Discovery potential for Heavy Neutral Leptons at the SAND detector of the DUNE experiment

Beyond its primary goal of determining the remaining neutrino oscillation parameters, the DUNE next-generation long-baseline neutrino experiment will pursue a broad program of precision measurements of neutrino cross-sections and standard model observables, as well as searches for Physics Beyond the Standard Model (BSM). At the DUNE Near Detector complex, the SAND detector will exploit the intense broadband neutrino beam and a high-resolution gas-detector tracker in a magnetic field coupled to a novel 1-ton liquid Argon detector and a high-granularity calorimeter to perform searches of BSM-phenomena. In particular, the high neutrino flux and a low-density tracker are optimal conditions to search for Dark Neutrinos, a class of Heavy Neutral Lepton (HNL) models in which HNLs are produced in the scattering of standard model neutrinos and undergo displaced decays to di-lepton pairs through the exchange of a dark sector gauge boson Z', or dark photon. This dissertation presents a first estimate of the discovery potential in SAND for dark neutrino models featuring decays to e+/e− pairs. To this end, a dedicated event generator was integrated in the standard SAND simulation chain, allowing the definition of the decay signatures in the tracker and the development of a dedicated track fitting algorithm for e+/e−. A multivariate classification was then performed to estimate the SAND sensitivity to the models under study. A significant improvement over the current limits after one year of exposure was found across the model range.