P. Wilhelm

Rescattering effects in coherent pion photoproduction on the deuteron in the Δ resonance region

Abstract 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 NΔ, NNπ 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.

Photodisintegration of the deuteron in the Δ-resonance region

Abstract Photodisintegration of the deuteron in the Δ-resonance region is calculated treating the final state interaction within a N N − NΔ coupled channel approach including explicit pion degrees of freedom. For the electromagnetic interaction the nucleonic one-body current, meson exchange currents and the direct Δ-excitation are taken into account. The γN Δ -coupling is fixed in accordance with the M 1+ ( 3 2 ) multipole amplitude for pion photoproduction on the nucleon. The resulting total cross section underestimates the experimental data for photon energies below 340 MeV, whereas a good agreement is achieved by uaing a modified γN Δ -coupling effectively including the nonresonant Born terms. Furthermore, results for differential cross section, photon asymmetry and nucleon polarization are compared with experiment.

The spin response of the deuteron and its implication for the Gerasimov-Drell-Hearn sum rule☆

Abstract The spin response of the deuteron, i.e., the asymmetry of photoabsorption with respect to parallel and antiparallel spins of photon and deuteron, is calculated allowing to explicitly evaluate the Gerasimov-Drell-Hearn sum rule by 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 1 S 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.

Signatures from polarization observables for photon absorption on a 1S0 proton pair

Abstract Photoabsorption on a bound 1 S 0 proton pair has been investigated with respect to possible specific signatures from polarization observables. Assuming the dominance of any two of the three lowest possible multipoles (E1,E2,M2) we obtain simple forms for the differential cross section dσ dΩ , the photon polarization asymmetry Σ and the proton polarization P y . Specifically we find distinct relations between dσ dΩ and Σ. They may be applied to test the assumption of a dominance of the two-body process in (γ, pp ) reactions in nuclei.

Meson retardation in deuteron photodisintegration above π-threshold

Abstract Photodisintegration of the deuteron above π -threshold is studied in a coupled channel approach including NΔ - and πd -channels with 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.

Unitary ambiguity in the extraction of the E2/M1 ratio for the gamma N Delta transition.

The resonant electric quadrupole amplitude in the transition

Extraction of electromagnetic properties of the {Delta}(1232) excitation from pion photoproduction

\gamma N\leftrightarrow\Delta(1232)

Photon absorption on a neutron-proton pair in 3He

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(

On the Gerasimov-Drell-Hearn sum rule for the deuteron

\gamma N\leftrightarrow\Delta

Pion vs. photon absorption on the deuteron

) ratio (bare or dressed) which is based on a dynamical model using a largely phenomenological