Search for Sub-GeV Invisible Particles in Inclusive Decays of $ J/\psi $ to $ \varphi $

A search for an invisible particle, $ X$, with a mass between 0 and $ 0.96\text{}\text{}\mathrm{GeV}/{c}^{2}$, is performed in the process $ J/\psi \to \varphi +X$ using $ (8774.0\pm 39.4)\times {10}^{6}\text{}\text{}J/\psi $ events collected with the BESIII detector from 2017 to 2019. The $ \varphi $ meson is fully reconstructed and an efficient veto of photons and neutral and charged hadrons up to twice the $ {K}_{L}^{0}$ mass is applied to the rest of the event and the recoil mass against the $ \varphi $ is obtained precisely from the kinematic constraint in the event. No significant signal over the expected background is observed in the investigated region and the upper limit on the inclusive branch fraction of $ J/\psi \to \varphi +X$ is determined to be $ 7.0\times {10}^{-8}$ at 90% confidence level. Upper limits at a 90% confidence level are also given for this branch fraction as a function of the invisible particle mass, varying from $ 4\times {10}^{-9}$ to $ 4\times {10}^{-8}$ over the investigated mass range. Additionally, a 90% confidence level upper limit on the branch fraction of $ \eta \to \text{invisible}$ is determined to $ 2.4\times {10}^{-5}$, which improves the previous best results by more than 4 times. The analysis technique in this Letter offers a clean window to search for sub-GeV invisible particles, which can be adapted for other $ J/\psi $ decays and direct $ {e}^{+}{e}^{-}$ annihilation experiments in future studies, and improve the sensitivity by orders of magnitude.