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

The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson <math display="inline"><msup><mi>A</mi><mo>′</mo></msup></math> were set by the NA64 experiment for the mass region <math display="inline"><msub><mi>m</mi><msup><mi>A</mi><mo>′</mo></msup></msub><mo>≲</mo><mn>250</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>, by analyzing data from the interaction of <math display="inline"><mn>2.84</mn><mo>×</mo><msup><mn>10</mn><mn>11</mn></msup></math> 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including <math display="inline"><msup><mi>A</mi><mo>′</mo></msup></math> 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 <math display="inline"><msup><mi>e</mi><mo>+</mo></msup></math> beam efforts.