Large Higgs-electron Yukawa coupling in 2HDM

TL;DR

This paper explores the implications of a potentially large electron Yukawa coupling in two Higgs doublet models, analyzing how it affects predictions, constraints, and phenomenology in particle physics.

Contribution

It investigates the consequences of an enhanced electron Yukawa coupling in 2HDM without natural flavor conservation, including predictions, constraints, and model variations.

Findings

Enhanced electron Yukawa coupling can predict other fermion couplings.

Light second Higgs doublet if softly broken Z2 symmetry; heavier if hard breaking.

LHC searches significantly constrain the scenario.

Abstract

The present upper bound on $\kappa_e$, the ratio between the electron Yukawa coupling and its Standard Model value, is of ${\cal O}(600)$. We ask what would be the implications in case that $\kappa_e$ is close to this upper bound. The simplest extension that allows for such enhancement is that of two Higgs doublet models (2HDM) without natural flavor conservation. In this framework, we find the following consequences: (i) Under certain conditions, measuring $\kappa_e$ and $\kappa_V$ would be enough to predict values of Yukawa couplings for other fermions and for the $H$ and $A$ scalars. (ii) In the case that the scalar potential has a softly broken $Z_2$ symmetry, the second Higgs doublet must be light, but if there is hard breaking of the symmetry, the second Higgs doublet can be much heavier than the electroweak scale and still allow the electron Yukawa coupling to be very different from its SM value. (iii) CP must not be violated at a level higher than ${\cal O}(0.01/\kappa_e)$ in both the scalar potential and the Yukawa sector. (iv) LHC searches for $e^+e^-$ resonances constrain this scenario in a significant way. Finally, we study the implications for models where one of the scalar doublets couples only to the first generation, or only to the third generation.