Many-body interactions of neutrinos with nuclei: Observables

Background: The total inclusive cross sections obtained for quasielastic (QE) scattering in the Mini Booster Neutrino Experiment (MiniBooNE) are significantly larger than those calculated by all models based on the impulse approximation and using the world average value for the axial mass of M-A approximate to 1 GeV. This discrepancy has led to various, quite different explanations in terms of increased axial masses, changes in the functional form of the axial form factor, increased vector strength in nuclei, and initial two-particle interactions. This is disconcerting since the neutrino energy reconstruction depends on the reaction mechanism. Purpose: We investigate whether exclusive observables, such as nucleon knockout, can be used to distinguish between the various proposed reaction mechanisms. We determine the influence of 2p-2h excitations on the neutrino energy reconstruction. Method: We extend the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) model by explicitly incorporating initial 2p-2h excitations. Results: We calculate inclusive cross sections and numbers and spectra of knockout nucleons and show their sensitivity to the presence of 2p-2h initial excitations. We also discuss the influence of 2p-2h excitations on the neutrino energy reconstruction. Conclusions: Inclusive double-differential cross sections, depending only on muon variables, are fairly insensitive to the reaction mechanism. 2p-2h excitations lead to an increase in the number n of knockout nucleons for n >= 2, while only the n = 1 knockout remains a clean signal of true QE scattering. The spectra of knockout nucleons are also changed, but their shape is hardly affected. In the energy reconstruction, 2p-2h interactions as well as Delta excitations lead to a downward shift of the reconstructed energy; this effect of 2p-2h excitations disappears at higher energies because the 2p-2h influence is spread out over a wider energy range.