Abstract
The 2HDM possesses a neutral scalar interaction eigenstate whose tree-level properties coincide with the Standard Model (SM) Higgs boson. In light of the LHC Higgs data which suggests that the observed Higgs boson is SM-like, it follows that the mixing of the SM Higgs interaction eigenstate with the other neutral scalar interaction eigenstates of the 2HDM should be suppressed, corresponding to the so-called Higgs alignment limit. The exact Higgs alignment limit can arise naturally due to a global symmetry of the scalar potential. If this symmetry is softly broken, then the Higgs alignment limit becomes approximate (although still potentially consistent with the current LHC Higgs data). In this paper, we obtain the approximate Higgs alignment suggested by the LHC Higgs data as a consequence of a softly broken global symmetry of the Higgs Lagrangian. However, this can only be accomplished if the Yukawa sector of the theory is extended. We propose an extended 2HDM with vector-like top quark partners, where explicit mass terms in the top sector provide the source of the soft symmetry breaking of a generalized CP symmetry. In this way, we can realize approximate Higgs alignment without a significant fine-tuning of the model parameters. We then explore the implications of the current LHC bounds on vector-like top quark partners for the success of our proposed scenario.
Highlights
Boson coincide with those of the Standard Model (SM) Higgs boson to within the current accuracy of the accumulated data, typically in the range of 10%–20% depending on the observable [3,4,5]
It is perhaps surprising that the fundamental theory of particles and their interactions at the energy scale of electroweak symmetry breaking (EWSB) consists of a scalar sector that is of minimal form
Of particular experimental interest is the case of approximate Higgs alignment without decoupling, where multiple states in the Higgs sector are light, but the neutral scalar interaction eigenstate, whose tree-level properties coincide with the SM Higgs field, does not mix strongly with the other scalar field degrees of freedom
Summary
Boson coincide with those of the SM Higgs boson to within the current accuracy of the accumulated data, typically in the range of 10%–20% depending on the observable [3,4,5]. The SM Higgs boson comes from a single electroweak complex-scalar doublet that yields precisely one physical degree of freedom after electroweak symmetry breaking This should be contrasted with the non-minimal structures inherent in a fermion sector that consist of three generations of quarks and leptons and a gauge sector based on a direct product of three separate gauge groups. In the so-called Higgs alignment limit [11,12,13,14], there exists one neutral scalar mass eigenstate that is aligned with the direction of the Higgs vacuum expectation value in field space This direction corresponds precisely to the interaction eigenstate with the tree-level properties of the SM Higgs boson. In light of the LHC Higgs data, if an extended Higgs sector exists the Higgs alignment limit must be approximately realized, which implies that the mixing of the SM Higgs interaction eigenstate with other neutral scalar mass eigenstates is suppressed
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