M. Martini

Low-energy excitations and quasielastic contribution to electron-nucleus and neutrino-nucleus scattering in the continuum random-phase approximation

We present a detailed study of a continuum random-phase approximation approach to quasielastic electron-nucleus and neutrino-nucleus scattering. We compare the (e, e) cross-section predictions with electron scattering data for the nuclear targets C-12, O-16, and Ca-40, in the kinematic region where quasielastic scattering is expected to dominate. We examine the longitudinal and transverse contributions to C-12(e, e) and compare them with the available data. We find an overall satisfactory description of the (e, e) data. Further, we study the C-12(nu(mu), mu(-)) cross sections relevant for accelerator-based neutrino-oscillation experiments. We pay special attention to low-energy excitations which can account for non-negligible contributions in measurements, and require a beyond-Fermi-gas formalism.

Electron-neutrino scattering off nuclei from two different theoretical perspectives

We analyze charged-current electron-neutrino cross sections on Carbon. We consider two different theoretical approaches, on one hand the Continuum Random Phase Approximation (CRPA) which allows a description of giant resonances and quasielastic excitations, on the other hand the RPA-based calculations which are able to describe multinucleon emission and coherent and incoherent pion production as well as quasielastic excitations. We compare the two approaches in the genuine quasielastic channel, and find a satisfactory agreement between them at large energies while at low energies the collective giant resonances show up only in the CRPA approach. We also compare electron-neutrino cross sections with the corresponding muon-neutrino ones in order to investigate the impact of the different charged-lepton masses. Finally, restricting to the RPA-based approach we compare the sum of quasielastic, multinucleon emission, coherent and incoherent one-pion production cross sections (folded with the electron-neutrino T2K flux) with the charged-current inclusive electron-neutrino differential cross sections on Carbon measured by T2K. We find a good agreement with the data. The multinucleon component is needed in order to reproduce the T2K electron-neutrino inclusive cross sections.

Influence of short-range correlations in neutrino-nucleus scattering

Background: Nuclear short-range correlations (SRCs) are corrections to mean-field wave functions connected with the short-distance behavior of the nucleon-nucleon interaction. These SRCs provide corrections to lepton-nucleus cross sections as computed in the impulse approximation (IA). nPurpose: We want to investigate the influence of SRCs on the one-nucleon (1N) and two-nucleon (2N) knockout channels for muon-neutrino induced processes on a C-12 target at energies relevant for contemporary measurements. nMethod: The model adopted in this work corrects the impulse approximation for SRCs by shifting the complexity induced by the SRCs from the wave functions to the operators. Due to the local character of the SRCs, it is argued that the expansion of these operators can be truncated at a low order. nResults: The model is compared with electron-scattering data, and two-particle two-hole responses are presented for neutrino scattering. The contributions from the vector and axial-vector parts of the nuclear current as well as the central, tensor, and spin-isospin parts of the SRCs are studied. nConclusions: Nuclear SRCs affect the 1N knockout channel and give rise to 2N knockout. The exclusive neutrino-induced 2N knockout cross section of SRC pairs is shown and the 2N knockout contribution to the QE signal is calculated. The strength occurs as a broad background which extends into the dip region.

Impact of low-energy nuclear excitations on neutrino-nucleus scattering at MiniBooNE and T2K kinematics

Background: Meticulous modeling of neutrino-nucleus interactions is essential to achieve the unprecedented precision goals of present and future accelerator-based neutrino-oscillation experiments. nPurpose: Confront our calculations of charged-current quasielastic cross sections with the measurements of MiniBooNE and T2K, and to quantitatively investigate the role of nuclear-structure effects, in particular, low-energy nuclear excitations in forward muon scattering. nMethod: The model takes the mean-field approach as the starting point, and solves Hartree-Fock (HF) equations using a Skyrme (SkE2) nucleon-nucleon interaction. Long-range nuclear correlations are taken into account by means of the continuum random-phase approximation (CRPA) framework. nResults: We present our calculations on flux-folded double differential, and flux-unfolded total cross sections off C-12 and compare them with MiniBooNE and (off-axis) T2K measurements. We discuss the importance of low-energy nuclear excitations for the forward bins. nConclusions: The HF and CRPA predictions describe the gross features of the measured cross sections. They underpredict the data (more in the neutrino than in the antineutrino case) because of the absence of processes beyond pure quasielastic scattering in our model. At very forward muon scattering, low-energy HF-CRPA nuclear excitations (omega < 50 MeV) account for nearly 50% of the flux-folded cross section. This extra low-energy strength is a feature of the detailed microscopic nuclear model used here, that is not accessed in a Fermi-gas based approach.

Assessing the role of nuclear effects in the interpretation of the MiniBooNE low-energy anomaly

We study the impact of the effect of multinucleon interactions in the reconstruction of the neutrino energy on the fit of the MiniBooNE data in terms of neutrino oscillations. We obtain some improvement of the fit of the MiniBooNE low-energy excess in the framework of two-neutrino oscillations and a shift of the allowed region in the sin 2 2ϑ-Δm 2 plane toward smaller values of sin 2 2ϑ and larger values of Δm 2 . However, this effect is not enough to solve the problem of the appearance-disappearance tension in the global fit of short-baseline neutrino oscillation data.

Neutrino versus antineutrino cross sections and CP violation

We discuss the nuclear interactions of neutrinos versus those of antineutrinos, a relevant comparison for CP violation experiments in the neutrino sector. We consider the MiniBooNE quasielasticlike double differential neutrinos and antineutrinos cross sections which are flux dependent and hence specific to the MiniBooNE set-up. We combine them introducing their sum (�+¯�) and their difference (� − ¯ �). We show that the last combination can bring a general information, which can be exploited in other experiments, on the nuclear matrix elements of the axial vector interference term. Our theoretical model reproduces well the two cross sections combinations. This confirms the need for a sizeable multinucleon component in particular in the interference term.

CRPA Calculations for Neutrino-Nucleus Scattering: From Very Low Energies to the Quasielastic Peak

We present continuum random phase approximation calculations (CRPA) for neutrino-induced quasielastic scattering off atomic nuclei. The validity of our formalism is checked by a careful confrontation of its results with semi-inclusive double-differential electron scattering data. We pay special attention to excitations in the giant resonance region. The CRPA is well-suited for the description of interactions in this energy range. We aim at providing a uniform description of one-nucleon knockout processes over the whole energy range from threshold to the quasielastic peak. Our calculations point to the fact that low-energy and giant-resonance excitations provide a non-negligible contribution to the interaction strength, especially at forward lepton-scattering angles.

Nuclear response functions with finite-range Gogny force: Tensor terms and instabilities

A fully-antisymmetrized random phase approximation calculation employing the continued fraction technique is performed to study nuclear matter response functions with the finite range Gogny force. The most commonly used parameter sets of this force, as well as some recent generalizations that include the tensor terms are considered and the corresponding response functions are shown. The calculations are performed at the first and second order in the continued fraction expansion and the explicit expressions for the second order tensor contributions are given. Comparison between first and second order continued fraction expansion results are provided. The differences between the responses obtained at the two orders turn to be more pronounced for the forces including tensor terms than for the standard Gogny ones. In the vector channels the responses calculated with Gogny forces including tensor terms are characterized by a large heterogeneity, reflecting the different choices for the tensor part of the interaction. For sake of comparison the response functions obtained considering a G-matrix based nuclear interaction are also shown. As first application of the present calculation, the possible existence of spurious finite-size instabilities of the Gogny forces with or without tensor terms has been investigated. The positive conclusion is that all the Gogny forces, but the GT2 one, are free of spurious finite-size instabilities. In perspective, the tool developed in the present paper can be inserted in the fitting procedure to construct new Gogny-type forces.