Searcharxiv⌕ Search

arXiv subjects

A. Fix

Publications and source records attributed to A. Fix.

At least 73 records · Page 4Linked to original sources
arXiv2006

Incoherent single pion electroproduction on the deuteron with polarization effects

Incoherent pion electroproduction on the deuteron is studied from threshold up to the second resonance region with special emphasis on the influence of final state interaction, in particular on polarization observables. The elementary $γN\toπN$ amplitude is taken from the MAID-2003 model. Final state interaction is included by considering complete rescattering in the final $NN$ and $πN$ subsystems. Their influence on the structure functions governing the semi-exclusive differential cross section, where besides the scattered electron only the produced pion is detected, is investigated in detail. For charged pion-production the effect of $NN$-rescattering is moderate whereas $πN$-rescattering is almost negligible, except very close to threshold. $NN$-rescattering appears much stronger in neutral pion production for which the primary mechanism is the elimination of a significant spurious coherent contribution in the impulse approximation. Sizeable effects are also found in some of the polarization structure functions for beam and/or target polarizations.

nucl-th↗
arXiv2005

Signature of the eta-NN configurations in coherent pi0 photoproduction on the deuteron

The photoproduction of a neutral pion on the deuteron is considered in the energy region around the eta threshold, where a bump-like structure was observed at very backward pion angles. Different dynamical aspects which may be responsible for this phenomenon are analysed within a theoretical frame which includes intermediate eta-NN configurations. The results show in particular, that a three-body treatment of the eta-NN interaction is of special importance.

nucl-th↗
arXiv2005

Incoherent pion photoproduction on the deuteron with polarization observables I: Formal expressions

Formal expressions are developed for the general five-fold differential cross section of incoherent $π$-photoproduction on the deuteron including beam and target polarization. The polarization observables of the cross section are described by various beam, target and beam-target asymmetries for polarized photons and/or polarized deuterons. They are given as bilinear hermitean forms in the reaction matrix elements divided by the unpolarized cross section. In addition, the corresponding observables for the semi-exclusive reaction $\vec d(\vec γ,π)NN$ are also given.

nucl-th↗
arXiv2005

Incoherent pion photoproduction on the deuteron with polarization observables II: Influence of final state rescattering

Incoherent pion photoproduction on the deuteron is studied for photon energies from threshold up to 1 GeV with special emphasis on polarization observables. The elementary $γN\toπN$ amplitude is taken from the MAID model. The influence of final state interactions on total and semi-exclusive cross sections $\vec d(\vecγ,π)NN$ is investigated by including complete rescattering in the final $NN$- and $πN$-subsystems. For charged pion-production the influence of $NN$-rescattering is moderate whereas $πN$-rescattering is almost negligible. Much stronger influences of $NN$-rescattering are seen in neutral pion production, which is due to the elimination of a significant spurious coherent contribution in the impulse approximation. Sizeable effects are also found in some of the beam, target and beam-target asymmetries of the differential cross section.

nucl-th↗
arXiv2005

Double pion photoproduction on nucleon and deuteron

Photoproduction of two pions on nucleon and deuteron is studied for photon energies from threshold up to $E_γ= 1.5$ GeV. For the elementary operator an effective Lagrangean approach is used with resonance and Born contributions. The model parameters are fixed by resonance decay widths and multipole analyses of single pion photoproduction. A satisfactory description of total cross sections of two-pion production on the proton for various charge channels is achieved, except for $π^0π^0$ production for which a significant underestimation is found near threshold. The operator then serves for the evaluation of this reaction on the deuteron in the impulse approximation. In addition, $NN$ rescattering in the final state is taken into account, but $πN$ and $ππ$ rescatterings are neglected. Results are presented for total cross sections and target asymmetries.

nucl-th↗
arXiv2004

Spin asymmetry and GDH sum rule for real and virtual photons for the deuteron

An explicit evaluation of the spin asymmetry of the deuteron and the associated GDH sum rule is presented which includes disintegration, single and double pion and eta production. For the GDH integral a large cancellation is found between the disintegration channel and the meson production channels. Furthermore, first results for the contribution of the disintegration channel to the generalized GDH integral at constant four-momentum transfer reveal a dominance of the isovector M1 transition to the $^1$S$_0$-state near threshold resulting in a negative contribution with a minimum around $Q^2\approx 0.2$ fm$^{-2}$ which is driven by the nucleon anomalous isovector magnetic moment.

nucl-th↗
arXiv2003

Final state interaction in the reaction NN -> eta NN

The production of $η$ mesons in $NN$ collisions in the near threshold region is analyzed with special emphasis on the role of final state interaction which is calculated using three-body scattering theory. The three-body aspects are shown to be very important in interpreting experimental results for the total cross section of $pp\toηpp$. Furthermore, energy spectra and angular distributions of the emitted particles are calculated and compared to experimental data.

nucl-th↗
arXiv2003

Do we understand the $ηN$ interaction from the near threshold $η$ photoproduction on the deuteron?

The effects of final state interaction in incoherent $η$ photoproduction on deuteron are studied within a three-body approach including a realistic $NN$ potential. The results are compared with available data, and differences with other theoretical predictions are analyzed. The role of the $ηN$ interaction and the possibility of extracting the $ηN$ scattering parameters from this reaction are discussed.

nucl-th↗
arXiv2003

Low-energy scattering and photoproduction of $η$-mesons on three-body nuclei

The optical potential approach for low-energy scattering of $η$-mesons on three-body nuclei is compared to an exact treatment of the $η3N$ system using four-body scattering theory with separable interactions in s-waves only. The higher-order terms including the interaction of the struck nucleon with the surrounding nuclear medium and virtual target excitations in between successive $η$-scatterings are found to cause important corrections. Effects of final state interaction in $η$-photoproduction on $^3$H and $^3$He are also studied and sizable contributions beyond the optical model approach are found.

nucl-th↗
arXiv2002

Three-Body Analysis of Incoherent Photoproduction of $η$ Mesons on the Deuteron near Threshold

The importance of three-body dynamics in the $ηnp$ system in elastic and inelastic $η$-deuteron scattering as well as coherent and incoherent $η$ photoproduction on the deuteron in the energy region from threshold up to 30 MeV above has been investigated. It is shown that a restriction to first order rescattering with respect to the $NN$- and $ηN$-final state interactions, i.e., restriction to rescattering in the two-body subsystems, does not give a sufficiently accurate approximation to the s-wave reaction amplitude and that higher order terms, as described by the three-body dynamics give very substantial contributions.

nucl-th↗
arXiv2002

Present status of electromagnetic reactions on the deuteron above pion threshold

The present status of the theoretical description of electromagnetic reactions on the deuteron above pion threshold is reviewed. Three major topics are considered: (i) retardation effects in $π$-meson exchange contributions to $NN$-interaction and meson exchange currents in deuteron photodisintegration above $π$-threshold in the $Δ$-resonance region, (ii) off-shell effects in the one-body current treated in a simple pion cloud model in deuteron photodisintegration in and above the $Δ$-resonance region, and (iii) final state interaction effects in photoproduction of $π$ and $η$ mesons on the deuteron.

nucl-th↗
arXiv2002

The $η$-3N problem with separable interactions

The $η$-3N-interaction is studied within the four-body Faddeev-Yakubovsky theory adopting purely separable forms for the two- and three-body subamplitudes, limiting the basic two-body interactions to s-waves only. The corresponding separable approximation for the integral kernels is obtained by using the Hilbert-Schmidt procedure. Results are presented for the $η$-$^3$H scattering amplitude and for the total elastic cross section for energies below the triton break-up threshold.

nucl-th↗
arXiv2002

Interaction of $η$ mesons with a three-nucleon system

A study of the $η$-3N interaction in the energy region near the $η$-^3H elastic scattering threshold is presented. The calculational scheme is based on the four-body scattering formalism. A manageable form of the integral equations is achieved by using a separable ansatz for the driving two-body $ηN$- and NN-forces as well as for the subamplitudes appearing in the (1+3) and (2+2) partitions of the $η$-3N system. Results presented for the $η$-^3H scattering length point to the existence of a virtual (antibound) s-wave $η$-^3H state. As a consequence, a large enhancement of the cross section for $η$-production from three-body nuclei is predicted.

nucl-th↗
arXiv2001

The $ηNN$-system at low energy within a three-body approach

The role of the $ηNN$-interaction is studied in the low energy regime in $η$-deuteron reactions as well as in coherent and incoherent $η$-photoproduction on the deuteron using a three-body model with separable two-body interactions. The three-body approach turns out to be quite essential in the most important lowest partial wave. Results are presented for differential and total cross sections as well as for the $η$-meson spectrum. They differ significantly from those predicted by a simple rescattering model in which only first-order $ηN$- and $NN$-interactions in the final state are considered. The major features of the experimental data of $η$-photoproduction in the near-threshold region are well reproduced.

nucl-th↗
arXiv2000

Three-body analysis of incoherent $η$-photoproduction on the deuteron in the near threshold region

A three-body calculation of the reaction $γd\toηnp$ in the energy region from threshold up to 30 MeV above has been performed. The primary goal of this study is to assess the importance of the three-body aspects in the hadronic sector of this reaction. Results are presented for the $η$-meson spectrum as well as for the total cross section. The three-body results differ significantly from those predicted by a simple rescattering model in which only first-order $ηN$- and $NN$-interactions in the final state are considered. The major features of the experimental data are well reproduced although right at threshold the rather large total cross section could not be explained.

nucl-th↗
arXiv2000

Analysis of the low-energy $η$NN-dynamics within a three-body formalism

The interaction of an $η$-meson with two nucleons is studied within a three-body approach. The major features of the $ηNN$-system in the low-energy region are accounted for by using a s-wave separable ansatz for the two-body $ηN$- and $NN$-amplitudes. The calculation is confined to the $(J^π;T)=(0^-;1)$ and $(1^-;0)$ configurations which are assumed to be the most promising candidates for virtual or resonant $ηNN$-states. The eigenvalue three-body equation is continued analytically into the nonphysical sheets by contour deformation. The position of the poles of the three-body scattering matrix as a function of the $ηN$-interaction strength is investigated. The corresponding trajectory, starting on the physical sheet, moves around the $ηNN$ three-body threshold and continues away from the physical area giving rise to virtual $ηNN$-states. The search for poles on the nonphysical sheets adjacent directly to the upper rim of the real energy axis gives a negative result. Thus no low-lying s-wave $ηNN$-resonances were found. The possible influence of virtual poles on the low-energy $ηNN$-scattering is discussed.

nucl-th↗
arXiv1998

Photopion reactions on deltas preexisting in nuclei

Reactions A(γ, pi^+p) are considered to proceed through the formation of pion-proton pairs on delta constituents in nuclei. We develop the nonrelativistic operator for Δ^{++}(γ, pi^+)p process in an arbitrary frame. The calculated cross section for C12(γ, π^+p) reaction is compared to the existing experimental data.

nucl-th↗