Meson-exchange currents in lepton-nucleus scattering

Neutrino physics represents one of the most dynamic and rapidly evolving areas in modern science. Understanding and accurately describing neutrinos -the most elusive particles in the Standard Model- is a central goal of the community. This effort focuses heavily on the precise determination of the PMNS oscillation parameters, particularly the CP-violating Dirac phase, which ongoing experiments like T2K and NOνA, as well as future projects such as DUNE and Hyper-Kamiokande, aim to measure. These experiments also seek to determine the mass hierarchy of the three neutrino eigenstates. Achieving these goals requires delving into the complexities of nuclear interactions and the behavior of nuclear systems, given that the very low interaction cross-section of neutrinos necessitates the use of nuclei as targets. Consequently, a detailed understanding of lepton-nucleus interactions has become critical. Lepton-nucleus interactions are characterized by a multitude of phenomena, which manifests through several interaction channels, active at different kinematics. In neutrino experiments, these channels cannot be disentangled, necessitating simultaneous descriptions of different reaction mechanisms. Among these, the two-particle–two-hole (2p2h) channel driven by meson-exchange currents (MEC) has emerged as a subdominant but crucial component for interpreting experimental data. This work provides a comprehensive exploration of the MEC formalism, starting with its Lagrangian formulation with elements derived from chiral perturbation theory. The model is fully relativistic, with the MEC calculations carried out within the Relativistic Fermi Gas framework, employing the infinite nuclear matter approximation. Therefore, the presented work provides a generalization to the weak sector of the electromagnetic inclusive model developed by the Torino group and an extension of the calculation to enable semi-inclusive predictions. Benchmark calculations for inclusive two-nucleon knockout processes validate the presented model, emphasizing the role of key elements such as strong form factors and baryonic resonances. The calculation is then extended to describe semi-inclusive processes. These measurements, which involve the detection of both the outgoing lepton and one or more hadrons in the final state, provide more specific and detailed insights into nuclear dynamics compared to inclusive measurements. However, very few microscopic calculations of this cross section in the 2p2h channel are available. The main scope of this work is to fill this gap, meeting the needs of the experimental neutrino physics community. As new high-precision data become available, the results of this work will play a significant role in their interpretation, contributing to a deeper understanding of lepton-nucleus interactions and advancing the precision era of neutrino physics.