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H. Arenhoevel

Publications and source records attributed to H. Arenhoevel.

At least 19 recordsLinked to original sources
arXiv2014

Complete set of observables for photoproduction of two pseudoscalars on a nucleon

The problem of determining completely the spin amplitudes of photoproduction of two pseudoscalar mesons on a nucleon from observables is studied. The procedure of reconstruction of the scattering matrix elements from a complete set of observables is based on the expressions of all observables as quadratic hermitean forms in the reaction matrix elements which are derived explicitly. Their inversion allows one to find explicit solutions for the reaction matrix elements in terms of observables. Two methods for finding a complete set of observables are presented. In particular, one set was found, which does not contain a triple polarization observable.

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arXiv2013

A Truncated partial wave analysis of a complete experiment for photoproduction of two pseudoscalar mesons on a nucleon

A truncated partial wave analysis for the photoproduction of two pseudoscalar mesons on a nucleon is discussed with respect to the determination of a complete set of observables. For the selection of such a set we have applied a criterion previously developed for photo- and electrodisintegration of a deuteron, which allows one to find a 'minimal' set of observables for determining the partial wave amplitudes up to possible discrete ambiguities. The question of resolving the remaining ambiguities by invoking additional observables is discussed for the simplest case, when the partial wave expansion is truncated at the lowest total angular momentum of the final state $J_{max}=1/2$. The resulting 'fully' complete set, allowing an unambiguous determination of the partial wave amplitudes, is presented.

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arXiv2008

Parity Violating Effects in Elastic Electron Deuteron Scattering

The general expressions for parity violation observables in elastic scattering of polarized and/or unpolarized electrons from unpolarized deuterons are given and are numerically evaluated for the kinematics of SAMPLE, PVA4 and G0 experiments. The dominant contribution from the interference of $γ$ and Z exchange as well as the smaller contributions from strangeness ($s{\bar{s}}$) components of the nucleon, parity odd admixtures in the deuteron wave function, anapole moments and radiative corrections are included and discussed in the context of parity violating electron scattering experiments of present interest.

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arXiv2008

The Charge Form Factor of the Neutron at Low Momentum Transfer from the $^{2}\vec{\rm H}(\vec{\rm e},{\rm e}'{\rm n}){\rm p}$ Reaction

We report new measurements of the neutron charge form factor at low momentum transfer using quasielastic electrodisintegration of the deuteron. Longitudinally polarized electrons at an energy of 850 MeV were scattered from an isotopically pure, highly polarized deuterium gas target. The scattered electrons and coincident neutrons were measured by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The neutron form factor ratio $G^{n}_{E}/G^{n}_{M}$ was extracted from the beam-target vector asymmetry $A_{ed}^{V}$ at four-momentum transfers $Q^{2}=0.14$, 0.20, 0.29 and 0.42 (GeV/c)$^{2}$.

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arXiv2007

Coulomb effects in polarization transfer in elastic antiproton and proton electron scattering at low energies

The influence of Coulomb distortion on the polarization transfer in elastic proton and antiproton electron scattering at low energies is calculated in a distorted wave Born approximation. For antiproton electron scattering Coulomb effects reduce substantially the spin transfer cross section compared to the plane wave Born approximation whereas for proton electron scattering they lead to a dramatic increase for kinetic proton lab energies below about 20 keV.

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arXiv2007

A surprising method for polarising antiprotons

We propose a method for polarising antiprotons in a storage ring by means of a polarised positron beam moving parallel to the antiprotons. If the relative velocity is adjusted to $v/c \approx 0.002$ the cross section for spin-flip is as large as about $2 \cdot 10^{13}$ barn as shown by new QED-calculations of the triple spin-cross sections. Two possibilities for providing a positron source with sufficient flux density are presented. A polarised positron beam with a polarisation of 0.70 and a flux density of approximately $1.5 \cdot 10^{10}$/(mm$^2$ s) appears to be feasible by means of a radioactive $^{11}$C dc-source. A more involved proposal is the production of polarised positrons by pair production with circularly polarised photons. It yields a polarisation of 0.76 and requires the injection into a small storage ring. Such polariser sources can be used at low (100 MeV) as well as at high (1 GeV) energy storage rings providing a time of about one hour for polarisation build-up of about $10^{10}$ antiprotons to a polarisation of about 0.18. A comparison with other proposals show a gain in the figure-of-merit by a factor of about ten.

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arXiv2006

Inclusive electron scattering off 4He

Inclusive electron scattering off 4He is calculated exactly with a complete treatment of the final state interaction within a simple semirealistic potential model. We discuss results for both the longitudinal and the transverse response functions, at various momentum transfers. A consistent meson exchange current is implemented. Good agreement with available experimental data is found for the longitudinal response function, while some strength is still missing in the transverse response function.

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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.

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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.

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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.

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arXiv2004

The importance of few-nucleon physics at low energy

This manuscript originated from the discussion at the workshop on the "Future of Few-body Low Energy Experimental Physics" (FFLEEP), which was held at the University of Trento on December 4-7, 2002 and has been written in its present form on March 19, 2003. It illustrates a selection of theoretical advancements in the nuclear few-body problem, including two- and many-nucleon interactions, the three-nucleon bound and scattering system, the four-body problem, the A-body (A$>$4) problem, and fields of related interest, such as reactions of astrophysical interest and few-neutron systems. Particular attention is called to the contradictory situation one experiences in this field: while theory is currently advancing and has the potential to inspire new experiments, the experimental activity is nevertheless rapidly phasing out. If such a trend will continue, advancements in this area will become critically difficult.

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arXiv2004

Rescattering effects and two-step process in kaon photoproduction on the deuteron

Kaon photoproduction on the deuteron is investigated by considering YN and KN rescattering and the two-step process gamma d --> pi NN --> KYN. A strong enhancement in the total cross section is found from the two-step process. YN rescattering has remarkable effects in the inclusive and exclusive cross section, while the effect of KN rescattering is much less important.

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arXiv2004

Ab initio calculation of Li7 photodisintegration

The Li7 total photoabsorption cross section is calculated microscopically. As nucleon-nucleon interaction the semi-realistic central AV4' potential with S- and P-wave forces is taken. The interaction of the final 7-nucleon system is fully taken into account via the Lorentz Integral Transform (LIT) method. For the calculation of the LIT we use expansions in hyperspherical harmonics (HH) in conjunction with the HH effective interaction (EIHH) approach. The convergence of the LIT expansion is discussed in detail. The calculated cross section agrees quite well with the available experimental data, which cover an energy range from threshold up to 100 MeV.

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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.

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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.

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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.

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arXiv2003

Final state interaction in spin asymmetry and GDH sum rule for incoherent pion production on the deuteron

The contribution of incoherent single pion photoproduction to the spin response of the deuteron, i.e., the asymmetry of the total photoabsorption cross section with respect to parallel and antiparallel spins of photon and deuteron, is calculated over the region of the $Δ$-resonance with inclusion of final state $NN$- and $πN$-rescattering. Sizeable effects, mainly from $NN$-rescattering, are found leading to an appreciable reduction of the spin asymmetry. Furthermore, the contribution to the Gerasimov-Drell-Hearn integral is explicitly evaluated by integration up to a photon energy of 550 MeV. Final state interaction reduces the value of the integral to about half of the value obtained for the pure impulse approximation.

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