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

TL;DR

This study analyzed a large dataset of neutral pion decays to search for dark photons, setting new limits on their properties and discussing future search prospects in kaon decays.

Contribution

The paper presents the most stringent limits to date on dark photon parameters in a specific mass range using pion decay data.

Findings

No dark photon signal observed.

Established new exclusion limits on dark photon mass and mixing parameter.

Discussed future search potential 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 prospects of the dark photon search in the $K^\pm\to\pi^\pm A'$ decay are also discussed.