ν_μ and ν_τ elastic scattering in Borexino

We perform a detailed study of neutrino-electron elastic scattering using the mono-energetic ^7Be neutrinos in Borexino, with an emphasis on exploring the differences between the contributions of ν_e, ν_μ, and ν_τ. We find that current data are capable of measuring these components such that the contributions from ν_μ and ν_τ cannot be zero, although distinguishing between them is challenging – the differences stemming from Standard Model radiative corrections are insufficient without significantly more precise measurements. In studying these components, we compare predicted neutrino-electron scattering event rates within the Standard Model (accounting for neutrino oscillations), as well as going beyond the Standard Model in two ways. We allow for non-unitary evolution to modify neutrino oscillations, and find that with a larger exposure (∼30x), Borexino may provide relevant information for constraining non-unitarity, and that JUNO may be able to accomplish this with its data collection of ^7Be neutrinos. We also consider novel ν_μ- and ν_τ-electron scattering from a gauged U(1)_L_μ - L_τ model, showing consistency with previous analyses of Borexino and this scenario, but also demonstrating the impact of uncertainties on Standard Model mixing parameters on these results.