Investigation of Two-Higgs-Doublet Models

Abstract

Much of our knowledge of particle physics has been incorporated into the Standard Model (SM), which is a gauge field theory that describes 17 fundamental, structureless particles. While the SM has been extremely successful, it leaves some striking deficiencies which we aim to address. The Two-Higgs-Doublet model (2HDM) is a simple extension of the SM with an additional Higgs doublet. Electroweak symmetry breaking in the 2HDM reveals a total of 5 massive scalar bosons including 2 CP-even scalars, a CP-odd scalar, and 2 degenerate charged scalars. We consider three types of 2HDMs; namely, the Type-I 2HDM, Type-II 2HDM and the Inert Doublet Model (IDM). The former two have additional flavour-changing charged currents which may help explain anomalies in flavour observables, while the latter includes a suitable dark matter (DM) candidate. Global fits are performed using the GAMBIT software at tree-level with results presented as frequentist profile likelihood ratio plots. The 2HDM was previously implemented in GAMBIT, but is further enhanced with new features, bug fixes, and performance improvements. Separate plots are presented for a variety of different theoretical and experimental constraints. The theoretical constraints we consider are S-matrix unitarity, perturbativity and vacuum stability, whereas the experimental constraints arise from measurements of the electroweak S, T, and U parameters; dark matter direct and indirect detection; flavour physics observables; and measurements of Higgs signal rates from collider experiments. Results are found to favour an SM-like scenario in general. The Type-II 2HDM provides the strongest lower bounds on the scalar masses due to flavour constraints, while the Type-I 2HDM has weaker limits which arise only from collider constraints. This is a result of a particular SM-like limit in Type-I model which weakens the flavour constraints. Results for the hidden-Higgs scenario, where the heavier CP-even scalar is considered to be SM-like, are presented separately. The main difference is that the scalar masses have a strict upper limit of 600 GeV set by the theoretical constraints. In the Type-II 2HDM, we find that this conflicts with the lower bound for charged scalar mass set by flavour constraints. Preliminary results using the 2-loop-level FlexibleSUSY spectrum generator are also presented with the main difference being a stricter limit on the couplings. The IDM is initially generated by the GUM software which writes the required code into GAMBIT to implement the new model. New features are also added to GUM and further additions are made in the IDM implementation in GAMBIT, including a tree-level spectrum generator, theoretical constraints and new bases. Results for the IDM scans are presented at tree-level. We apply the observed relic density as an upper bound, to allow the possibility of other dark matter candidates. We find lower bounds on the scalar masses arising from experimental constraints except when the DM-SM couplings are close to zero, or near the Higgs resonance.