Search for invisible decays of sub-GeV dark photons in missing-energy events at the CERN SPS

TL;DR

This paper reports a search for sub-GeV dark photons in missing-energy events at CERN SPS, setting new limits on their properties and excluding certain models as explanations for the muon g-2 anomaly.

Contribution

First direct search for invisible dark photons in the sub-GeV range using the NA64 experiment at CERN SPS, establishing new exclusion limits.

Findings

No evidence for dark photon decays was observed.

Excluded invisible dark photons with mass below 100 MeV as muon g-2 explanation.

Set new upper limits on the kinetic mixing parameter.

Abstract

We report on a direct search for sub-GeV dark photons (A') which might be produced in the reaction e^- Z \to e^- Z A' via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The A's would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75\cdot 10^{9} electrons on target. We set new limits on the \gamma-A' mixing strength and exclude the invisible A' with a mass < 100 MeV as an explanation of the muon g_\mu-2 anomaly.