Cosmological Aspects of Minimal and Extended Higgs Sectors

Abstract

Despite the huge success of the Standard Model (SM) of particle physics, many open problems indicate the need for New Physics. One particular extension of the SM is the Two Higgs doublet model with an additional complex scalar singlet (2HDMS). This model provides an enlarged scalar sector resulting in seven physical Higgs particles, of which one is a suitable candidate to explain dark matter (DM). In this work the DM and collider phenomenology of the 2HDMS is investigated. Representative benchmark points with light, intermediate-range and heavy masses for the DM candidate are chosen. The benchmark points are consistent with all theoretical and experimental constraints and, in some cases, also accommodate the excess around 95 GeV, observed in the b ̄b channel at the Large Electron-Positron Collider (LEP) and in the γγ channel at the Large Hadron Collider (LHC). In terms of DM phenomenology, the focus is on matching the required DM relic density, as well as direct and indirect detection bounds of DM. In terms of collider phenomenology, the focus is on relevant signatures at the LHC and at proposed future lepton colliders, such as electron-positron colliders and a proposed muon collider. The 2HDMS model, as well as other beyond-the-Standard Model (BSM) theories, can provide a first-order electroweak phase transition (EWPT), which proceeds via the formation of bubbles. A second focus of this work is the investigation of the wall velocity of expanding bubbles from first-order phase transitions (FOPTs) with symmetry breaking and with symmetry restoration. In standard symmetry-breaking FOPTs, the frictional pressure on the expanding bubble walls can be dominated by transition radiation, the process of one particle from the surrounding plasma passing into the bubble and radiating off an additional particle. This process is enhanced in the soft-momentum limit of the radiated particle, and is known to produce significant frictional effects that are proportional to the Lorentz factor γ of the bubble wall. This dependence of the frictional pressure on the Lorentz factor prohibits runaway behaviour of the bubble walls. The focus of this work is on the analogous pressure for phase transitions with symmetry restoration. It is shown that, in such transitions the pressure due to transition radiation can be negative for small and intermediate Lorentz factors, leading to a further acceleration of the bubble wall. However, for very large Lorentz factors the pressure approaches the same positive scaling as the standard symmetry-breaking scenarios. Therefore, the terminal Lorentz factor of the bubble wall can be significantly larger (by more than an order of magnitude) than in the corresponding symmetry-breaking scenario. This can carry important implications for various phenomenological applications, from gravitational waves (GWs) up to BSM physics.