Charged-current neutrino interactions with nucleons and nuclei at intermediate energies

Nowadays, the interest in neutrinos extends to a large variety of fields in Astrophysics, Nuclear Physics and Particle Physics. One of the open questions in theoretical physics is the description of neutrino oscillations for which an accurate interpretation of neutrino- nucleus reactions is crucial. In this context, recent years have witnessed an intense ex- perimental and theoretical activity to determine the properties of neutrinos and their interaction with matter. This PhD Thesis is thus focused on the analysis of charged-current neutrino-nucleus reactions at kinematics of interest for neutrino oscillation experiments, where the neu- trino energy is typically in the GeV region. Additionally, weak interactions in the nuclear medium at intermediate energies are an extraordinary opportunity to study the dynamics of the nuclear many-body system, beyond the information accesible from electron and hadron probes, and to gain a deeper knowledge of the axial structure and the strangeness content of the nucleons. In any accelerator-based neutrino oscillation experiment, neutrinos are produced as the decay products of successive reactions, thus implying a wide-ranged energy beam. Hence, when interacting with the nuclear matter, a large variety of nuclear effects come into play, going from quasielastic scattering to deep inelastic processes, multi-nucleon ex- citations or meson production via nucleon resonances. Accordingly, robust models that properly describe neutrino-nucleus interactions over the whole experimental range (of the order of 10s of MeV up to 10s of GeV) are required for the experimental analyses. Notice also that the kinematics involved demand a relativistic description of the microscopic nu- clear structure. In this thesis, the analysis of these processes are addressed by using realistic models that provide, within a fully relativistic framework, an accurate description of the different reaction mechanisms of relevance for neutrino oscillation measurements. We begin ana- lyzing neutrino scattering off free nucleons and describing the weak hadronic responses together with the inner structure of the nucleons. With the aim of achieveing a consis- tent analysis of charged-current quasielastic (CCQE) neutrino interactions with nuclei, we present the so-called SuSAv2 model, which is based on the superscaling behavior exhib- ited by electron scattering data and makes use of the relativistic mean field (RMF) theory to describe the nuclear effects arising in neutrino-nucleus interactions. This prescription accounts for the final-state interactions (FSI) between the outgoing nucleon and the resid- ual nucleus and allows for a description in terms of the different isovector/isoscalar and axial/vector reaction channels that play a role in weak interactions. At very high kine- matics, where FSI are negligible, we approach our model to the relativistic plane wave impulse approximation (RPWIA) where no FSI affect the outgoing nucleon. Furthermore, a basic feature in this thesis also concerns the evaluation of multi-nucleon excitations, in particular two-body meson exchange currents (2p-2h MEC) contributions, which are proved to be an essential ingredient to interpret neutrino cross section measure- ments at intermediate energies. In this regard, we develop a highly accurate parametriza- tion of the 2p-2h MEC nuclear responses based on a fully relativistic microscopic calcu- lation. In order to test the reliability of this SuSAv2-MEC model, we firstly compare our pre- dictions with the large amount of existing inclusive 12C(e, e ′) data over the whole energy spectrum. In this connection, we also extend our description to the complete inelastic regime performing a detailed analysis of the inelastic structure functions for protons and neutrons. All this provides a solid benchmark to assess the validity of our model for the analysis of charged-current neutrino-nucleus cross sections. Regarding this point, we com- pare our calculations with recent CCQE and inclusive _μ and ¯_μ measurements on 12C from different collaborations: MiniBooNE, T2K, MINER_A, NOMAD and SciBooNE, covering an energy range from a few MeV to tens of GeV. This comparison also allows us for a deeper understanding of the nuclear reaction mechanisms at different kinematics as well as their influence in terms of the energy and momentum transfers to the nucleus. In this regard, the SuSAv2-MEC approach is applied to the analysis of diverse nuclei of relevance for future neutrino oscillation experiments with the aim of shedding light on the experimental uncertainties arising from nuclear effects in both initial and final states. Furthermore, the SuSAv2-MEC can easily make predictions at high kinematics in which other microscopic-based models would require demanding, time-consuming calculations. Moreover, we also focus on the difference between electron and muon neutrino reac- tions, where a detailed knowledge of _μ and _e cross sections is decisive in connection to the _μ ! _e oscillation experiments aiming at the determination of the neutrino mass hierarchy and the search for CP violation in the leptonic sector. In summary, this PhD thesis constitutes an extensive analysis of the different neutrino- nucleus interaction mechanisms of interest for neutrino oscillation experiments and con- forms an open window for further works and collaborations in hadronic and nuclear physics.