Search for sub-GeV dark particles in $\hat{I}\cdot \to \ddot{I}{€}^0+\mathrm{invisible}$ decay

BESIII Collaboration, M. Ablikim, M. N. Achasov +685

Using (10087$\pm$44)$\times$10${}^6$ $J/\ddot{I}ˆ$ events collected with the BESIII detector at the BEPCII collider at the center-of-mass energy of $\sqrt{s}=3.097\mathrm{GeV}$, we report th…