Fitting the ZbbˉZ b \bar b vertex in the two-Higgs-doublet model and in the three-Higgs-doublet model

We investigate the new contributions to the parameters $g_L$ and $g_R$ of the $Z b \bar b$ vertex in a multi-Higgs-doublet model (MHDM). We emphasize that those contributions generally worsen the fit of those parameters to the experimental data. We propose a solution to this problem, wherein $g_R$ has the opposite sign from the one predicted by the Standard Model; this solution, though, necessitates light scalars and large Yukawa couplings in the MHDM.Comment: 24 pages, 10 figures; v2: added two footnotes and two reference

The ZbbˉZ b \bar b vertex in a left-right model

We consider the one-loop corrections to the $Z b \bar b$ vertex in a $CP$-conserving left--right model (LRM), $viz$. a model with gauge group $SU(2)_L \times SU(2)_R \times U(1)$. We allow the gauge coupling constants of $SU(2)_L$ and $SU(2)_R$ to be different. The spontaneous symmetry breaking is accomplished only by doublets and/or singlets of $SU(2)_L$ and $SU(2)_R$. The lightest massive neutral gauge boson of our LRM is assumed to have the same Yukawa couplings to bottom-quark pairs as the $Z$ of the Standard Model (SM); this assumption has the advantage that, then, the infrared divergences automatically cancel down in the subtraction of the $Z b \bar b$ vertex in the SM from the same vertex in the LRM. We effect a proper renormalization of the $Z b \bar b$ vertex and check explicitly both its gauge invariance and the cancellation of all the ultraviolet divergences. We find out that a LRM with the above assumptions cannot achieve a better fit to the $Z b \bar b$ vertex than a multi-Higgs extension of the SM, $viz$. both models can only achieve a decent fit when one admits scalar particles with very low masses $\lesssim 50$ GeV. This is true even when we allow for markedly different gauge coupling constants of $SU(2)_L$ and $SU(2)_R$.Comment: 57 pages, 8 figures; v2: added section 3, figure 2 and three references; v3: matches published versio

Two-body lepton-flavour-violating decays in a 2HDM with soft family-lepton-number breaking

We evaluate the decays ℓ1→ℓ2γ, Z→ℓ1ℓ2, and h→ℓ1ℓ2, where ℓ1 and ℓ2 are charged leptons with different flavours and h is the scalar particle with mass 125.25 GeV, in a two-Higgs-doublet model where all the Yukawa-coupling matrices conserve the lepton flavours but the Majorana mass terms of the right-handed neutrinos break the flavour lepton numbers. We find that (1) the decays ℓ1→ℓ2γ require large Yukawa couplings and very light right-handed neutrinos in order to be visible, (2) the decays Z→ℓ1ℓ2 will be invisible in all the planned experiments, except in a very restricted range of circumstances, but (3) the decays h→ℓ1ℓ2 may be detected in future experiments for rather relaxed sets of input parameters

Higgs masses and couplings in a general 2HDM with unitarity bounds

We investigate the general two Higgs doublet model imposing both the unitarity conditions and the bounded-from-below conditions. Both types of conditions restrict the ranges of the parameters of the scalar potential. We study the model in the Higgs basis, i.e. in the basis for the scalar doublets where only one doublet has vacuum expectation value. We use the experimental bounds on the oblique parameter T, to produce scalar particles with masses and cubic and quartic couplings of the Higgs in agreement with the phenomenology. The numerical calculations show that the cubic coupling may be up to 1.6 times larger than in the Standard Model, but it may also be zero or even negative. The quartic coupling is always positive and may be up to four times larger than in the Standard Model

More models for lepton mixing with four constraints

We propose new lepton-mixing textures that may be enforced through well-defined symmetries in renormalizable models. Each of our textures has four sum rules for the neutrino mass observables. The models are based on the type-I seesaw mechanism; their charged-lepton mass matrices are diagonal because of the symmetries imposed. Each model has three versions, depending on the identification of the charged leptons. Testing all the models, we have found that five of them agree with the data at the 1 sigma level when the neutrino-mass ordering is normal, and two models agree with the data for an inverted ordering. We detail the predictions of each of those seven models

Light neutrino mass spectrum with one or two right-handed singlet fermions added

We analyse two cases of the minimal extension of the Standard Model when one or two right-handed fields are added to the three left-handed fields. A second Higgs doublet (two Higgs doublet model – 2HDM) is included in our model. We calculate one-loop radiative corrections to the mass parameters which produce mass terms for the neutral leptons. In both cases we numerically analyse light neutrino masses as functions of the heavy neutrino masses. Parameters of the model are varied to find light neutrino masses that are compatible with experimental data of solar Δm^2_solar and atmospheric Δm^2_atm neutrino mass differences for normal hierarchy. We choose values for the parameters of the tree-level by numerical scans, where we look for the best agreement between computed and experimental neutrino oscillation angles

Numerical analysis of SO(10) models with flavor symmetries

We consider a supersymmetric SO(10) Grand Unified Theory (GUT) in which the fermion masses are generated by renormalizable Yukawa couplings. Consequently, the scalar multiplets under consideration belong to the irreps 10, 126, and 120 of SO(10). We perform a complete investigation of the possibilities of imposing flavour symmetries in this scenario; the purpose is to reduce the number of Yukawa coupling constants in order to identify potentially predictive models. We have found that there are 14 inequivalent cases of Yukawa coupling matrices, out of which 13 cases pertain to one-generator Abelian groups and only one case has a two-generator symmetry group. Supersymmetry enters through the numerical examination of those cases, in which we have used the charged-fermion masses evaluated at the GUT scale through renormalization-group running in the context of the Minimal Supersymmetric Standard Model. However, the numerical analysis rules out almost all the cases, leaving only few viable ones which are compatible with the data on the fermion masses and mixings. In order to test the viability of each case, and to find adequate numerical values for its parameters, we construct a minimization function chi^2 which relate experimental data with the observables (masses and mixing parameters) to be fitted. For the numerical minimization we have employed the Differential Evolution algorithm. By modifying errors in the chi^2 function and diversely restricting parameters space we have thus been able to test, for each case, more local minima, and to find the minima closer to the global minimu

The one-loop improved Lagrangian of the Grimus-Neufeld model

Adding a single gauge singlet fermion and a second Higgs doublet to the original Standard Model allows an explanation for the observed smallness of the neutrino masses using the seesaw mechanism. This model predicts two neutral fermions with vanishing mass. The one-loop contribution to the neutral fermion masses due to the second Higgs doublet lifts this degeneracy and allows to fit the model parameters to the observed neutrino mass differences. We determine the values of the additional Yukawa couplings by requiring the correct prediction of the mass differences and mixings in the neutrino sector. We also discuss the ambiguities of the model parameters