Quasielastic electromagnetic scattering cross sections and world data comparisons in the GENIE Monte Carlo event generator

Abstract

The usage of Monte Carlo neutrino event generators (MCνEGs) is a norm within the high-energy ν scattering community. The relevance of quasielastic energy regimes to ν oscillation experiments implies that accurate calculations of ν-nucleus cross sections in this regime will be a key contributor to reducing the systematic uncertainties affecting the extraction of oscillation parameters. In spite of this, many MCνEGs utilize highly phenomenological, parameterized models of quasielastic scattering cross sections. Moreover, a culture of validation of MCνEGs against prolific electron scattering data has been historically lacking. In this work, we implement new electron-nucleus cross sections obtained from nuclear ab initio approaches in GENIE, the primary MCνEG utilized by the FNAL community. In particular, we utilize results from Quantum Monte Carlo computational methods which solve the many-body nuclear problem in the Short-Time Approximation (STA), allowing consistent retention of two-nucleon dynamical effects that are crucial to explain available nuclear electromagnetic (electroweak) data over a wide range of energy and momentum transfers. This new implementation in GENIE is fully tested against the world quasielastic electromagnetic data, finding agreement with available data below ∼ 2 GeV of beam energy with the aid of a scaling function formalism. The STA is currently limited to study A ≤ 12 nuclei, however, its exclusive multibody identity components are exportable to other many-body computational techniques such as Auxiliary Field Diffusion Monte Carlo which can reach A ≤ 40 systems while continuing to realize the semifinal states contained within the STA's multinucleon dynamics. Together, these developments promise to make future experiments such as the Deep Underground Neutrino Experiment ever more accurate in their assessment of MCνEG systematics, ν properties, and potentially empower the discovery of physics beyond the Standard Model. 2. We discuss our new, holistic framework within GENIE based on calculated electromagnetic (electroweak) nuclear response functions and supplemented by interpolation schemes to derive double differential electron scattering cross sections;

Comparisons and challenges of modern neutrino-scattering experiments (TENSIONS 2019 report)

“…Consistency in the isospin decomposition of emitted nucleons is an ongoing problem. While some advanced theoretical models [31] have explored interference between one-and two-body currents, the implementation of these in event generators is just beginning to be explored [32].…”

Theoretical tools for neutrino scattering: interplay between lattice QCD, EFTs, nuclear physics, phenomenology, and neutrino event generators

“…The computational algorithm exploits a factorization scheme to consistently retain two-body physics, namely two-body correlations and associated two-body currents. Despite limiting the description of the scattering process to interactions of the probe with pairs of correlated nucleons, the STA is found to be in good agreement with both GFMC predictions and experimental data for electron scattering from the alpha particle and the trinucleon systems [65,393,394]. Importantly, the STA can account for interference effects between one-and two-body current contributions that are found to be essential to explain, e.g., the observed excess in the transverse electromagnetic nuclear response [244,313].…”

“…An example of the latter has been the use of tables of pre-computed inclusive response functions to evaluate neutrino cross sections [394,[571][572][573]. This strategy enables a straightforward implementation of multiple models, including those which would be computationally impractical otherwise.…”

Event Generators for High-Energy Physics Experiments

“…However, it is computationally expensive, requiring many CPU hours to achieve precise results. A first STA implementation in GENIE, able to reproduce the model predictions for inclusive electron scattering on 4 He, relies upon interpolation of tables of precomputed nuclear responses [1044]. Efforts to extend the technique to simulate the hadronic final state (while preserving proper correlations with the sampled lepton kinematics) are underway.…”