Search for Light Dark Matter with NA64 at CERN.

Area of Science:

• Particle Physics
• Cosmology
• Astroparticle Physics

Background:

• Thermal dark matter models propose particles (χ) with masses below the electroweak scale could explain the universe's dark matter density.
• These models necessitate a weak interaction between dark matter and ordinary matter.

Purpose of the Study:

• To search for sub-GeV dark matter particle (χ) production via a dark photon (A') mediator.
• To probe previously unexplored parameter space in benchmark thermal scalar and fermionic dark matter models.

Main Methods:

• Utilized 100 GeV electron collisions with an active target at the NA64 experiment (CERN SPS).
• Collected 9.37×10^11 electrons on target data across 2016-2022 experimental runs.
• Searched for signals of dark matter production through the dark photon interaction.

Main Results:

• No evidence for dark matter particle production was observed.
• Established the most sensitive limits on dark photon (A') couplings to photons for masses below 0.35 GeV.
• Excluded specific parameter regions for scalar and Majorana dark matter with χ-A' coupling (αD ≤ 0.1) within the sub-GeV mass range.

Conclusions:

• The NA64 experiment has provided the most stringent constraints to date on certain thermal dark matter models.
• The results highlight the sensitivity of fixed-target experiments in searching for feebly interacting particles.
• Further exploration of dark matter interactions at lower mass scales is warranted.