arXiv subjects
Publications and source records attributed to H. Arenhoevel.
Photodisintegration of the deuteron above pi-threshold is studied in a coupled channel approach including N Delta- and pi d-channels with pion retardation in potentials and exchange currents. A much improved description of total and differential cross sections in the energy region between pi-threshold and 400-450 MeV is achieved.
For deuteron electroweak disintegration, parity violating effects are investigated which 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. The general formalism for the differential cross section and polarization observables of electromagnetic deuteron disintegration is extended to incorporate parity violating contributions. Formal expressions for the additional structure functions are derived. Results are presented for these parity violating structure functions for quasifree kinematics neglecting final state interaction and two-body effects. Both types of parity violating contributions to the asymmetry of the inclusive reaction with respect to longitudinally polarized electrons are evaluated. The one from parity violating deuteron components is negligible over the whole range of momentum transfers considered.
Several methods for the treatment of pion photoproduction in the region of the Delta(1232) resonance are discussed, in particular the effective Lagrangian approach and the speed plot analysis are compared to a dynamical treatment. As a main topic, we discuss the extraction of the genuine resonance parts of the magnetic dipole and electric quadrupole multipoles of the electromagnetic excitation of the resonance. To this end, we try to relate the various values for the ratio R_{EM} of the E2 to M1 multipole excitation strengths for the Delta(1232) resonance as extracted by the different methods to corresponding ratios of a dynamical model. Moreover, it is confirmed that all methods for extracting resonance properties suffer from an unitary ambiguity which is due to some phenomenological contributions entering the models.
Relativistic effects and the role of heavy meson exchange in deuteron photodisintegration are studied systematically for photon energies below the pion production threshold. In a (p/M)-expansion, all leading order relativistic one-body and pi-exchange as well as all static heavy meson exchange currents consistent with the Bonn OBEPQ model are included. In addition, one- and two-body boost effects have been investigated. Sizeable effects from the various two-body contributions beyond pi-exchange have been found in almost every observable considered, i.e., differential cross section and single polarization observables.
Incoherent $η$-photoproduction on the deuteron is studied for photon energies from threshold up to 720 MeV. The elementary $γN \to ηN$ amplitude is constructed in the frame of a conventional isobar model. Effects of final state interaction are investigated and their role in the cross section as well as in polarization observables is found to be important. Inclusion of such effects leads to a satisfactory agreement with experimental data in the near threshold region. In addition, recent experimental results on the coherent reaction $d(γ,η)d$ are discussed.
Coherent $η$ meson photoproduction on $^4$He and $^{12}$C is considered in the near-threshold region. The elementary $η$ photoproduction operator includes contributions from the $S_{11}(1535)$ and $D_{13}(1520)$ resonances as well as $t$-channel vector meson exchange and the nucleon pole terms. Due to the suppression of the dominant $S_{11}(1535)$ resonance for spin and isospin saturated nuclei, the reaction is mainly governed by $ω$ exchange. Furthermore, the influence of Fermi motion and of different prescriptions for the choice of the invariant reaction energy $W_{γN}$ in the elementary amplitude is studied.
The contribution of the $Δ$(1232) isobar to the electromagnetic current of the two-nucleon system and its role in ($γ$,NN) processes is investigated. The difference between the genuine $Δ$-excitation current and that part of the current connected to the deexcitation of a preformed $Δ$ in the target nucleus is stressed. The latter cannot lead to a resonant behaviour of matrix elements for energies in the $Δ$ region. The reaction $^{16}O(γ,pp)^{14}C$, where the $Δ$ contribution is dominant at intermediate energies, is considered. The large variations found in the cross sections for different treatments stress the need for a proper treatment of the $Δ$ current for a clear understanding of the reaction mechanism of two-nucleon emission processes.
The Gerasimov-Drell-Hearn sum rule is evaluated for the deuteron by explicit integration up to a photon energy of 550 MeV including contributions from the photodisintegration channel and from coherent and incoherent single pion production as well. The photodisintegration channel converges fast enough in this energy range and gives a large negative contribution, essentially from the $^1S_0$ resonant state near threshold. Its absolute value is about the same size than the sum of proton and neutron GDH values. It is only partially cancelled by the single pion production contribution. But the incoherent channel has not reached convergence at 550 MeV.
The influence of relativistic contributions to deuteron electrodisintegration is systematically studied in various kinematic regions of energy and momentum transfer. As theoretical framework the equation-of-motion and the unitarily equivalent S-matrix approaches are used. In a (p/M)-expansion, all leading order relativistic $π$-exchange contributions consistent with the Bonn OBEPQ model are included. In addition, static heavy meson exchange currents including boost terms, $γπρ/ω$-currents, and $Δ$-isobar contributions are considered. Sizeable effects from the various relativistic two-body contributions, mainly from $π$-exchange, have been found in inclusive form factors and exclusive structure functions for a variety of kinematic regions.
The intrinsic electromagnetic nuclear meson exchange charge and current operators arising from a separation of the center-of-mass motion are derived for a one-boson-exchange model for the nuclear interaction with scalar, pseudoscalar and vector meson exchange including leading order relativistic terms. Explicit expressions for the meson exchange operators corresponding to the different meson types are given in detail for a two-nucleon system. These intrinsic operators are to be evaluated between intrinsic wave functions in their center-of-mass frame.
Coherent eta photoproduction on the deuteron is studied in the $S_{11}$ resonance region neglecting eta rescattering and two-body processes. For the elementary reaction on the nucleon, we have considered the dominant $S_{11}(1535)$ resonance and as background the nucleon pole terms as well as $ρ$ and $ω$ meson exchange. We have studied the influence of different choices for the neutron resonance amplitude, different prescriptions for fixing the invariant mass of the resonance amplitude and different methods for deriving the elementary production amplitude for an arbitrary reference frame.
Photo production of pions on the deuteron is studied in the spectator nucleon model. The Born terms of the elementary production amplitude are determined in pseudovector $π$N coupling and supplied with a form factor. The $Δ$ resonance is considered both in the $s$ and the $u$ channel. The parameters of the $Δ$ resonance and the cutoff of the form factors are fixed on the leading photoproduction multipoles. Results for total and differential cross sections are compared with experimental data. Particular attention is paid to the role of Pauli correlations of the final state nucleons in the quasifree case. The results are compared with those for pion photoproduction on the nucleon.
The resonant electric quadrupole amplitude in the transition $γN\leftrightarrowΔ(1232)$ is of great interest for the understanding of baryon structure. Various dynamical models have been developed to extract it from the corresponding photoproduction multipole of pions on nucleons. It is shown that once such a model is specified, a whole class of unitarily equivalent models can be constructed, all of them providing exactly the same fit to the experimental data. However, they may predict quite different resonant amplitudes. Therefore, the extraction of the E2/M1($γN\leftrightarrowΔ$) ratio (bare or dressed) which is based on a dynamical model using a largely phenomenological $πN$ interaction is not unique.
Within a dynamical model, rescattering effects are studied in coherent pion photoproduction on the deuteron in the $Δ$(1232) resonance region. The model consists of a system of coupled equations for the N$Δ$, NN$π$ and NN channels. Pion rescattering leads in general to a significant reduction of total and differential cross sections resulting in a better description of experimental data. Only a few polarization observables are sensitive to rescattering effects. In particular $T_{20}$ allows an analysis of the reaction mechanism for specific kinematics. The question how rescattering affects the sensitivity to the E2 excitation of the $Δ$ resonance is also studied.
The importance of subnuclear degrees of freedom in photon absorption and electron scattering by nuclei is discussed. After a short introduction into the basic properties of one-photon processes and a very brief survey on the nuclear response in the various regions of energy and momentum transfers, the particular role of subnuclear degrees of freedom in terms of meson and isobar degrees of freedom is considered. Their importance is illustrated by several reactions on the deuteron which are currently under study at c.w.\ electron machines like MAMI in Mainz.
The axial strange form factor $F^s_A$ of the nucleon is assumed to be dominated at low momentum transfer by the isoscalar axial vector mesons $f_1(1285)$ and $f_1(1420)$. The importance of the $a_0πN$-triangular vertex correction is demonstrated.
Three-body photodisintegration of 3He is calculated in the photon energy range 200 - 400 MeV assuming quasifree absorption on np pairs both in initial quasideuteron and singlet configurations. The model includes the normal nucleonic current, explicit meson exchange currents and the Delta(1232)-isobar excitation. The total cross section is increased by a factor of about 1.5 compared with free deuteron photodisintegration. Well below and above the Delta region also some spin observables differ significantly from the ones of free deuteron disintegration due to the more compressed wave function of the correlated np pairs in 3He compared to the deuteron. The initial singlet state causes a significant change in the analyzing power Ay. These differences could presumably be seen at the conjugate angles where two-body effects are maximized and where photoreactions could complement similar pion absorption experiments. Figures by fax or post from janiskanen@phcu.helsinki.fi