Search for sub-GeV dark particles in $\eta\to\pi^0+\rm{invisible}$ decay

TL;DR

This study searches for sub-GeV dark particles in eta decays using BESIII data, setting new limits on dark scalar bosons and dark matter interactions, with no significant signals observed.

Contribution

First search for eta decay into a dark scalar and invisible particles, establishing new upper limits and improving constraints on dark matter interactions.

Findings

No significant signals observed in eta to pi0 plus invisible decay.

Set upper limits on branching fractions and coupling strengths.

Improved dark-matter-nucleon scattering cross section constraints by about 5 orders of magnitude.

Abstract

Using (10087$\pm$44)$\times$10$^{6}$ $J/\psi$ events collected with the BESIII detector at the BEPCII collider at the center-of-mass energy of $\sqrt{s}=3.097~\rm{GeV}$, we report the first search for $\eta\to\pi^0S\to\pi^0\chi\bar{\chi}$ with $S$ denotes an on-shell dark scalar boson and $\chi$ an invisible dark matter particle. No significant signals are observed with $S$ mass ranging from 0 to 400 $\rm{MeV}/c^2$. The upper limits on the branching fractions and the new physics coupling strengths between $S$ and quarks are set to be $(1.8\sim5.5)\times10^{-5}$ and $(1.3\sim3.2)\times10^{-5}$ at the 90% confidence level, respectively. The constraints on the dark-matter-nucleon scattering cross section is improved by approximately 5 orders of magnitude over previous dark-matter-nucleon scattering experiments, providing unique insights into sub-GeV dark matter.