Search for Sub-GeV Dark Particles in η→π^{0}+Invisible Decay

Using (10087±44)×10^{6}  J/ψ events collected with the BESIII detector at the BEPCII collider at the center-of-mass energy of sqrt[s]=3.097  GeV, we report the first search for η→π^{0}S→π^{0}χχ[over ¯] with S denoting an on-shell dark scalar boson and χ an invisible dark matter particle. No significant signals are observed with S mass ranging from 0 to 400  MeV/c^{2}. The upper limits on the branching fractions and the new physics coupling strengths between S and quarks are set to be (1.8-5.5)×10^{-5} and (1.3-3.2)×10^{-5} at the 90% confidence level, respectively. The constraints on the dark-matter-nucleon scattering cross section is improved by approximately five orders of magnitude over previous dark-matter-nucleon scattering experiments, providing unique insights into sub-GeV dark matter.