A natural mechanism for approximate Higgs alignment in the 2HDM

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.

Fine-Tuning in the 2HDM

“…( 26) where the alignment is exact. Notice, in particular, that Eq (27) is satisfied for some simple relations between the potential parameters, such as {λ 1 = λ 2 & m 2 11 = m 2 22 }, which can be achieved by imposing certain global symmetries in the potential [20][21][22][23]. However, for phenomenological reasons these symmetries cannot be exact.…”

“…Nevertheless, as pointed out in ref. [22], this requires to extend the Yukawa sector, e.g. with vector-like top quark partners coupled to the Higgses, which goes beyond the scope of this paper.…”

“…In this analysis we will adopt an "agnostic" point of view, in the sense that we will not consider additional symmetries which constrain the model (beyond the usual Z 2 parity to avoid FCNC). There have been several analyses exploring this alternative direction, in particular to get an exact or approximate alignment [20][21][22][23][24][25]. As pointed out in ref.…”

“…As pointed out in ref. [22], an approximate alignment can arise from a softly broken global symmetry of the scalar potential, but this requires to extend the Yukawa sector, e.g. with vector-like top quark partners.…”

Two-loop Prediction of the Anomalous Magnetic Moment of the Muon in the Two-Higgs Doublet Model with GM2Calc 2

“…Although the 2HDM has been studied for many decades and the only Higgs boson discovered is closely SM-like [44,45], interest in this model has not waned, and recent progress has been achieved e.g. on LHC interpretations [46][47][48][49][50], B-physics [51][52][53][54][55], theoretical constraints [56][57][58][59][60], electroweak phase transitions in the early universe [61][62][63][64][65], precision calculations of Higgs decays [66][67][68][69][70][71][72][73][74]. Remarkably, the 2HDM is one of the very few single field extensions of the Standard Model that can explain the deviation between a Exp µ and a SM µ [75] while satisfying existing constraints from collider physics searches and other observables.…”