'}})$ and ${}^{10}\mathrm{B}({e,e}^{\ensuremath{'}}p)$ measurements evolve from an interior depression for small ${E}_{m}$, characteristic of short-range correlations, to a surface-peaked enhancement for larger ${E}_{m}$, characteristic of long-range correlations. This observation can be interpreted in terms of the nucleon effective mass.

Electron-induced proton knock-out from 30Si, 31P and 32S

Electron-induced proton knock-out from the isotones 30Si, 31P and 32S is studied. The extracted r.m.s. radii of the bound-state wave functions, momentum distributions and spectral functions are presented and the systematics are discussed. The momentum distributions and r.m.s. radii for knock-out from the 2s12 and 1d52 orbitals are found to be similar for all three isotones. The 2s12 orbit is filled smoothly in going from 30Si to 32S, but it is neither completely empty nor completely filled in 30Si and 32S, respectively. The experimental results are compared to shell-model calculations and to the results of a quasi-particle phonon model calculation. Although the distribution of spectroscopic strength as a function of energy can be reproduced reasonably well by the latter model, its value, integrated up to 24 MeV excitation energy, is not compatible with the model value.