Searching for lepton portal dark matter with colliders and gravitational waves

A bstract We study the lepton portal dark matter (DM) model in which the relic abundance is determined by the portal coupling among the Majorana fermion DM candidate χ , the singlet charged scalar mediator S ± and the Standard Model (SM) right-handed lepton. The direct and indirect searches are not sensitive to this model. This article studies the lepton portal coupling as well as the scalar portal coupling (between S ± and SM Higgs boson), as the latter is generally allowed in the Lagrangian. The inclusion of scalar portal coupling not only significantly enhances the LHC reach via the gg → h * → S + S − process, but also provides a few novel signal channels, such as the exotic decays and coupling devi- ations of the Higgs boson, offering new opportunities to probe the model. In addition, we also study the Drell-Yan production of S + S − at future lepton colliders, and find out that the scenario where one S ± is off-shell can be used to measure the lepton portal coupling directly. In particular, we are interested in the possibility that the scalar potential triggers a first-order phase transition and hence provides the stochastic gravitational wave (GW) signals. In this case, the terrestrial collider experiments and space-based GW detectors serve as complementary approaches to probe the model.