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

TL;DR

This study searches for dark photons in neutral pion decays using a large dataset, setting new limits on their properties and exploring potential production in kaon decays, with no positive detection.

Contribution

It provides the most stringent limits to date on dark photon parameters in the 9-70 MeV/c^2 mass range using pion decay data.

Findings

No dark photon signal observed.

Established new exclusion limits on mixing parameter ε^2.

Evaluated sensitivity for dark photon production in kaon 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 ($A'$) production in the $\pi^0\to\gamma A'$ decay followed by the prompt $A'\to e^+e^-$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_{A'}$ and mixing parameter $\varepsilon^2$ is established. The obtained upper limits on $\varepsilon^2$ are more stringent than the previous limits in the mass range $9~{\rm MeV}/c^2<m_{A'}<70~{\rm MeV}/c^2$. The NA48/2 sensitivity to the dark photon production in the $K^\pm\to\pi^\pm A'$ decay is also evaluated.