Positronium Decays with Dark $Z$ and Fermionic Dark Matter

TL;DR

This paper explores how positronium decays could reveal light fermionic dark matter mediated by a dark Z boson, proposing that such decays might be detectable if dark matter is lighter than the electron.

Contribution

It introduces a model where positronium decay rates are enhanced via dark Z interactions, providing a new experimental probe for light dark matter beyond standard weak interactions.

Findings

Positronium decay rate can be significantly increased in the dark Z model.

Light fermionic dark matter lighter than the electron is viable under this scenario.

Constraints from Big Bang Nucleosynthesis are considered with an alternative thermal history.

Abstract

We investigate the invisible decay of positronium to probe the fermionic light dark matter mediated by the dark $Z$ boson. Too tiny is the invisible decay rate of positronium through weak interaction in the standard model to be detected in the experiment. We show that it can be enhanced to be observed in the future if the dark matter is lighter than the electron in the dark $Z$ model. We also compute the relic abundance of such light dark matter and discuss the Big Bang Nucleosynthesis constraint with an alternative thermal history scenario.