arXiv subjects
Publications and source records attributed to H. Arenhoevel.
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 some selected processes are discussed.
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.
Incoherent photoproduction of pions on the deuteron in the Delta(1232) resonance region is studied, where besides the impulse approximation complete two-body rescattering in the nucleon-nucleon and pion-nucleon final state subsystems is included.
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.
The influence of final state $NN$- and pi N-rescattering in incoherent pion photoproduction on the deuteron has been investigated. For the elementary photoproduction operator an effective Lagrangian model is used which describes well the elementary reaction. The interactions in the final two-body subsystems are taken in separable form. While NN-rescattering shows quite a significant effect, particularly strong for neutral pion production, pi N-rescattering is almost negligible. Inclusion of such effects leads to an improved and quite satisfactory agreement with experiment.
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.
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.
The 2H(e,e'p)n cross section was measured in Hall A of the Thomas Jefferson National Accelerator Facility (JLab) in quasielastic kinematics (x=0.96) at a four-momentum transfer squared, Q^2=0.67 (GeV/c)^2. The experiment was performed in fixed electron kinematics for recoil momenta from zero to 550 MeV/c. Though the measured cross section deviates by 1-2 sigma from a state-of-the-art calculation at low recoil momenta, it agrees at high recoil momenta where final state interactions (FSI) are predicted to be large.
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.
Coherent $η$-meson photoproduction on the deuteron has been studied, where the emphasis is on the relative importance of two-body contributions from hadronic rescattering and electromagnetic meson exchange currents besides the impulse approximation. For the elementary photoproduction amplitude a coupled resonance model developed by Bennhold and Tanabe has been used which fits reasonably well the experimental data. The rescattering effects are treated within a coupled channel approach considering the intermediate excitation of the $P_{11}(1440)$, $D_{13}(1520)$, and $S_{11}(1535)$ nucleon resonances. The hadronic interaction between nucleon and resonances is modeled by one boson exchange potentials, which we have considered both in the static approximation as well as fully retarded. The sum of all considered two-body effects results in an enhancement of the total cross section between 10 in the maximum and 25 percent closer to threshold around 680 MeV if the hadronic interaction is treated retarded. This enhancement shows up in the differential cross sections mainly at backward angles. It increases steadily from only a few percent at $0^\circ$ to more than a factor two at $180^\circ$ for a photon energy of 680 MeV. Two-body effects become also significant in certain polarization observables. Finally, no discrepancy has been found for the ratio of the isoscalar amplitude to the proton amplitude between coherent and incoherent $η$-photoproduction on the deuteron due to a nonvanishing complex and energy dependent phase relation.
Measurements of the ${^2}H(\vec{e},e^{\prime}p)n$ reaction were performed using an 800-MeV polarized electron beam at the MIT-Bates Linear Accelerator and with the out-of-plane magnetic spectrometers (OOPS). The longitudinal-transverse, $f_{LT}$ and $f_{LT}^{\prime}$, and the transverse-transverse, $f_{TT}$, interference responses at a missing momentum of 210 MeV/c were simultaneously extracted in the dip region at Q$^2$=0.15 (GeV/c)$^2$. On comparison to models of deuteron electrodisintegration, the data clearly reveal strong effects of relativity and final-state interactions, and the importance of the two-body meson-exchange currents and isobar configurations. We demonstrate that these effects can be disentangled and studied by extracting the interference response functions using the novel out-of-plane technique.
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.
As first topic, the GDH sume rule is discussed in the context of a more general class of sum rules associated with the various contributions to the total photoabsorption cross section for target and beam polarization. Then I address the question of whether the GDH sum rule for the neutron can be determined from the one for the deuteron. It appears that this will not be possible in a simple manner. The spin response of the deuteron is calculated including contributions from the photodisintegration channel and from coherent and incoherent single pion production as well, and the GDH integral is evaluated up to a photon energy of 550 MeV. The photodisintegration channel converges fast enough and gives a large negative contribution, essentially from the $^1S_0$ state near threshold and its absolute size is about the same 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 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.
Rescattering effects in coherent eta photoproduction on the deuteron are studied in the region of the S_11(1535) resonance. For the elementary reaction mechanism on the nucleon, an effective isobar model with inclusion of resonances and Born terms is used. The resonance parameters are fixed by considering pion photoproduction only. For the coherent eta production on the deuteron we have considered the impulse approximation taking into account the P_11(1440), D_13(1520), and S_11(1535) resonances, rescattering, and meson exchange currents. For the rescattering mechanism a coupled channel model including the dominant S_11(1535) resonance is used.
We report on a first measurement of tensor analyzing powers in quasi-elastic electron-deuteron scattering at an average three-momentum transfer of 1.7 fm$^{-1}$. Data sensitive to the spin-dependent nucleon density in the deuteron were obtained for missing momenta up to 150 MeV/$c$ with a tensor polarized $^2$H target internal to an electron storage ring. The data are well described by a calculation that includes the effects of final-state interaction, meson-exchange and isobar currents, and leading-order relativistic contributions.
After a brief introduction into the basic ingredients of electroweak theory as a spontaneously broken local, non-Abelian gauge symmetry, the general properties of the electromagnetic current and two-photon operators are discussed. The specific forms of vector and axial one-nucleon currents are given, together with lowest order $π$ meson exchange and isobar currents as well as meson production currents. A brief overview is given on the most important one- and two-boson processes. Electron scattering in the one-boson- approximation is then considered in greater detail. Formal expressions of the cross section for inclusive and exclusive processes are given, including parity violating contributions from $γ$-$Z$ interference as well as from parity violating components in the hadronic wave function. Specific electromagnetic reactions on the deuteron are then discussed with respect to the influence of meson exchange currents, isobar configurations in the deuteron groundstate, relativistic contributions and the role of $π$-meson retardation. Furthermore, recent results on coherent and incoherent $π$- and $η$-photoproduction are presented as well as a discussion of the Gerasimov-Drell-Hearn sum rule and the effect of a parity violating deuteron component on inclusive electron scattering off the deuteron for quasifree kinematics. The review closes with a summary and a brief outlook.
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.