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The current most stringent constraints for the existence of sub-GeV darkmatter coupling to Standard Model via a massive vector boson $A^\prime$ wereset by the NA64 experiment for the mass region $m_{A^\prime}\lesssim 250$ MeV,by analyzing data from the interaction of $2.84\cdot10^{11}$ 100-GeV electronswith an active thick target and searching for missing-energy events. In thiswork, by including $A^\prime$ production via secondary positron annihilationwith atomic electrons, we extend these limits in the $200$-$300$ MeV region byalmost an order of magnitude, touching for the first time the dark matter relicdensity constrained parameter combinations. Our new results demonstrate thepower of the resonant annihilation process in missing energy dark-mattersearches, paving the road to future dedicated $e^+$ beam efforts.

Improved exclusion limit for light dark matter from $e^+e^-$ annihilation in NA64