Model independent bounds on heavy sterile neutrinos from the angular distribution of $\mathbf{B}\to {\mathbf{D}}^{\ast }\mathbf{\ell }\nu$ decays

Abstract

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 $B\to D^{\ast }\ell {\bar{\nu }}_{\ell }$ decays, with $\ell =e,\mu$. Indeed, a sterile neutrino $N$ may lead to competing $B\to D^{\ast }\ell \bar{N}$ 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 $M_{\mathrm{miss}}^2$ Belle distribution to search for evidences of an additional massive neutrino. We found in such a way a small hint at $M_{\mathrm{miss}}^2\sim (350\mathrm{MeV}{)}^2$. 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.

Comments: 23 pages, 6 figures, 2 tables. Matching published version to appear on JHEP