14 research outputs found
Relativistic eikonal approximation in high-energy A(e,e'p) reactions
A fully relativistic model for the description of exclusive (e,e'p) reactions
off nuclear targets at high energies and momentum transfers is outlined. It is
based on the eikonal approximation for the ejectile scattering wave function
and a relativistic mean-field approximation to the Walecka model. Results for
^{12}C(e,e'p) and ^{16}O(e,e'p) differential cross sections and separated
structure functions are presented for four-momenta in the range 0.8 \leq Q^{2}
\leq 20 (GeV/c)^{2}. The regions of applicability of the eikonal approximation
are studied and observed to be confined to proton knockout in a relatively
small cone about the momentum transfer. A simple criterium defining the
boundaries of this cone is determined. The Q^2 dependence of the effect of
off-shell ambiguities on the different (e,e'p) structure functions is
addressed. At sufficiently high values of Q^2 their impact on the cross
sections is illustrated to become practically negligible. It is pointed out
that for the whole range of Q^2 values studied here, the bulk of the
relativistic effects arising from the coupling between the lower components in
the wave functions, is manifesting itself in the longitudinal-transverse
interference term.Comment: 13 pages,11 figure
The role of hyperon resonances in p(gamma,K+)Lambda processes
We discuss the role of hyperon resonances in the u-channel when modeling p
(gamma,K+)Lambda processes in an effective Lagrangian approach. Without the
introduction of hyperon resonances, one is forced to use soft hadronic form
factors with a cutoff mass which is at best two times the kaon mass. After
inclusion of the hyperon resonances in the u-channel, we obtain a fair
description of the data with a cutoff mass of the order of 1.8 GeV.Comment: 7 pages, 7 eps figures, submitted to Eur. Phys. J.
Electroinduced two-nucleon knockout and correlations in nuclei
We present a model to calculate cross sections for electroinduced two-nucleon
emission from finite nuclei. Short-range correlations in the wave functions and
meson-exchange contributions to the photoabsorption process are implemented.
Effects of the short-range correlations are studied with the aid of a
perturbation expansion method with various choices of the Jastrow correlation
function. The model is used to investigate the relative importance of the
different reaction mechanisms contributing to the A(e,epn) and A(e,epp)
process. Representative examples for the target nuclei C and O
and for kinematical conditions accessible with contemporary high-duty cycle
electron accelerators are presented. A procedure is outlined to calculate the
two-nucleon knockout contribution to the semi-exclusive (e,ep) cross
section. Using this technique we investigate in how far semi-exclusive
(e,ep) reactions can be used to detect high-momentum components in the
nuclear spectral function.Comment: 51 pages, Latex, uses epsf.sty and elsart.sty, 17 figures (in eps
format
Polarization degrees of freedom in photoinduced two-nucleon knockout from finite nuclei
The polarization degrees of freedom in photoinduced two-nucleon knockout from
finite nuclei are studied. It is pointed out that they open good perspectives
to study the dynamics of dinucleons in the medium in detail. The ()
and () angular cross sections, photon asymmetries and outgoing
nucleon polarizations are calculated for the target nuclei O and
C and photonenergies ranging from 100 up to 500 MeV. It is investigated
to which degree the two-nucleon emission reaction is dominated by
photoabsorption on proton-neutron and proton-proton
pairs in the nuclear medium. The calculations demonstrate that dominance of
wave photoabsorption in the () channel does not necessarily imply
that the reaction mechanism is similar to what is observed in deuteron
photodisintegration.Comment: 27 pages, REVTeX 3.0 with epsf.sty, 11 figures in EPS forma
Meson and Isobar Degrees of Freedom in A() reactions at
The effect of meson and isobar degrees of freedom in A()
and A(e,e'n) is studied for four-momentum transfers Q^2 in the range between
0.2 and 0.8 (GeV/c)^2. The calculations are performed in a non-relativistic
framework with explicit (N,\Delta,\pi) degrees-of-freedom. For the whole range
of momentum transfers under investigation the relative effect of the
meson-exchange and isobar degrees of freedom is significant. At low missing
momenta and quasi-elastic conditions, a tendency to reduce the (e,e'p) and
(e,e'n) differential cross sections is noticed. The greatest sensitivity is
found in the interference structure functions and . The recoil
polarization observables, on the other hand, are moderately affected by the
meson-exchange and -isobar currents.Comment: 16 pages (Revtex) + 18 figures (eps file
The
Differential cross-sections for exclusive 16O(e,e‘pp)14C processes are computed within a distorted-wave framework which includes central short-range correlations and intermediate excitations. The cross-sections are compared to high-resolution data from the MAMI facility at Mainz for a central energy and momentum transfer of MeV and MeV, respectively. A fair agreement between the numerical calculations and data is reached when using spectroscopic information extracted from a 15N C experiment. The comparison between the calculations and the data provides additional evidence that short-range correlations exclusively affect nucleon pairs with a small center-of-mass momentum residing in a relative S state
Tensor correlations in nuclei and exlusive electron scattering
The effect of tensor nucleon-nucleon correlations upon exclusive and semi-exclusive electronuclear reactions is studied. Differential cross sections for the semi-exclusive 16 O(e, e ′ p) and exclusive 16 O(e,e ′ pn) processes are computed by explicitly evaluating the dynamical electromagnetic coupling to a tensor correlated nucleon pair. In both reaction channels the tensor correlations contribute in a very substantial way. Tensor correlations are found to generate more electronuclear strength than central Jastrow correlations do. In the history of nuclear physics, it has been notoriously difficult to detect signals that directly point towards phenomena beyond the scope of the effective mean-field theories. This holds in particular for the short-range correlations that reflect the remnants of the hard-core part of the nucleon-nucleon (NN) force in th