Gauge Singlet Vector-like Fermion Dark Matter, LHC Diphoton Rate and Direct Detection

We study a gauge-singlet vector-like fermion hidden-sector dark matter model, in which the communication between the dark matter and the visible standard model sector is via the Higgs-portal scalar-Higgs mixing, and also via a hidden-sector scalar with loop-level couplings to two gluons and also to two hypercharge gauge bosons induced by a vector-like quark. We find that the Higgs-portal possibility is stringently constrained to be small by the recent LHC di-Higgs search limits, and the loop-induced couplings are important to include. In the model parameter space, we present the dark matter relic-density, the dark-matter-nucleon direct-detection scattering cross-section, the LHC diphoton rate from gluon-gluon fusion, and the theoretical upper-bounds on the fermion-scalar couplings from perturbative unitarity.