The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson $A^{\prime }$ were set by the NA64 experiment for the mass region $m_{A^{\prime }}\lesssim 250\mathrm{MeV}$, by analyzing data from the interaction of $2.84\times {10}^{11}$ 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including $A^{\prime }$ production via secondary positron annihilation with atomic electrons, we extend these limits in the 200–300 MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated $e^{+}$ beam efforts.

• Received 16 August 2021
• Accepted 15 October 2021

DOI:https://doi.org/10.1103/PhysRevD.104.L091701

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society