The study of neutrino-nucleus interactions continues to be a dynamic and critical field at the interface of nuclear and particle physics. Measurements of neutrino-nucleus scattering cross sections are driven mainly by the needs of current and future neutrino oscillation experiments which aim for unprecedented precision and sensitivity in next generation experiments. This workshop followed on the success of a previous INT workshop on this topic (INT-13-57W) in bringing together approximately 40 scientists from neutrino physics, electron scattering physics, and theoretical high energy and nuclear physics, in an informal and open-ended discussion on some of the critical issues facing the field.

Highlights included:

• A systematic comparison of electron scattering data with several theoretical approaches (ab initio, superscaling, etc.) including the impact of relativistic effects using several approaches. In particular, there was a methodical comparison between formalisms that are intrinsically non-relativistic (e.g. ab initio methods) with other methods where various relativistic corrections can be systematically incorporated, leading to better clarity on the role and interplay of various effects. Many important developments in (super)scaling and universality were discussed, including the A-dependence of short-range correlations.

• A summary on a recent workshop at the University of Pittsburgh on a systematic comparison of neutrino interaction measurements to pinpoint whether datasets are in tension. Numerous challenges remain in specifying the reported observables as well as consistently applying existing event generator in different experimental environments. New analysis tools (i.e. NUISANCE) promise to facilitate consistent inter-experiment comparisons in the future.

• A major outcome of the last INT workshop was the establishment of enhancement in the transverse response in the axial sector using Greens Function Monte Carlo methods. Such an enhancement also naturally, though enigmatically, results from the Relativistic Mean Field approach. This effect is critical towards understanding and reconciling cross section enhancements observed in electron and neutrino-scattering data; the workshop included a comprehensive discussion based on these two theoretical methods.

• A significant frontier for neutrino interaction modelling, particularly with the advent of liquid argon time projection chambers, should be the ability to model and predict detailed hadronic final states for heavy nuclei. Of particular interest in the study of quasi-elastic scattering are nucleon emission and kinematics, as opposed to the inclusive lepton kinematics. Three different approaches to the program were presented and discussed, which clarified both the continued challenges and potential paths towards validating these methods.

A round table discussion involving a broad cross section of participants also highlighted several points from the workshop, including further clarification and consistency of terminology across fields and the presentation of measurements, the need for a more systematic approach towards analyzing and utilizing electron scattering data in order to validate and build confidence in models, and the potential for lattice calculations to provide an underpinning in the parametrization of axial form factors.

This collective gathering at INT of the theoretical and experimental communities studying electron and neutrino interactions has been the sole venue where such extended and unbounded discussions have occurred and remains a crucial means of both assessing progress and establishing the path forward in improving how we measure and model neutrino-nucleus scattering.

This diagram illustrates the multitude of processes that can occur when a lepton(neutrino or electron) scatters from a nucleus. In electron scattering, where the incoming lepton energy and outgoing scattering angle are specified, the various processes separate in variables such as the energy transfer as shown in this diagram. From left to right, the processes vary from elastic scattering, where the lepton scatters from the nucleus as a whole, to quasi-elastic scattering of bound nucleons, and to resonance production and other inelastic scattering processes. We are confronting the challenges of developing robust theoretical models for these processes in neutrino scattering, where the initial and final states are far less constrained.