A natural mechanism for approximate Higgs alignment in the 2HDM

TL;DR

This paper proposes a natural mechanism for approximate Higgs alignment in the 2HDM by extending it with vector-like top quark partners and softly breaking a global symmetry, aligning theoretical predictions with LHC data.

Contribution

It introduces an extended 2HDM with vector-like top partners that achieves approximate Higgs alignment through soft symmetry breaking, reducing fine-tuning.

Findings

The extended 2HDM can naturally produce Higgs alignment consistent with LHC data.

Current LHC bounds on vector-like top quarks constrain the model parameters.

The mechanism avoids significant fine-tuning in the Higgs sector.

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.