Abstract
The present upper bound on κe, the ratio between the electron Yukawa coupling and its Standard Model value, is of mathcal{O}(600) . We ask what would be the implications in case that κe 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 κe and κV 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 Z2 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 mathcal{O}left(0.01/{kappa}_eright) 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.
Highlights
While measurements of the third generation Yukawa couplings imply that κt,b,τ = O(1) [1,2,3,4], direct measurements still allow the Yukawa couplings of the first two generations to be very different from the SM values
We find the following consequences: (i) Under certain conditions, measuring κe and κV 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 Z2 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 O(0.01/κe) in both the scalar potential and the Yukawa sector. (iv) LHC searches for e+e− resonances constrain this scenario in a significant way
In this subsection we assume that CP is a good symmetry of the scalar potential, such that the neutral mass eigenstates are the even (h and H) and odd (A) CP eigenstates
Summary
We assume that CP is a good symmetry of the scalar potential and of the Yukawa sector. In this subsection we identify conditions under which the diagonal couplings of fermions f = e are related to that of the electron For this purpose, we employ the βe-basis, defined in the previous section. This basis plays no special role for the other fermions, and both y1f and y2f are different from zero In such a case, there is no further predictive power for κf (unless a specific flavor model is assumed). [15] for a review of the various NFC models.) Our findings here are, much more general than NFC They apply whenever, in the fermion mass basis and some basis for the two Higgs doublets, two (or more) diagonal entries vanish.
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