Sensitivity to Heavy Neutral Leptons with the SAND detector at the DUNE ND complex

Heavy Neutral Leptons (HNLs) have been an interesting topic for experimental particle physics in the past few years. A study has been performed within the framework of the multi-instrument DUNE near detector complex, specifically regarding the on-Axis System, to assess the sensitivity to HNL within six years of exposure. By utilizing two MC generators, and charmed heavy meson decay channels, the sensitivity to HNL masses between 0.3 and 1.8 GeV/$c^2$ has been explored. A Mad-Graph/Mad-Dump model has been implemented based on the $\nu MSM$ Lagrangian, and used to obtain accurate kinematics for the decay of mesons and HNL. The simulated final-state particles have been propagated through the detector; a track reconstruction algorithm, based on the Kalman Filter technique, along with a simple two-body decay selection, is implemented to estimate efficiency and background rejection. The HNL sensitivity has been estimated both from purely phenomenological as well as experimental point of view, reaching O(${10}^{-9}$) for higher HNL masses, with about a factor 3 deterioration between the phenomenological and the experimental case. In this paper, the results for direct and indirect decay channels of charmed meson $D_s$ to HNL has been investigated and the potential for further improvements has been discussed.