Meson-exchange currents and nuclear correlations in neutrino and electron scattering with nuclei

TL;DR

Paloma Rodríguez Casalé develops a unified, MEC-and-SRC-aware framework for lepton-nucleus scattering by first refining the superscaling approach (SuSAM*) with a relativistic effective mass and a smeared, Fermi-like momentum distribution to avoid unphysical extrapolations, enabling consistent inclusion of MEC in the 1p1h channel. The thesis then demonstrates, across electron and CC neutrino scattering, that MEC typically reduce transverse responses in the 1p1h sector, with Δ-exchange dominating the interference and leaving pure MEC contributions small. In the second part, SRC are rigorously treated via the Bethe-Goldstone equation with the Granada 2013 potential, yielding high-momentum components and exposing tensor-driven np dominance in SRC tails. Finally, the combined MEC+SRC analysis shows that SRC can enhance the transverse response in the presence of MEC, addressing longstanding transverse-strength enhancements observed in electron scattering and informing neutrino-cross-section modeling. Together, these pieces provide a more complete, microscopic understanding of QE lepton-nucleus scattering and offer improved tools for neutrino oscillation analyses and electron-scattering benchmarks.

Abstract

This thesis is dedicated to the study of electron and neutrino scattering on nuclei, with special emphasis on meson-exchange currents (MEC) and short-range correlations (SRC) between nucleon pairs in one-particle emission processes. In chapter 2, the SuSAM* model is improved by redefining the single nucleon hadronic tensor, averaging it over a Fermi distribution instead of using the previous extrapolation from the relativistic Fermi gas. This formulation removes the inconsistency associated with negative contributions in kinematics far from the QE peak. The new definition allows extending the SuSAM* formalism to include MEC, which is the focus of chapter 3. In chapter 3, a scaling analysis of 12C data is performed incorporating MEC explicitly at the single nucleon level, leading to a new phenomenological scaling function. Using this model, the effect of MEC on EM responses is studied and compared with the relativistic Fermi gas (RFG) and the relativistic mean field (RMF) models. Chapter 4 presents a detailed analysis of the interference between 1b2b in the RT. It is shown that this interference is always negative within the independent-particle approximation. Chapter 5 extends the study of MEC to the CCQE neutrino scattering within the RFG, RMF, and SuSAM* frameworks. It is found that OB-MEC interference reduces both the RT and the neutrino cross section. Chapter 6 addresses SRC in the wave function of a nucleon pair in nuclear matter. The BG equation is solved using the NN potential developed by the Granada group, allowing for the calculation of high-momentum components. Finally, chapter 7 combined the effect of MEC and SRC, which extends the FG by including the high-momentum components of nucleon pairs affected by MEC. It is found that SRC enhance the RT, in contrast with what is observed in uncorrelated models.

Figures (89)

• Figure 1: Integration path in momentum space of the initial nucleon for different values of the energy transfer $\omega$ (indicated in MeV in the key for each panel) and for three values of the momentum transfer.
• Figure 2: Super scaling analysis with relativistic effective mass (SuSAM*) of $^{12}$C data. Top panel: experimental scaling data $f^*_{exp}$ plotted against $\psi^*$. Middle panel: data surviving after cleanup of non-quasielastic sparse points. The black curve is Gaussian fit made in this work, $f^*_{QE}(\psi^*)$. In the bottom panel we compare the two scaling functions obtained with two different definitions of the averaged single-nucleon responses: using the extrapolated Fermi gas responses and performing the average with a Fermi distribution defined in Section \ref{['aver']}.
• Figure 3: Averaged and extrapolated longitudinal and transverse response functions for proton plus neutron, as a function of $\omega$ and of the scaling variable $\psi^*$, for three values of the momentum transfer.
• Figure 4: Averaged and extrapolated longitudinal and transverse response functions for protons and neutrons, as a function of the scaling variable and for three values of the momentum transfer.
• Figure 5: Averaged and extrapolated transverse response functions for protons and neutrons, for $G_M^*=0$, as a function of the scaling variable and for three values of the momentum transfer. Averaged and extrapolated longitudinal response functions for protons and neutrons, for $G_E^*=0$, as a function of the scaling variable and for three values of the momentum transfer.

Theorems & Definitions (2)

• Proposition 1
• Proposition 2