Scrutinizing the Alignment Limit in Two-Higgs-Doublet Models. Part 1: $m_h = 125$ GeV

In the alignment limit of a multidoublet Higgs sector, one of the Higgs mass eigenstates aligns in field space with the direction of the scalar field vacuum expectation values, and its couplings approach those of the Standard Model (SM) Higgs boson. We consider CP-conserving two-Higgs-doublet models (2HDMs) of type I and type II near the alignment limit in which the heavier of the two CP-even Higgs bosons, H, is the SM-like state observed with a mass of 125 GeV, and the couplings of H to gauge bosons approach those of the SM. We review the theoretical structure and analyze the phenomenological implications of this particular realization of the alignment limit, where decoupling of the extra states cannot occur given that the lighter CP-even state h must, by definition, have a mass below 125 GeV. For the numerical analysis, we perform scans of the 2HDM parameter space employing the software packages 2HDMC and Lilith, taking into account all relevant pre-LHC constraints, constraints from the measurements of the 125 GeV Higgs signal at the LHC, as well as the most recent limits coming from searches for other Higgs-like states. Implications for Run 2 at the LHC, including expectations for observing the other scalar states, are also discussed.