Franco Davide Pacati

Nuclear response in electromagnetic interactions with complex nuclei

Abstract The response of nucleons and complex nuclei to an external electromagnetic probe at intermediate and high energy is illustrated by considering both inclusive and semi-inclusive electron scattering. Gauge invariance and current conservation allow one to define form factors for elastic scattering and structure functions for inelastic scattering which contain the relevant information on the structure and dynamics of the target hadron system under investigation. They are derived and discussed in some specific examples, also taking into account polarization observables.

Spin-orbit distortion of the emerging nucleon in quasi-free electron scattering

Abstract Spin-orbit distortion of the emerging nucleon in quasi-free electron scattering is considered in the frame of a program aiming at examining the approximations involved in the theoretical treatment. The imaginary part of the spin-orbit coupling in the optical-model potential felt by the knocked out nucleon appears here to have a significant role. Its consequences in the case of the reaction 12 C(e, e′p) 11 B are discussed.

Orthogonality, antisymmetry and c.m. effects in direct photoreactions

Abstract The relevance of different approximations usually adopted in the description of direct knockout of nucleons in photoreactions at medium energies is investigated. Orthogonality between initial and final states in DWIA and antisymmetrization of the outgoing particle are studied in the presence of recoil terms arising from a correct treatment of the c.m. motion. The different effects are discussed in the specific example of the 12 C target nucleus for different combinations of bound and scattering states. Whereas orthogonality and antisymmetrization appear to play a rather relevant role only at backward angles, the recoil term is particularly important for the (γ, n) reaction and cannot be simulated by the use of multipole energy-independent effective charges.

Treatment of the Δ current in electromagnetic two-nucleon knockout reactions

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 16O(γ, pp)14C, 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.

Exclusive (e,e′pp) knockout reactions

Abstract Exclusive cross sections of the 16 O(e,e′pp) 14 C knockout reaction are calculated for transitions to the low-lying discrete final states of the residual nucleus. Short-range correlations and two-body currents, due to the excitation of a Δ resonance in the intermediate state, are included in the calculations. Final-state interactions are taken into account by means of phenomenological spin-dependent optical potentials. Recoil-momentum distributions for transitions to states with different angular momentum exhibit different shapes, which are basically determined by the cm orbital angular momentum of the initial proton pair.

Meson exchange currents in a relativistic model for electromagnetic one nucleon emission

We analyze the role of meson exchange currents (MEC) in photon- and electron-induced one nucleon emission reactions in a fully relativistic model. The relativistic mean field theory is used for the bound state and the Pauli reduction for the scattering state. Direct 1-body and exchange 2-body terms in the nuclear current are considered. Results for the 12C(gamma,p) and 16O(gamma,p) differential cross sections and photon asymmetries are displayed in an energy range between 60 and 196 MeV. The 2-body seagull current affects the cross section less than in nonrelativistic analyses. In the case of the 16O(gamma,n) differential cross section MEC effects are large but not sufficient to reproduce the data. MEC have a small effect on (e,ep) calculations.

Relativistic corrections in(γ,N)knockout reactions

We develop a fully relativistic DWIA model for photonuclear reactions using the relativistic mean field theory for the bound state and the Pauli reduction of the scattering state which is calculated from a relativistic optical potential. Results for the 12C(gamma,p) and 16O(gamma,p) differential cross sections and photon asymmetries are displayed in a photon energy range between 60 and 257 MeV, and compared with nonrelativistic DWIA calculations. The effects of the spinor distortion and of the effective momentum approximation for the scattering state are discussed. The sensitivity of the model to different prescriptions for the one-body current operator is investigated. The off-shell ambiguities are large in (gamma,p) calculations, and even larger in (gamma,n) knockout.

Multi-scattering effects in (e,e′p) knockout

Abstract Multistep direct scattering of the ejectile proton is considered in (e,e′p) reactions in the quasi-free region as an improvement over the usual treatment of final-state interactions by means of an optical potential. The theory is applied to 40Ca(e,e′p) as a case example. Important contributions of two- and three-step processes are found at high missing energy and momentum in agreement with the experimental trend.

Charge exchange in final-state interactions of (e,e′ pp) reactions

Abstract The charge-exchange mechanism due to the isospin-dependent part of the optical potential is investigated for the (e,e′ pp) reaction and added to the direct one-step knockout. A small contribution is found, which is practically independent of the kinematics and of the different ingredients of the calculation.

Meson-exchange current effects in nucleon photoemission

Abstract The role of meson-exchange currents in (γ, p) and (γ, n) reactions at intermediate energies is investigated in the frame of a direct knockout model where the photon interacts with a pair of correlated nucleons. Realistic short-range correlations and final-state interactions are included in the calculations. Direct and exchange terms are considered. The results for 16 O(γ, p) and 16 O(γ, n) indicate that the two-body seagull current significantly affects the cross sections. In both cases the size and the shape of the experimental angular distributions are fairly well reproduced.