arXiv subjects

Hartmuth Arenhoevel

Publications and source records attributed to Hartmuth Arenhoevel.

17 recordsLinked to original sources

arXiv2008

Spin degrees and Polarization Observables in Electromagnetic Reactions

After a brief introduction into the general importance of polarization observables for the analysis of a reaction, the basic density matrix formalism for the description of polarization phenomena is outlined and illustrated by explicit examples for spin one-half and one. Then various electromagnetic processes like photoreactions and electron scattering are discussed in some detail. Finally, the question of how to determine a complete set of polarization observables providing the maximum information on a reaction is considered.

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arXiv2007

Inclusive electron scattering off 4He

Inclusive electron scattering off 4He is investigated for low and medium energy and momentum transfers. The final state interaction, given by the simple semirealistic Malfliet-Tjon potential, is treated rigorously applying the Lorentz Integral Transform (LIT) method. Besides the nonrelativistic one-body current a consistent meson exchange current is constructed and implemented. Results are presented for both longitudinal and transverse response functions at various momentum transfers. Good agreement with experimental data is found for the longitudinal response function, while some strength is missing in the transverse response function on the low-energy side of the quasi-elastic peak.

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arXiv2004

Interaction effects in ${\bm K^+}$ photoproduction on the deuteron

Kaon photoproduction on the deuteron is studied with respect to a specific two-body contribution, namely a pion mediated production process, besides other final state interaction contributions from kaon-nucleon and hyperon-nucleon scattering. In this process, a pion is first photoproduced on one nucleon and then interacts with the spectator nucleon in a strangeness exchange reaction leading to a kaon and a hyperon. A sizeable effect from this pion mediated contribution is found, considerably larger than the previously studied hyperon-nucleon rescattering, whereas kaon-nucleon rescattering is much less important. Besides total and semi-inclusive differential cross sections, tensor target asymmetries are studied with respect to the influence of such interaction effects.

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arXiv2004

The generalized Gerasimov-Drell-Hearn sum rule for deuteron electrodisintegration

The generalized Gerasimov-Drell-Hearn sum rule $I^{GDH}_{γ^*d}(Q^2)$ for deuteron electrodisintegration $d(e,e')np$ as function of the squared four-momentum transfer $Q^2$ is evaluated by explicit integration. The calculation is based on a conventional nonrelativistic framework using a realistic $NN$-potential and including contributions from meson exchange currents, isobar configurations and leading order relativistic terms. Good convergence is achieved. The prominent feature is a deep negative minimum, $I_{γ^* d}^{GDH}=-9.5$ mb, at low $Q^2\approx 0.2$ fm$^{-2}$ which is almost exclusively driven by the nucleon isovector anomalous magnetic moment contribution to the magnetic dipole transition to the $^1S_0$-state. Above $Q^2=20$ fm$^{-2}$ the integral $I^{GDH}_{γ^*}(Q^2)$ approaches zero rapidly.

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arXiv2002

General Multipole Expansion of Polarization Observables in Deuteron Electrodisintegration

Formal expressions are derived for the multipole expansion of the structure functions of a general polarization observable of exclusive electrodisintegration of the deuteron using a longitudinally polarized beam and/or an oriented target. This allows one to exhibit explicitly the angular dependence of the structure functions by expanding them in terms of the small rotation matrices $d^j_{m'm}(θ)$, whose coefficients are given in terms of the electromagnetic multipole matrix elements. Furthermore, explicit expressions for the coefficients of the angular distributions of the differential cross section including multipoles up to $L_{max}=3$ are listed in tabular form.

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arXiv2001

Construction of Consistent Meson Exchange Currents by Laplace Transform

We propose a new method for the construction of a consistent meson exchange current in $r$-space for the spin-isospin dependent central and tensor part of phenomenological nucleon-nucleon potentials by using a Laplace transformation, which allows the representation by a finite number of Yukawa functions. This method is applied to the Paris and the recent Argonne $V_{18}$ potentials. Results are presented for electrodisintegration of the deuteron near threshold.

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arXiv2001

The two-nucleon system in the $Δ$ region including full meson retardation

A model is developed for the hadronic and electromagnetic interaction in the two-nucleon system above pion threshold in the framework of meson, nucleon and $Δ$ degrees of freedom. It is based on time-ordered perturbation theory and includes full meson retardation in potentials and exchange currents as well as loop contributions to the nucleonic one-body current. Results for $NN$ scattering and deuteron photodisintegration are presented.

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arXiv2001

Offshell effects in electromagnetic reactions on the deuteron

Offshell contributions to the electromagnetic nuclear current are evaluated within a nonrelativistic approach by incorporation one-pion loop contributions in time-ordered perturbation theory. By construction, the correct experimental onshell properties of the nucleon current are ensured so that only the genuine offshell effects appear as model dependent. For a qualitative assessment of such offshell effects, this model is applied to photodisintegration of the deuteron for photon energies up to 500 MeV. While at low energies offshell contributions are small, above 300 MeV they lead to sizeable effects in observables up to about 30 percent pointing to the necessity of incorporating such effects if one aims at theoretical predictions of high precision.

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arXiv2000

Polarization observables in elastic electron deuteron scattering including parity and time reversal violating contributions

The general formalism for polarization observables in elastic electron deuteron scattering is extended to incorporate parity and time reversal violating contributions. Parity violating effects arise from the interference of $γ$ and $Z$ exchange as well as from the hadronic sector via a small parity violating component in the deuteron. In addition we have allowed for time reversal invariance violating contributions in the hadronic sector. Formal expressions for the additional structure functions are derived, and their decomposition into the various multipole contributions are given explicitly.

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arXiv2000

The Role of Meson Retardation in the NN Interaction above Pion Threshold

A model is developed for the hadronic interaction in the two-nucleon system above pion threshold which is based on meson, nucleon and $Δ$ degrees of freedom and which includes full meson retardation in the exchange operators. For technical reasons, the model allows maximal one meson to be present explicitly. Thus the Hilbert space contains besides $NN$ and $NΔ$ also configurations consisting of two nucleons and one meson. For this reason, only two- and three-body unitarity is obeyed, and the model is suited for reactions in the two nucleon sector only, where one pion is produced or absorbed. Starting from a realistic pure nucleonic retarded potential, which had to be renormalized because of the additional $π$ and $Δ$ degrees of freedom, a reasonable fit to experimental $NN$-scattering data could be achieved.

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arXiv2000

The Role of Meson Retardation in Deuteron Photodisintegration above Pion Threshold

Photodisintegration of the deuteron above $π$ threshold is studied in a coupled channel approach including $N Δ$ and $πd$ channels with consideration of pion retardation in potentials and exchange currents. A much improved description of total and differential cross sections in the energy region between $π$ threshold and 400-450 MeV is achieved. With respect to polarization observables, the description of the linear photon asymmetry and the proton polarization remains problematic.

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arXiv1999

Elastic electron deuteron scattering with consistent meson exchange and relativistic contributions of leading order

The influence of relativistic contributions to elastic electron deuteron scattering is studied systematically at low and intermediate momentum transfers ($Q^2\leq 30$ fm$^{-2}$). In a $(p/M)$-expansion, all leading order relativistic $π$-exchange contributions consistent with the Bonn OBEPQ models are included. In addition, static heavy meson exchange currents including boost terms and lowest order $ρπγ$-currents are considered. Sizeable effects from the various relativistic two-body contributions, mainly from $π$-exchange, have been found in form factors, structure functions and the tensor polarization $T_{20}$. Furthermore, static properties, viz. magnetic dipole and charge quadrupole moments and the mean square charge radius are evaluated.

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arXiv1999

Coherent pion electroproduction on the deuteron in the $Δ(1232)$ resonance region

Coherent pion electroproduction on the deuteron is studied in the $Δ(1232)$ resonance region in the impulse approximation, i.e., neglecting pion rescattering and two-body effects. The elementary reaction on the nucleon is described in the framework of an effective Lagrangian approach including the dominant $P_{33}(1232)$ resonance and the usual background terms of the Born contributions for $π^0$ production. We have studied the influence of these different contributions on the various structure functions which determine the unpolarized exclusive differential cross section in a variety of kinematic regions.

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arXiv1999

Invariant amplitudes for coherent electromagnetic pseudoscalar production from a spin-one target (II): crossing, multipoles and observables

The formal properties of the recently derived set of linearly independent invariant amplitudes for the electromagnetic production of a pseudoscalar particle from a spin-one particle have been further exploited. The crossing properties are discussed in detail. Since not all of the amplitudes have simple crossing behaviour, we introduce an alternative set of basic amplitudes which are either symmetric or antisymmetric under crossing. The multipole decomposition is given, and the representation of the multipoles as integrals over the invariant functions weighted with Legendre polynomials is derived. Furthermore, differential cross section and polarization observables are expressed in terms of the corresponding invariant functions.

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arXiv1999

Complete Sets of Polarization Observables in Electromagnetic Deuteron Break-up

For deuteron photo- and electrodisintegration the selection of complete sets of polarization observables is discussed in detail by applying a recently developed new criterion for the check of completeness of a chosen set of observables. The question of ambiguities and their resolution by considering additional observables is discussed for a numerical example, for which the role of experimental uncertainties is also investigated. Furthermore, by inversion of the expressions of the observables as hermitean forms in the $t$-matrix elements a bilinear term of the form $t_{j'}^*t_j$ can be given as a complex linear form in the observables from which an explicit solution for $t_j$ in terms of observables can be obtained. These can also be used to select sets of observables for the explicit representation of the $t$-matrix.

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arXiv1998

Invariant amplitudes for coherent electromagnetic pseudoscalar production from a spin-one target

A set of 13 linearly independent invariant amplitudes for the electromagnetic production of a pseudoscalar particle from a spin-one particle is derived which respect Lorentz and gauge invariance. The $T$-matrix can be represented by a linear superposition of these amplitudes with invariant functions as coefficients, which depend on the Mandelstam variables only. Their explicit form is determined by the underlying dynamics. Nine of these amplitudes are purely transverse and describe photoproduction. The remaining four appear in electroproduction in addition, describing charge and longitudinal current contributions. Furthermore, a reduction of these amplitudes to operators acting in non-relativistic spin space is given.

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arXiv1998

On Complete Sets of Polarization Observables

A new criterion is developed which provides a check as to whether a chosen set of polarization observables is complete with respect to the determination of all independent $T$-matrix elements of a reaction of the type $a+b\to c+d+...$. As an illustrative example, this criterion is applied to the longitudinal observables of deuteron electrodisintegration.

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