Constraints on Light Leptophilic Dark Matter Mediators from Decay Experiments

We study the influence of leptophilic dark matter interactions on decays of muons and ground state mesons in existing experiments. We consider a secluded dark sector exclusively interacting with leptons via either a (leptophilic) scalar or vector mediator. These interactions will therefore influence leptonic decays and deform the energy spectra. We first study the Michel decay of muons, $\mu^+\rightarrow e^+\nu_e \bar{\nu}_\mu$, which allow us to constrain the parameter space reasonably well. Secondly, the rare $\pi^\pm$, $K^\pm$, $D^\pm$ and $D_s^\pm$ decays to $e\nu$ will be considered. Scalar mediators would remove the Standard Model helicity suppression, so that strong constraints can be derived. The resulting bounds on the couplings of the light mediators to electrons and muons still turn out to be somewhat weaker than those from searches at low--energy $e^+e^-$ colliders and the magnetic moment of the muon, respectively. Finally, we show that kaon and pion decays basically exclude a ''Co--SIMP'' scenario where a scalar dark matter particle has a dimension--5 coupling to electrons.