Lepton kinematics in low-energy neutrino-argon interactions

Background: Neutrinos in the low-energy regime provide a gateway to a wealth of interesting physics. While plenty of literature exists on detailing the calculation and measurement of total reaction strengths, relatively little attention is paid to the measurement and modeling of the final lepton through differential cross sections at low energies, despite the experimental importance. Purpose: We calculate differential cross sections for low-energy neutrino-nucleus scattering. We examine the role played by forbidden transitions in these distributions and how this differs across different energies and nuclear target masses. Attention is also paid to predictions for typical experimental neutrino spectra. Method: The differential cross sections are calculated within a continuum random-phase approximation framework, which allows us to include collective excitations induced by long-range correlations. The Coulomb interaction of the final lepton in charged current events is treated in an effective way. Results: Kinematic distributions are calculated for O-16, Ar-40, and Pb-208. The Ar-40 model results are compared for charged current (CC) (nu(e), e(-)) reactions to events generated by the Modeling of Argon Reaction Low-energy Yields (MARLEY) event generator [S. Gardiner, Ph.D. thesis, University of California, Davis (2018)], with noticeable discrepancies. Conclusion: Forbidden transitions have a marked effect on the kinematic distributions of the final lepton at low-energy kinematics, such as for decay-at-rest neutrinos or for a Fermi-Dirac spectrum at low temperature. This could introduce biases in experimental analyses. Backward scattering is noticeably more prominent than with MARLEY