[Paper Review] Large pseudoscalar Yukawa couplings in the complex 3HDM

Boto, Rafael, Lourenco, Luis+2 more|arXiv (Cornell University)|Jul 29, 2024

Atomic and Subatomic Physics Research32 references6 citations

TL;DR

The paper analyzes a complex three-Higgs-doublet model (C3HDM) with a Z2×Z2 symmetry and explicit CP violation, showing that a mostly CP-even top coupling and mostly CP-odd bottom coupling for h125 can be partly revived, prompting experimental exploration.

ABSTRACT

As LHC's Run3 unfolds, we peer deeper into the couplings of the 125GeV Higgs ($h_{125}$) and also test for extra scalars. Even before such extra scalars are found, one can probe them by using the fact that theories with multiple Higgs doublets allow for substantial deviations of the couplings of $h_{125}$ from their SM values. Indeed, the observed couplings already place stringent limits on such theories. We investigate the curious possibility that $h_{125}$ couples to the top quark mostly as a scalar, while it couples to the bottom quark mostly as a pseudoscalar. This possibility was allowed by 2017 data for the so-called C2HDM; a two Higgs doublet model with a single source of explicit CP violation. It was shown recently that this possibility disappears when using the full experimental data of 2024. Here we discuss a three Higgs doublet with explicit CP violation (C3HDM), and show that the curious CP-even/CP-odd $t$/$b$ possibility is partly resuscitated, prompting further experimental exploration of this prospect.

Motivation & Objective

• Motivate precision h125 coupling measurements to probe multi-Higgs scenarios beyond the SM.
• Investigate the viability of a mostly CP-even top coupling and mostly CP-odd bottom coupling for h125 within a 3HDM.
• Develop a detailed parameterization of the C3HDM including complex potential terms and CP-violating effects.
• Map theoretical and experimental constraints on the C3HDM and assess the parameter space compatibility.

Proposed method

• Define the scalar potential with Z2×Z2 symmetry and soft-breaking terms for the C3HDM.
• Parameterize the neutral and charged scalar mass matrices and perform basis changes to reach a Higgs basis.
• Derive the Yukawa Lagrangian under the type-Z NFC assignment and express couplings of neutral scalars to fermions.
• Diagonalize the neutral scalar mass matrix with a general orthogonal rotation and relate mass eigenstates to potential parameters.
• Express Higgs-fermion couplings in terms of observable couplings (κV, cei, coi) and rotation matrices, enabling inversion to model parameters.

Experimental results

Research questions

• RQ1 Can the C3HDM accommodate a h125 coupling to the top quark that is mostly scalar while the coupling to bottom quarks is mostly pseudoscalar?
• RQ2 To what extent do the full 2024 experimental constraints allow or disfavor the CP-violating t/b coupling pattern observed in the C2HDM, when extended to a 3HDM (C3HDM)?
• RQ3 How does the type-Z NFC structure in a complex 3HDM influence the correlations between h125 couplings and other scalar spectrum observables?
• RQ4 What parameter relations and mass spectra are required to realize the CP-odd bottom and CP-even top coupling pattern while satisfying theoretical/experimental bounds?

Key findings

• A 3HDM with explicit CP violation can partly revive the CP-even top / CP-odd bottom coupling pattern for h125 that was disfavored in the 2HDM.
• The paper provides a comprehensive parameterization of the complex scalar potential, including the relations among mass matrices and tadpole conditions.
• A detailed Yukawa structure (type-Z) assigns each right-handed fermion species to a different scalar, enabling distinct CP properties in h125 couplings.
• Constraints—theoretical and experimental—are analyzed and incorporated into a scan to determine viable parameter space.
• The work demonstrates the continued relevance of pursuing CP-violating h125 couplings in upcoming measurements and searches.