Yukawa Sector Research Articles

Lightness of Higgs boson and spontaneousCPviolation in the Lee model

We proposed a mechanism in which the lightness of Higgs boson and the smallness of charge parity ($CP$) violation are correlated based on the Lee model, namely, the spontaneous $CP$-violation two-Higgs-doublet model. In this model, the mass of the lightest Higgs boson ${m}_{h}$ as well as the quantities $K$ and $J$ are $\ensuremath{\propto}{t}_{\ensuremath{\beta}}{s}_{\ensuremath{\xi}}$ in the limit ${t}_{\ensuremath{\beta}}{s}_{\ensuremath{\xi}}\ensuremath{\rightarrow}0$ (see text for definitions of ${t}_{\ensuremath{\beta}}$ and $\ensuremath{\xi}$), namely, the $CP$ conservation limit. Here, $K$ and $J$ are the measures for $CP$-violation effects in scalar and Yukawa sectors, respectively. It is a new way to understand why the Higgs boson discovered at the LHC is light. We investigated the important constraints from both high energy LHC data and numerous low energy experiments, especially the measurements of electric dipole moments of electron and neutron as well as the quantities of B meson and kaon. Confronting all data, we found that this model is still viable. It should be emphasized that there is no standard-model limit for this scenario; thus it is always testable for future experiments. In order to pin down the Lee model, it is important to discover the extra neutral and charged Higgs bosons and measure their $CP$ properties and the flavor-changing decays. At the LHC with $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$, this scenario is favored if there is significant suppression in the $b\overline{b}$ decay channel or any vector boson fusion, $\mathrm{V}+\mathrm{H}$ production channels. On the contrary, it will be disfavored if the signal strengths are standard-model--like more and more. It can be easily excluded at $(3--5)\ensuremath{\sigma}$ level with several ${\mathrm{fb}}^{\ensuremath{-}1}$ at future ${e}^{+}{e}^{\ensuremath{-}}$ colliders, via accurately measuring the Higgs boson production cross sections.

Viability of minimal left–right models with discrete symmetries

We provide a systematic study of minimal left–right models that are invariant under P, C, and/or CP transformations. Due to the high amount of symmetry such models are quite predictive in the amount and pattern of CP violation they can produce or accommodate at lower energies. Using current experimental constraints some of the models can already be excluded. For this purpose we provide an overview of the experimental constraints on the different left–right symmetric models, considering bounds from colliders, meson-mixing and low-energy observables, such as beta decay and electric dipole moments. The features of the various Yukawa and Higgs sectors are discussed in detail. In particular, we give the Higgs potentials for each case, discuss the possible vacua and investigate the amount of fine-tuning present in these potentials. It turns out that all left–right models with P, C, and/or CP symmetry have a high degree of fine-tuning, unless supplemented with mechanisms to suppress certain parameters. The models that are symmetric under both P and C are not in accordance with present observations, whereas the models with either P, C, or CP symmetry cannot be excluded by data yet. To further constrain and discriminate between the models measurements of B-meson observables at LHCb and B-factories will be especially important, while measurements of the EDMs of light nuclei in particular could provide complementary tests of the LRMs.

Higgs boson masses and mixings in the complex MSSM with two-loop top-Yukawa-coupling corrections

Results for the leading two-loop corrections of $\mathcal{O}{\left(\alpha_t^2\right)}$ from the Yukawa sector to the Higgs-boson mass spectrum of the MSSM with complex parameters are presented, with details of the analytical calculation performed in the Feynman-diagrammatic approach using a mixed $\left.\text{on-shell}\middle/\,\overline{\text{DR}}\right.$ scheme that can be directly matched onto the higher-order terms in the code ${\tt FeynHiggs}$. Numerical results are shown for the masses and mixing effects in the neutral Higgs-boson sector and their variation with the phases of the complex parameters. Furthermore, the analytical expressions of the two-loop self-energies and the required renormalization constants are recorded. The new results can consistently be implemented in ${\tt FeynHiggs}$.

Towards predictive flavour models in SUSY SU(5) GUTs with doublet-triplet splitting

We discuss how the double missing partner mechanism solution to the doublet-triplet splitting problem in four-dimensional supersymmetric SU(5) Grand Unified Theories (GUTs) can be combined with predictive models for the quark-lepton Yukawa coupling ratios at the GUT scale. It is argued that towards this goal a second SU(5) breaking Higgs field in the adjoint representation is very useful and we discuss all possible renormalizable superpotentials with two adjoint Higgs fields and calculate the constraints on the GUT scale and effective triplet mass from a two-loop gauge coupling unification analysis. Two explicit flavour models with different predictions for the GUT scale Yukawa sector are presented, including shaping symmetries and a renormalizable messenger sector. Towards calculating the rates for proton decay induced by the exchange of colour triplets, the required Clebsch-Gordan coefficients for their couplings are calculated for the possible dimension five and six operators. They are provided in detailed tables in the appendix, together with additional helpful material for GUT flavour model building.

Radiative neutrino mass in 3-3-1 scheme

We propose a new radiative mechanism for neutrino mass generation based on the $\mathrm{SU}(3{)}_{c}\ensuremath{\bigotimes}\mathrm{SU}(3{)}_{L}\ensuremath{\bigotimes}\mathrm{U}(1{)}_{X}$ electroweak gauge group.Lepton number is a symmetry of the Yukawa sector which is spontaneously broken in the gauge sector. As a result light Majorana masses arise from neutral gauge boson exchanges at the one-loop level. In addition to the isosinglet neutrinos that may be produced at the LHC through the extended gauge boson portals, the model contains new quarks which can also lie at the TeV scale, and which can provide a plethora of accessible collider phenomena.

Two-loop top-Yukawa-coupling corrections to the Higgs boson masses in the complex MSSM

Results for the leading two-loop corrections of O(αt2) from the Yukawa sector to the Higgs-boson masses of the MSSM with complex parameters are presented. The corresponding self-energies and their renormalization have been obtained in a Feynman-diagrammatic approach. A numerical analysis of the new contributions is performed for the mass of the lightest Higgs boson, supplemented by the full one-loop result and the O(αtαs) terms including complex phases. In the limit of the real MSSM a previous result is confirmed.

Littlest Higgs with T-parity: status and prospects

The Littlest Higgs model with T-parity is providing an attractive solution to the fine-tuning problem. This solution is only entirely natural if its intrinsic symmetry breaking scale f is relatively close to the electroweak scale. We examine the constraints using the latest results from the 8 TeV run at the LHC. Both direct searches and Higgs precision physics are taken into account. The constraints from Higgs couplings are by now competing with electroweak precision tests and both combined exclude f up to 694 GeV or 560 GeV depending on the implementation of the down-type Yukawa sector. Direct searches provide robust and complementary limits and constrain f to be larger than 638 GeV. We show that the Littlest Higgs model parameter space is slowly driven into the TeV range. Furthermore, we develop a strategy on how to optimise present supersymmetry searches for the considered model, with the goal to improve the constraints and yield more stringent limits on f.

Towards the minimal renormalizable supersymmetric E 6 model

We find an explicit renormalizable supersymmetric $E_6$ model with all the ingredients for being realistic. It consists of the Higgs sector $351'+\overline{351'}+27+\overline{27}$, which breaks $E_6$ directly to the Standard Model gauge group. Three copies of $27$ dimensional representations then describe the matter sector, while an extra $27+\overline{27}$ pair is needed to successfully split the Standard Model Higgs doublet from the heavy Higgs triplet. We perform the analysis of the vacuum structure and the Yukawa sector of this model, as well as compute contributions to proton decay. Also, we show why some other simpler $E_6$ models fail to be realistic at the renormalizable level.

Erratum: Yukawa independent constraints for two-Higgs-doublet models with a 125 GeV Higgs boson [Phys. Rev. D88, 055027 (2013)

In this paper various constraints for the parameter spaces of two variants of the Two Higgs Doublet Model with a Z_2-symmetric potential are reconsidered, including the LHC data on existence of a 125 GeV Higgs-like boson. We analyze the model in which only one of the doublets develops a nonzero vacuum expectation value (VEV) -- the Inert Doublet Model (IDM) and the Mixed Model where both of the doublets have nonzero VEVs. Positivity constraints, conditions determining the type of the vacuum, perturbative unitarity condition, constraints following from electroweak precision tests together with the LEP bounds on masses of the scalars are included in the analysis. The analysis is performed without specific assumptions regarding the Yukawa sector. For the IDM constraints on quartic couplings and masses of the scalars as well as their implications for Dark Matter scenarios are presented. A new type of bound on the mass parameter of the potential coming from the condition for the existence of the Inert vacuum is given. In the Mixed Model a strong bound on the value of tan\beta, 0.18<tan\beta<5.59, is found. It depends on the mass of the Higgs boson and is independent of the Yukawa interactions. Also Standard Model (SM)-like scenarios with either h or H playing the role of the SM-like Higgs boson are analyzed.

Yukawa independent constraints for two-Higgs-doublet models with a 125 GeV Higgs boson

In this paper various constraints for the parameter spaces of two variants of the two-Higgs-doublet model with a ${\mathbb{Z}}_{2}$-symmetric potential are reconsidered, including the LHC data on existence of a 125 GeV Higgs-like boson. We analyze the model in which only one of the doublets develops a nonzero vacuum expectation value (VEV)---the inert doublet model (IDM) and the mixed model where both of the doublets have nonzero VEVs. Positivity constraints, conditions determining the type of the vacuum, perturbative unitarity condition, constraints following from electroweak precision tests together with the LEP bounds on masses of the scalars are included in the analysis. The analysis is performed without specific assumptions regarding the Yukawa sector. For the IDM constraints on quartic couplings and masses of the scalars as well as their implications for dark matter scenarios are presented. A new type of bound on the mass parameter of the potential coming from the condition for the existence of the inert vacuum is given. In the mixed model a strong bound on the value of $\mathrm{tan}\ensuremath{\beta}$, $0.18\ensuremath{\lesssim}\mathrm{tan}\ensuremath{\beta}\ensuremath{\lesssim}5.59$, is found. It depends on the mass of the Higgs boson and is independent of the Yukawa interactions. Also standard model (SM)-like scenarios with either $h$ or $H$ playing the role of the SM-like Higgs boson are analyzed.

Yukawa sector of multi-Higgs-doublet models in the presence of Abelian symmetries

A general method for classifying the possible quark models of a multi-Higgs-doublet model, in the presence of Abelian symmetries, is presented. All the possible sets of textures that can be present in a given sector are shown, thus turning the determination of the flavor models into a combinatorial problem. Several symmetry implementations are studied for two and three Higgs doublet models. Some models implementations are explored in great detail, with a particular emphasis on models known as Branco-Grimus-Lavoura and nearest-neighbour-nnteraction. Several considerations on the flavor changing neutral currents of multi-Higgs models are also given.

K→ππhadronic matrix elements of left-right current-current operators

Effective $\ensuremath{\Delta}S=1$ four-fermion operators involving left- and right-handed currents are relevant in left-right gauge extensions of the Standard Model and scalar extension of the Yukawa sector. They induce $K\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}$ decays which are strictly constrained by experimental data, typically resulting in strong bounds on the new physics scales or parameters. We evaluate the $K\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}$ hadronic matrix elements of such operators within the phenomenological framework of the chiral quark model. The results are consistent with the estimates used in a previous work on TeV scale left-right symmetry, thus confirming the conclusions obtained there.

A non supersymmetric SO(10) grand unified model for all the physics below M GUT

Abstract We present a renormalizable non supersymmetric Grand Unified SO(10) model which, at the price of a large fine tuning, is compatible with all compelling phenomenological requirements below the unification scale and thus realizes a minimal extension of the SM, unified in SO(10) and describing all known physics below M GUT. These requirements include coupling unification at a large enough scale to be compatible with the bounds on proton decay; a Yukawa sector in agreement with all the data on quark and lepton masses and mixings and with leptogenesis as the origin of the baryon asymmetry of the Universe; an axion arising from the Higgs sector of the model, suitable to solve the strong CP problem and to account for the observed amount of Dark Matter. The above constraints imposed by the data are very stringent and single out a particular breaking chain with the Pati-Salam group at an intermediate scale M I ~ 1011 GeV.

Charged lepton flavor violation in supersymmetric low-scale seesaw models

We study charged lepton flavour violation in low-scale seesaw models of minimal supergravity, which realize large neutrino Yukawa couplings thanks to approximate lepton-number symmetries. There are two dominant sources of lepton flavour violation in such models. The first source originates from the usual soft supersymmetry-breaking sector, whilst the second one is entirely supersymmetric and comes from the supersymmetric neutrino Yukawa sector. Within the framework of minimal supergravity, we consider both sources of lepton flavour violation, soft and supersymmetric, and calculate a number of possible lepton-flavour-violating transitions, such as the photonic decays of muons and taus, mu -> e gamma, tau -> e gamma and tau -> mu gamma, their neutrinoless three-body decays, mu -> e e e, tau -> e e e, tau -> mu mu mu, tau -> e e mu and tau -> e mu mu, and the coherent mu -> e conversion in nuclei. After taking into account the exclusion bounds placed by present experiments of lepton flavour violation, we derive combined theoretical limits on the universal heavy Majorana mass scale mN and the light-to-heavy neutrino mixings. Supersymmetric low-scale seesaw models offer distinct correlated predictions for lepton-flavour-violating signatures, which might be discovered in current and projected experiments, such as MEG, COMET/PRISM, Mu2e, super-BELLE and LHCb.

The Standard Model with one universal extra dimension

Effects of universal extra dimensions on Standard Model observables first arise at the one-loop level. The quantization of this class of theories is therefore essential in order to perform predictions. A comprehensive study of the SU C(3) × SU L(2) × U Y(1) Standard Model defined in a space-time manifold with one universal extra dimension, compactified on the oribifold \(S^1/Z_2\), is presented. The fact that the four-dimensional Kaluza–Klein theory is subjected to two types of gauge transformations is stressed and its quantization under the basis of the BRST symmetry discussed. A SU C(3) × SU L(2) × U Y(1)-covariant gauge-fixing procedure for the Kaluza–Klein excitations is introduced. The connection between gauge and mass eigenstate fields is established in an exact way. An exhaustive list of the explicit expressions for all physical couplings induced by the Yang–Mills, Currents, Higgs, and Yukawa sectors is presented. The one-loop renormalizability of the standard Green’s functions, which implies that the Standard Model observables do not depend on a cut-off scale, is stressed.

Yukawa hierarchies from spontaneous breaking of the SU(3) L × SU(3) R flavour symmetry?

The tree level potential for a scalar multiplet of `Yukawa fields' $Y$ for one type of quarks admits the promising vacuum configuration $<Y> \propto {\rm diag}(0,0,1)$ that breaks spontaneously $SU(3)_L\times SU(3)_R$ flavour symmetry. We investigate whether the vanishing entries could be lifted to nonvanishing values by slightly perturbing the potential, thus providing a mechanism to generate the Yukawa hierarchies. For theories where at the lowest order the only massless states are Nambu-Goldstone bosons we find, as a general result, that the structure of the tree-level vacuum is perturbatively stable against corrections from scalar loops or higher dimensional operators. We discuss the reasons for this stability, and give an explicit illustration in the case of loop corrections by direct computation of the one-loop effective potential of Yukawa fields. Nevertheless, a hierarchical configuration $<Y>\propto {\rm diag}(\epsilon',\epsilon,1)$ (with $\epsilon', \epsilon\ll 1$) can be generated by enlarging the scalar Yukawa sector. We present a simple model in which spontaneous breaking of the flavour symmetry can give rise to the fermion mass hierarchies.

Single top and Higgs associated production as a probe of the $Ht\overline{t}$ coupling sign at the LHC

The LHC sensitivity to an anomalous Higgs coupling to the top quark in the Higgs-top associated production is analyzed. Thanks to the strong destructive interference in the t-channel for standard model couplings, this process can be very sensitive to both the magnitude and the sign of a nonstandard top-Higgs coupling. We analyze cross sections and the main irreducible backgrounds for the $H \to \gamma \gamma$ decay channel. Sensitivities to an anomalous sign for the top Yukawa coupling are found to be large. In particular, at \sqrt{s}=14 TeV, assuming a universal rescaling in the Yukawa sector, a parton-level analysis with realistic selection cuts gives a signal-to-background ratio S/B ~ 5, for -1.5 < Y_t/Y_t^{SM} < 0. A number of events S ~ 10 (with corresponding significances ~ 3 \sigma) are expected for 60 fb^{-1}, to be compared with the standard-model expectation S ~ 0.3.

A minimal model of neutrino flavor

Abstract Models of neutrino mass which attempt to describe the observed lepton mixing pattern are typically based on discrete family symmetries with a non-Abelian and one or more Abelian factors. The latter so-called shaping symmetries are imposed in order to yield a realistic phenomenology by forbidding unwanted operators. Here we propose a supersymmetric model of neutrino flavor which is based on the group T 7 and does not require extra $ \mathbb{Z} $ N or U(1) factors in the Yukawa sector, which makes it the smallest realistic family symmetry that has been considered so far. At leading order, the model predicts tribimaximal mixing which arises completely accidentally from a combination of the T 7 Clebsch-Gordan coefficients and suitable flavon alignments. Next-to-leading order (NLO) operators break the simple tribimaximal structure and render the model compatible with the recent results of the Daya Bay and Reno collaborations which have measured a reactor angle of around 9°. Problematic NLO deviations of the other two mixing angles can be controlled in an ultraviolet completion of the model. The vacuum alignment mechanism that we use necessitates the introduction of a hidden flavon sector that transforms under a $ \mathbb{Z} $ 6 symmetry, thereby spoiling the minimality of our model whose flavor symmetry is then T 7 × $ \mathbb{Z} $ 6.

Super flavorsymmetry with multiple Higgs doublets

AbstractThe meaning of non‐Abelian super flavorsymmetry in the presence of Multiple Higgs Doublets is twofold: The first one is to order the Yukawa sector of the standard model, while generating the Cheng‐Sher suppression of tree‐level Flavor Changing Neutral Currents (FCNCs), and the second one is to soften the fine tuning problem in the scalar potential and at the same time to suppress FCNCs coming from the soft breaking of supersymmetry. If spontaneous CP violation is combined with non‐Abelian super flavorsymmetry the CP phases in the soft supersymmetry breaking sector can disappear. This feature of super flavorsymmetry is reviewed and elaborated in a supersymmetric model of flavor based on the finite group Q6. The predictions of the model in ferimion mass and mixing, and flavor and CP violation can be tested in future experiments.

Lepton mixing and discrete symmetries

The pattern of lepton mixing can emerge from breaking a flavor symmetry in different ways in the neutrino and charged lepton Yukawa sectors. In this framework, we derive the conditions imposed on the mixing matrix by the structure of discrete groups of the von Dyck type which include A4, S4 and A5. We show that, in general, these conditions reduce to two equations for the mixing parameters (angles and CP phase \delta). These constraints, which correspond to unbroken residual symmetries, are consistent with non-zero 13 mixing and deviations from maximal 2-3 mixing. For the allowed values of mixing angles we predict \delta = 60 - 90^{\circ}.