Search for Dark Matter in 2HDM+complex singlet at HL-LHC and future Lepton Colliders

We investigate the phenomenological prospects of the Two Higgs Doublet and Complex Singlet Scalar Extension (2HDMS) in the context of dark matter (DM) and Higgs phenomenology. The 2HDMS provides an extended Higgs sector along with a DM candidate. In this work, we perform an exhaustive scan to find representative benchmarks that are consistent with all theoretical and experimental constraints. We choose benchmarks with light, intermediate, and massive DM masses and in some cases, also accommodate the 95 GeV excess observed at the Large Electron-Positron Collider (LEP) and Large Hadron Collider (LHC). We investigate and compare different DM signal topologies for different DM mass regions and focus on the relevant signatures at all current and future colliders, such as the High Luminosity LHC, and at proposed future lepton colliders, including electron-positron and muon colliders. Using a cut and count analysis, we show that while the High Luminosity LHC (HL-LHC) may hint towards new physics, future lepton colliders prove to be efficient discovery probes for dark matter searches at colliders.