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

TL;DR

This paper enhances the exclusion limits for light dark matter coupling to Standard Model particles by incorporating positron annihilation processes, significantly improving constraints in the 200-300 MeV mass range.

Contribution

It introduces the inclusion of secondary positron annihilation in NA64 data analysis, extending exclusion limits and approaching dark matter relic density constraints.

Findings

Extended exclusion limits in the 200-300 MeV mass range.

Demonstrated the effectiveness of resonant positron annihilation in dark matter searches.

Paved the way for future $e^+$ beam experiments.

Abstract

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.