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

Understand

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 $m_{A^\prime}\lesssim 250$ MeV, by analyzing data from the interaction of $2.84\cdot10^{11}$ 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.