Positronium decays with a dark Z and fermionic dark matter

Abstract

We investigate the invisible decay of positronium to probe the fermionic light dark matter mediated by the dark Z boson. The invisible decay rate of positronium through weak interaction in the standard model is too small 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.