Probing a dark photon using rare leptonic kaon and pion decays

TL;DR

This paper explores how rare leptonic kaon and pion decays can be used to detect a 16.7-MeV dark photon, potentially confirming its role in explaining anomalies observed in beryllium-8 transitions.

Contribution

It evaluates the sensitivity of future experiments to a specific dark photon model and demonstrates how angular distributions can help distinguish signals from background noise.

Findings

Future experiments can probe most of the dark photon parameter space relevant to the Atomki anomaly.

Angular distributions provide a method to differentiate dark photon signals from background.

A significant portion of the 16.7-MeV dark photon parameter space can be tested.

Abstract

Rare leptonic kaon and pion decays $K^+(\pi^+) \to \mu^+\, \nu_{\mu}\, e^+ e^-$ can be used to probe a dark photon of mass ${\cal O}(10)$~MeV, with the background coming from the mediation of a virtual photon. This is most relevant for the 16.7-MeV dark photon proposed to explain a 6.8$\sigma$ anomaly recently observed in $^8$Be transitions by the Atomki Collaboration. We evaluate the reach of future experiments for different scenarios of how the dark photon couples with the standard model particles, and show that a great portion of the preferred 16.7-MeV dark photon parameter space can be decisively probed. We also show the use of angular distributions to further distinguish the signal from the background.