Search for Sub-GeV Dark Particles in 𝜂→𝜋0+Invisible Decay

Using $(10087 \pm 44)\times10^{6}$ $J/\psi$ events collected with the BESIII detector at the BEPCII collider at a center-of-mass energy of $\sqrt{s}=3.097~\mathrm{GeV}$, we report the first search for the decay $\eta\to\pi^0S\to\pi^0\chi\bar{\chi}$, where $S$ denotes an on-shell dark scalar boson and $\chi$ an invisible dark matter particle. No significant signal is observed for $S$ masses ranging from $0$ to $400~\mathrm{MeV}/c^2$. The upper limits on the branching fraction and on the new-physics coupling strength between $S$ and quarks are determined to be $$(1.8\text{--}5.5)\times10^{-5}$$ and $$(1.3\text{--}3.2)\times10^{-5},$$ respectively, at the $90\%$ confidence level. The corresponding constraints on the dark-matter--nucleon scattering cross section improve upon previous dark-matter--nucleon scattering experiments by approximately five orders of magnitude, providing unique sensitivity to sub-GeV dark matter.