The 750 GeV diphoton excess in a two Higgs doublet model and a singlet scalar model, with vector-like fermions, unitarity constraints, and dark matter implications

We explore the possibility of a beyond the standard model scalar ($phi$) as a possible explanation of the diphoton resonance at 750~GeV invariant mass reported by the ATLAS and CMS collaborations at the large hadron collider (LHC). We first present in a model-independent way the scalar-gluon-gluon and scalar-photon-photon effective couplings needed for obtaining the required diphoton cross-section at the LHC for different total widths. We investigate here two new-physics possibilities that can generate these effective couplings, namely, (i) the 2-Higgs-doublet model (2HDM) in the alignment limit, and (ii) a singlet scalar, with vector-like fermions added and playing a crucial role in generating the effective couplings. We present the regions of model parameter space which are allowed by direct LHC and perturbative unitarity constraints, and that give the required diphoton cross-section at the LHC for various total widths. In the singlet case, we include the possibility that $phi$ decays into a pair of neutral stable vector-like fermions that could be dark matter. We find regions of parameter-space of the singlet model that gives the required diphoton rate, have the correct dark matter relic-density, have dark matter direct-detection rates compatible with current direct-detection experiments, and satisfy LHC bounds and perturbative unitarity constraints.