A search for feebly-interacting particles in $B$ decays with missing energy at Belle

TL;DR

This study searches for invisible particles produced in B meson decays using Belle data, setting new upper limits on their branching fractions and constraining new physics models with no significant signals observed.

Contribution

First direct constraints on certain B decay channels involving invisible particles, improving exclusion limits and exploring new parameter space in physics beyond the Standard Model.

Findings

No significant signal observed in the search.

Set 90% CL upper limits on branching fractions between 10^{-4} and 10^{-6}.

Provided the most stringent limits to date on these decay modes.

Abstract

We present a search for an invisible hidden-sector particle $ X_{\rm{inv}}$, produced in $B^0 \to \bar{D}^0 X_{\rm{inv}}$ and $B^\pm\to h X_{\rm{inv}}$ decays, where $h = \pi^\pm$, $K^\pm$, $D_{s}^\pm$, $p^\pm$. The search is performed using $e^+ e^-$ collision data recorded with the Belle detector, corresponding to 711~fb$^{-1}$. No significant signal is observed. We set 90\% confidence level upper limits ranging between $10^{-4}$ and $10^{-6}$ on the branching fraction $\mathcal{B}(B \to h X_{\rm{inv}})$ as a function of $m_{ X_{\rm{inv}}}$. Corresponding limits are set on $\mathcal{B}(B \to hX)\times\mathcal{B}(X\to\gamma\gamma)$ for lifetimes $c\tau_X$ between $10~\mu$m and 10 m. Many of these limits are the first direct constraints on their respective decays. Our results provide the most stringent exclusion limits to date on the branching fractions for all search channels, and exclude previously unexplored regions of parameter space relevant to several new physics models.