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

Using (10087±44)×106 J/ψ events collected with the BESIII detector at the BEPCII collider at the center-of-mass energy of s=3.097 GeV, we report the first search for η→π0S→π0χχ¯ 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/c2. 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.