Neutrinos masses are clear evidence of physics beyond-the-SM. In this talk, we will review mechanisms of neutrino mass generation, focusing on the seesaw mechanism. This mechanism predicts the existence of heavy neutral leptons (or right-handed neutrinos) with rich phenomenology that can be probed with current and future experiments.
We discuss the analysis procedure of the BeEST Phase III and its current status. The discussion will include the application of likelihood profiling for the multiple spectra and various systematic uncertainties that may affect the final limits for the heavy neutrino search.