arXiv subjects

Wim Van Nespen

Publications and source records attributed to Wim Van Nespen.

5 recordsLinked to original sources

arXiv1999

Spin-dependent correlations and the semi-exclusive ^{16}O(e,e'p) reaction

The effect of central, tensor and spin-isospin nucleon-nucleon correlations upon semi-exclusive ^{16}O(e,e'p) reactions is studied for Q^2 and Bjorken x values in the range $0.2 \lesssim Q^2 \lesssim 1.1 (GeV/c)^2$ and 0.15 $\lesssim$ x $\lesssim$ 2. The fully unfactorized calculations are performed in a framework that accounts not only for the dynamical coupling of virtual photons to correlated nucleon pairs but also for meson-exchange and $Δ_{33}$-isobar currents. Tensor correlations are observed to produce substantially larger amounts of semi-exclusive ^{16}O(e,e'p) strength than central correlations do and are predominantly manifest in the proton-neutron knockout channel. With the exception of the $x \approx 2$ case, in all kinematical situations studied the meson-exchange and isobar currents are a strong source of A(e,e'p) strength at deep missing energies. This feature gives the A(e,e'p) strength at deep missing energies a pronounced transverse character.

nucl-th↗

arXiv1999

Polarization degrees of freedom in two-nucleon knockout from finite nuclei

Polarization observables for the A(e,e'NN) and A(γ,NN) reactions are a powerful tool to study nucleon-nucleon correlations in the nuclear medium. In this paper, model calculations for the ^{4}He(e,e'pp), ^{16}O(\vec{e},e'\vec{p}p), ^{16}O(\vec{e},e' \vec{p}n) and ^{12}C(\vecγ,\vec{p}N) reactions are presented. The sensitivity of the differential cross sections and polarization observables to central and spin-dependent nucleon-nucleon correlations is discussed.

nucl-th↗

arXiv1999

Meson and Isobar Degrees of Freedom in A($\vec{e},e'\vec{p}$) reactions at $0.2 \leq Q^2 \leq 0.8 (GeV/c)^2$

The effect of meson and isobar degrees of freedom in A($\vec{e},e'\vec{p}$) 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,Δ,π) 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 $W_{LT}$ and $W_{TT}$. The recoil polarization observables, on the other hand, are moderately affected by the meson-exchange and $Δ$-isobar currents.

nucl-th↗

arXiv1998

Polarization observables in electronuclear two-nucleon knockout

Differential (e,e'pp) measurements are presently recognized as a way of studying short-range correlations in finite nuclei. The ($\vec{e},e'\vec{p}p$) and ($\vec{e},e'\vec{p}n$) differential cross section and polarization observables are studied in a microscopic model that accounts for the short-range correlations, outgoing-nucleon distortions, meson-exchange and $Δ$-isobar currents. It is pointed out that polarization observables represent an attractive alternative for absolute electronuclear two-nucleon knockout measurements. In the polarization transfer $P'_t$ for ($\vec{e},e'\vec{p}p$), the effect of central short-range correlations is predicted to be large while at the same time the final-state interaction effects are small.

nucl-th↗

arXiv1997

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 ($γ,pp$) and ($γ,pn$) angular cross sections, photon asymmetries and outgoing nucleon polarizations are calculated for the target nuclei $^{16}$O and $^{12}$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 $^3S_1(T=0)$ proton-neutron and $^1S_0(T=1)$ proton-proton pairs in the nuclear medium. The calculations demonstrate that dominance of $S$ wave photoabsorption in the ($γ,pn$) channel does not necessarily imply that the reaction mechanism is similar to what is observed in deuteron photodisintegration.

nucl-th↗