Dark-Sector Search via Pion-Produced η and η^{'} Mesons Decaying Invisibly in the NA64h Detector.

We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar η and η^{'} mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV π^{-} on nuclei of an active target as the source of neutral mesons. The η,η^{'}→invisible events would exhibit themselves via a striking signature-the complete disappearance of the incoming beam energy in the detector. No evidence for such events has been found with 2.9×10^{9} pions on target accumulated during one day of data taking. This allows us to set a stringent limit on the branching ratio Br(η^{'}→invisible)<2.1×10^{-4} improving the current bound by a factor of ≃3. We also set a limit on Br(η→invisible)<1.1×10^{-4} comparable with the existing one. These results demonstrate the great potential of our approach and provide clear guidance on how to enhance and extend the sensitivity for dark sector physics from future searches for invisible neutral meson decays.