Investigation of Higgs portal dark matter models: from collider, indirect and direct searches to electroweak baryogenesis

Abstract

This thesis addresses two limitations of the Standard Model (SM) of particle physics, namely dark matter (DM) and the origin of matter-antimatter asymmetry. Specifically, we study the Higgs portal DM models where the DM-SM interaction proceeds via a SM Higgs boson. Such models lead to a rich DMphenomenology that can be tested at collider, indirect and direct search experiments. This thesis is composed of three parts. In the first part, we provide a brief background on the SM and follow the road that led to the Higgs boson discovery. We also present evidence for the existence of DM and motivate the observed baryon asymmetry in our universe. In the second part of this thesis, we present results from a combined analysis of effective scalar, vector, Majorana and Dirac fermion Higgs portal DM models. For the fermion models, we include both CP-even and CP-odd terms. The parameter space of all models is constrained using the DM relic density, limits on the Higgs invisible branching ratio from the Large Hadron Collider (LHC) as well as indirect and direct DM detection experiments. In line with previous studies, we find that direct detection experiments will continue to exclude much of the model parameter space. For the CP-odd case, indirect searches are the only probe for accessing the high mass range of the theory. We also study the scalar singlet model in light of electroweak baryogenesis (EWBG). By requiring a large scalar-SM Higgs coupling, the model can explain the observed matter-antimatter asymmetry via a strong first-order electroweak phase transition. This has important implications for EWBG that can be tested using collider, gravitational wave (GW) and direct detection signals. We find that the new scalar cannot simultaneously account for the observed DM abundance and matter-antimatter asymmetry. However, a large portion of the model parameter space can lead to a sizeable GW signal. In the third part of this thesis, we focus on global fits. In particular, we perform a global fit of the extended scalar singlet model with a fermionic DM candidate. In this model, the new scalar mixes with the SM Higgs boson, leading to two scalar mediators. By coupling to the new scalar, a Dirac fermion field can play the role of a DM candidate. From our 7-dimensional scans of the model using only the EWBG constraint, we find that EWBG is viable in all parts of the model parameter space provided the scalar-fermion DM coupling gS ≤ 5.62. On the other hand, the combined constraints from the DM relic density, direct detection limit from the PandaX-II experiment, EWBG, electroweak precision observables and Higgs searches at colliders place an upper limit on some of the model parameters. We also compute the GW spectra of viable points and check their detection prospects at current or future GW experiments. Lastly, we present preliminary results from global fits of the vector and Dirac fermion Higgs portal DM models using the GAMBIT software. After motivating and outlining the benefits of using GAMBIT for global fits, we perform scans of the model parameter space using the same set of constraints, model parameter ranges, nuclear and astrophysical parameter values as our previous study. For the Dirac fermion model, we allow the scalar-pseudoscalar mixing parameter ⇠ to vary in our scans. We find that our preliminary results using GAMBIT are in good agreement with those obtained in our previous study. This is used to motivate a future study of these models using the GAMBIT software.