[Paper Review] Two particle-two hole excitations in charged current quasielastic neutrino-nucleus interactions

M. Martini|arXiv (Cornell University)|Oct 26, 2011

Neutrino Physics Research2 references3 citations

TL;DR

This paper reviews theoretical models incorporating two-particle-two-hole (2p-2h) excitations in charged current quasielastic (CCQE) neutrino-nucleus interactions to explain the MiniBooNE experiment's anomalously high cross-section measurements. By including multinucleon emission via two-body currents and medium effects such as RPA correlations and Δ-hole excitations, the models achieve good agreement with MiniBooNE data without requiring a modified axial mass, resolving the long-standing discrepancy in the standard Fermi gas model.

ABSTRACT

We review the theoretical status of the models including the multinucleon emission channel in the calculation of quasielastic neutrino cross sections at MiniBooNE kinematics.

Motivation & Objective

• To resolve the discrepancy between MiniBooNE's measured CCQE cross-section and predictions from the relativistic Fermi gas model with standard axial mass $M_A = 1.03$ GeV/$c^2$.
• To investigate whether multinucleon emission, particularly 2p-2h excitations, can account for the excess in the observed quasielastic-like events.
• To evaluate the theoretical consistency and predictive power of different models incorporating 2p-2h contributions across multiple groups (Martini, Amaro, Nieves, Bodek).
• To assess the impact of medium effects—such as RPA correlations, Δ-hole degrees of freedom, and nucleon-nucleon correlations—on the cross-section predictions.
• To determine whether the inclusion of 2p-2h mechanisms can reconcile experimental data with the standard axial mass, avoiding the need for ad hoc adjustments.

Proposed method

• Theoretical calculations of 2p-2h contributions are derived from microscopic two-body current models, including pion-in-flight, contact, and Δ-intermediate state terms.
• The models employ relativistic or non-relativistic frameworks with RPA to include medium polarization and collective effects, including π and ρ meson exchange and $g'$ Landau-Migdal parameters.
• Two-body current matrix elements are computed using effective interactions derived from pion absorption and electromagnetic response data, ensuring gauge invariance and consistency with electron scattering data.
• The 2p-2h contributions are combined with genuine quasielastic processes, and the total cross-section is compared to MiniBooNE and SciBooNE data across energy and angular distributions.
• Different model implementations are compared: Martini et al. use non-relativistic reduction with Δ-MEC, Amaro et al. use a fully relativistic superscaling approach with MEC only, and Nieves et al. generalize electron scattering models to neutrino scattering with full axial and vector contributions.
• The models are validated by comparing flux-averaged double-differential cross-sections in $T_\mu$ and $\cos\theta$ to MiniBooNE measurements, assessing agreement across kinematic regions.

Experimental results

Research questions

• RQ1 Can 2p-2h excitations explain the MiniBooNE data without modifying the axial mass $M_A$?
• RQ2 How do different theoretical treatments of two-body currents and medium effects affect the predicted cross-sections?
• RQ3 Why does the agreement with MiniBooNE data vary across different models, particularly in high-angle regions?
• RQ4 To what extent do nucleon-nucleon correlations and interference terms with MECs influence the 2p-2h cross-section?
• RQ5 How does the inclusion of 2p-2h contributions affect the reliability of neutrino energy reconstruction in oscillation experiments?

Key findings

• The inclusion of 2p-2h excitations in the theoretical model leads to excellent agreement with MiniBooNE's flux-averaged double-differential cross-section data across all measured $T_\mu$ and $\cos\theta$ values.
• The total 'quasielastic' cross-section predicted by Martini et al. and Nieves et al. matches MiniBooNE data without requiring an increase in the axial mass beyond the standard value of $M_A = 1.03$ GeV/$c^2$.
• The 2p-2h contribution is particularly important in the 'dip' region between the quasielastic peak and the Δ-resonance, where it fills the missing strength observed in one-body models.
• Amaro et al.'s model shows improved agreement at low angles but still exhibits residual discrepancies at higher angles, likely due to the absence of nucleon-nucleon correlation contributions.
• The antineutrino cross-section is less sensitive to 2p-2h effects due to the negative axial-vector interference, but the models still show a measurable enhancement, with differences in magnitude depending on the treatment of medium effects.
• Theoretical results from Martini et al., Nieves et al., and Bodek et al. all confirm that the observed MiniBooNE excess can be explained by multinucleon emission, not by a change in $M_A$.