# Probing sterile neutrino in $B$ ($D$) meson decays at Belle II (BESIII)

## TL;DR

This paper explores how studying specific meson decays at Belle II and BESIII can detect or constrain heavy sterile neutrinos, offering competitive or improved bounds on their mixing with active neutrinos and insights into their nature.

## Contribution

It proposes a systematic method to detect or constrain heavy sterile neutrinos in meson decays, with improved sensitivity for certain mass ranges and the ability to distinguish Dirac versus Majorana neutrinos.

## Key findings

- Constraint on |U_{μN}|^2 comparable to LHCb with full Belle II data.
- Better constraint on |U_{μN}|^2 for m_N < 2 GeV.
- Potential to distinguish Dirac and Majorana neutrinos through decay signatures.

## Abstract

We present, how a systematic study of $B \to D\ell N$ ($D \to K \ell N$) decays with $\ell=\mu,\tau$, at Belle II (BESIII) can provide unambiguous signature of a heavy neutrino $N$ and/or constrain its mixing with active neutrinos $\nu_\ell$, which is parameterized by $|U_{\ell N}|^2$. Our constraint on $|U_{\mu N}|^2$ that can be achieved from the full Belle II data is comparable with what can be obtained from the much larger data set of the upgraded LHCb. Additionally, our method offers better constraint on $|U_{\mu N}|^2$ for mass of sterile neutrino $m_N < 2$ GeV. We can also probe the Dirac and Majorana nature of $N$ by observing the sequential decay of $N$, including suppression from observation of a displaced vertex as well as helicity flip, for Majorana $N$.