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

TL;DR

This paper derives model-independent bounds on heavy sterile neutrinos from Belle's angular analysis of B to D* l nu decays, revealing potential hints of a massive neutrino and setting stringent constraints on new physics couplings.

Contribution

It provides the first comprehensive bounds on heavy sterile neutrinos from angular distributions in B decays, including a search for a massive neutrino and analysis of new physics couplings.

Findings

Hints of a sterile neutrino at 350 MeV mass from missing mass distribution.

Most stringent bounds to date on vector and left-handed scalar operators.

Potential impact of sterile neutrinos on angular distributions analyzed.

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