In this paper we study the bounds that can be inferred on New Physics couplings to heavy sterile neutrinos N from the recent measurements performed by the Belle collaboration of the angular analysis of \( B\to {D}^{\ast}\ell {\overline{\nu}}_{\ell } \) decays, with ℓ = e, μ. Indeed, a sterile neutrino N may lead to competing \( B\to {D}^{\ast}\ell \overline{N} \) decays and Belle might have measured an incoherent sum of these two independent channels. After reviewing the theoretical formalism required to describe this phenomenon in full generality, we first perform a bump hunt in the \( {M}_{\textrm{miss}}^2 \) Belle distribution to search for evidences of an additional massive neutrino. We found in such a way a small hint at \( {M}_{\textrm{miss}}^2 \) ∼ (354 MeV)2. However, the Belle angular analysis is sensitive to N masses up to \( \mathcal{O} \) (50 MeV), preventing us to further inspect this hint. Nevertheless, we study the potential impact of this additional channel in the allowed mass range on the measured angular distributions and extract model-independent bounds on the new-physics couplings which could mediate such an interaction. In particular, in the mass window here inspected, we obtain the most stringent bounds for vector and left-handed scalar operators to date.
