Carlotta Giusti

Evidence for short-range correlations in O-16

Abstract The reaction 16 O( e , e ′ pp ) 14 C has been investigated at three values of the transferred energy ω . The differential cross sections were determined as a function of the missing energy and the missing momentum. Evidence for short-range correlations in 16 O has been obtained from the transition to the ground state of 14 C. The cross sections for this transition are well reproduced by two independent parameter-free microscopic calculations. The results of both calculations show that the reaction is dominated by knockout of a proton pair in a 1 S 0 state, driven by short-range-correlations.

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.

Relativistic descriptions of final-state interactions in neutral-current neutrino-nucleus scattering at MiniBooNE kinematics.

Dipartimento di Fisica Nucleare e Teorica, Universita` degli Studi di Pavia andINFN, Sezione di Pavia, via A. Bassi 6, I-27100 Pavia, Italy(Dated: October 19, 2011)The analysis of the recent neutral-current neutrino-nucleus scattering cross sections measuredby the MiniBooNE Collaboration requires relativistic theoretical descriptions also accounting forthe role of final state interactions. In this work we evaluate differential cross sections with therelativistic distorted-wave impulse-approximation and with the relativistic Green’s function modelto investigate the sensitivity to final state interactions. The role of the strange-quark content of thenucleon form factors is also discussed.

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.

Relativistic descriptions of final-state interactions in charged-current quasielastic antineutrino-nucleus scattering at MiniBooNE kinematics

The results of two relativistic models with different descriptions of the final-state interactions are compared with the MiniBooNE data of charged-current quasielastic cross sections. The relativistic mean field model uses the same potential for the bound and ejected nucleon wave functions. In the relativistic Greens function model, the final-state interactions are described in the inclusive scattering consistently with the exclusive scattering using the same complex optical potential. The relativistic Greens function results describe the experimental data for total cross sections without the need to modify the nucleon axial mass.

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.