Probing Light Dark Matter with positron beams at NA64

TL;DR

This paper reports a pioneering search for Light Dark Matter using positron beams at NA64, employing resonant annihilation to detect potential dark photon signals, setting new exclusion limits and demonstrating the technique's effectiveness.

Contribution

First experimental search for Light Dark Matter with positron beams using resonant annihilation, establishing new exclusion limits and showcasing the method's potential.

Findings

No dark photon signals observed in the data.

Set new exclusion limits on dark photon parameters.

Demonstrated the effectiveness of positron beam techniques.

Abstract

We present the results of a missing-energy search for Light Dark Matter which has a new interaction with ordinary matter transmitted by a vector boson, called dark photon $A^\prime$. For the first time, this search is performed with a positron beam by using the significantly enhanced production of $A^\prime$ in the resonant annihilation of positrons with atomic electrons of the target nuclei, followed by the invisible decay of $A^\prime$ into dark matter. No events were found in the signal region with $(10.1 \pm 0.1)~\times~10^{9}$ positrons on target with 100 GeV energy. This allowed us to set new exclusion limits that, relative to the collected statistics, prove the power of this experimental technique. This measurement is a crucial first step toward a future exploration program with positron beams, whose estimated sensitivity is here presented.