Search for an exotic light particle at Belle

TL;DR

This paper reports a search for a hypothetical light particle, $X^0$, in B meson decays, motivated by previous HyperCP experiment results suggesting a possible new particle around 214 MeV/$c^2$.

Contribution

The study conducts the first search for the $X^0$ particle in B meson decays using a large dataset collected by Belle, providing new constraints on its existence.

Findings

No evidence for $X^0$ in B decays was found.

Set upper limits on the branching fractions of $zkx$ and $zrhox$ decays.

Results constrain models predicting such a light exotic particle.

Abstract

The HyperCP experiment reported the observation of three events for the $\Sigma^+ \rightarrow p \mu^+\mu^-$ decay. The dimuon masses of the observed events are clustered within the detector resolution of 1 MeV/$c^2$. These decays might be interpreted as a two-body decay, $\Sigma^+ \rightarrow pX^0$, $X^0 \rightarrow \mu^+\mu^-$, where $X^0$ is a new particle with mass (214.3 $\pm$ 0.5) MeV/$c^2$. We report on a search for the $X^0$ particle in $\bzkx$ and $\bzrhox$ decays using 656 million $B$ meson pairs collected with the Belle detector at the KEKB asymmetric energy $e^+e^-$ collider.