Search for the dark photon in $π^0$ decays

A sample of $1.69 imes 10^7$ fully reconstructed $pi^0 ogamma e^+e^-$ decay candidates collected by the NA48/2 experiment at CERN in 2003--2004 is analysed to search for the dark photon ($A'$) production in the $pi^0 ogamma A'$ decay followed by the prompt $A' o e^+e^-$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_A'$ and mixing parameter $arepsilon^2$ is established. The obtained upper limits on $arepsilon^2$ are more stringent than the previous limits in the mass range $9~ m MeV/c^2<70~ m MeV/c^2$. The NA48/2 sensitivity to the dark photon production in the $K^pm opi^pm A'$ decay is also evaluated.