Michel parameters for elucidating the neutrinos nature and searching for new physics

We use the most general four-lepton effective interaction Hamiltonian to investigate the impact of massive Dirac and Majorana neutrinos on the leptonic decays of muons and taus. Our analysis encompasses the specific energy and angular distribution of the resulting charged lepton, accounting for both the initial and final polarizations of the charged leptons. Additionally, we identify the emergence of novel generalized Michel parameters and concentrate on the influence of the heavy neutrino masses, which can make significant contributions in cases where new sterile neutrinos exhibit non-negligible mixing. Our analysis reveals that the most promising scenario occurs in the case of τ decays, featuring one heavy neutrino with a mass approximately ranging from 10^2 to 10^3 MeV. In this setting, the discrepancy between the Dirac and Majorana cases could reach an order of magnitude of 10^−4, which is significant enough to be detected in present and future experiments.