Scalar Dark Matter in Leptophilic Two-Higgs-Doublet Model

Two-Higgs-Doublet Model of Type-X in the large $\tan\beta$ limit becomes leptophilic to allow a light pseudo-scalar $A$ and thus provides an explanation of the muon $g-2$ anomaly. Introducing a singlet scalar dark matter $S$ in this context, one finds that two important dark matter properties, nucleonic scattering and self-annihilation, are featured separately by individual couplings of dark matter to the two Higgs doublets. While one of the two couplings is strongly constrained by direct detection experiments, the other remains free to be adjusted for the relic density mainly through the process $SS\to AA$. This leads to the $4\tau$ final states which can be probed by galactic gamma ray detections.