Search for production of an invisible dark photon in π0 decays

The results of a search for π 0 decays to a photon and an invisible massive dark photon at the NA62 experiment at the CERN SPS are reported. From a total of 4.12 × 108 tagged π 0 mesons, no signal is observed. Assuming a kinetic-mixing interaction, limits are set on the dark photon coupling to the ordinary photon as a function of the dark photon mass, improving on previous searches in the mass range 60–110 MeV/c 2. The present results are interpreted in terms of an upper limit of the branching ratio of the electro-weak decay π⁰→ γ ν ν̄ π 0 → γ ν ν ¯ , improving the current limit by more than three orders of magnitude.