Search for the dark photon in ${\pi }^0$ decays

A sample of $1.69\times {10}^7$ fully reconstructed ${\pi }^0\to \gamma e^{+}{e}^{-}$ decay candidates collected by the NA48/2 experiment at CERN in 2003–2004 is analysed to search for the dark photon ($A^{\mathrm{\prime }}$) production in the $\pi^0\to\gamma A’$decayfollowedbytheprompt$A’\to e^+e^-$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_{A^{\mathrm{\prime }}}$ and mixing parameter ${\epsilon }^2$ is established. The obtained upper limits on ${\epsilon }^2$ are more stringent than the previous limits in the mass range $9\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2<m_{A^{\mathrm{\prime }}}<70\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$. The NA48/2 sensitivity to the dark photon production in the $K^{\pm }\to {\pi }^{\pm }{A}^{\mathrm{\prime }}$ decay is also evaluated.