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

Abstract

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 ($A^{\prime }$) production in the ${\pi }^0\to \gamma A^{\prime }$ decay followed by the prompt $A^{\prime }\to e^{+}{e}^{-}$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_{A^{\prime }}$ 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 $9\mathrm{MeV}/c^2<m_{A^{\prime }}<70\mathrm{MeV}/c^2$. The NA48/2 sensitivity to the dark photon production in the $K^{\pm }\to {\pi }^{\pm }{A}^{\prime }$ decay is also evaluated.

Revised version accepted for publication in Physics Letter B

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