Renormalizable Yukawa Couplings Research Articles

Sterile neutrinos and neutrinoless double beta decay in effective field theory

We investigate neutrinoless double beta decay (0νββ) in the presence of sterile neutrinos with Majorana mass terms. These gauge-singlet fields are allowed to interact with Standard-Model (SM) fields via renormalizable Yukawa couplings as well as higher-dimensional gauge-invariant operators up to dimension seven in the Standard Model Effective Field Theory extended with sterile neutrinos. At the GeV scale, we use Chiral effective field theory involving sterile neutrinos to connect the operators at the level of quarks and gluons to hadronic interactions involving pions and nucleons. This allows us to derive an expression for 0νββ rates for various isotopes in terms of phase-space factors, hadronic low-energy constants, nuclear matrix elements, the neutrino masses, and the Wilson coefficients of higher-dimensional operators. The required hadronic low-energy constants and nuclear matrix elements depend on the neutrino masses, for which we obtain interpolation formulae grounded in QCD and chiral perturbation theory that improve existing formulae that are only valid in a small regime of neutrino masses. The resulting framework can be used directly to assess the impact of 0νββ experiments on scenarios with light sterile neutrinos and should prove useful in global analyses of sterile-neutrino searches. We per- form several phenomenological studies of 0νββ in the presence of sterile neutrinos with and without higher-dimensional operators. We find that non-standard interactions involving sterile neutrinos have a dramatic impact on 0νββ phenomenology, and next-generation experiments can probe such interactions up to scales of mathcal{O} (100) TeV.

Systematic analysis of Dirac neutrino masses from a dimension five operator

We perform a systematic study of the Dirac neutrino masses which arise from a dimension five effective operator with a singlet scalar. We identify all possible realizations of this operator at tree level and one-loop level. The corresponding predictions for the particle content and neutrino mass matrix are presented. We add a ${Z}_{2}$ symmetry to forbid the renormalizable Yukawa coupling, and non-Abelian discrete flavor symmetry can be used to forbid tree level diagrams in non-genuine one-loop models. The electrically neutral particle mediating the loop diagram could be dark matter candidate. We give the possible dark matter particles and the restrictions on the parameter $\ensuremath{\alpha}$ for each model.

Revisiting fermion mass and mixing fits in the minimal SUSY SO(10) GUT model

Supersymmetric $SO(10)$ grand unified models with renormalizable Yukawa couplings involving only ${\bf 10}$ and $\overline{\bf 126}$ Higgs fields have been shown to realize the fermion masses and mixings economically. In previous works, the sum rule of the fermion mass matrices are given by inputting the quark matrices, and the neutrino mixings are predicted in this framework. Now the three neutrino mixings have been measured, and in this paper, we give the sum rule by inputting the lepton mass matrices, which makes clear certain features of the solution, especially if the vacuum expectation values of ${\bf 126}+ \overline{\bf126}$ ($v_R$) are large and the right-handed neutrinos are heavy. We perform the $\chi^2$ analyses to fit the fermion masses and mixings using the sum rule. In previous works, the best fit appears at $v_R \sim 10^{13}$ GeV, and the fit at the large $v_R$ scale ($\sim 10^{16}$ GeV) has been less investigated. Our expression of the sum rule has a benefit to understand the flavor structure in the large $v_R$ solution. Using the fit results, we perform the calculation of the $\mu \to e\gamma$ process and the electric dipole moment of electron, and the importance of $v_R$ dependence emerges in low energy phenomena. We also show the prediction of the CP phase in the neutrino oscillations, which can be tested in the near future.

Doublet-triplet fermionic dark matter

We extend the Standard Model (SM) by adding a pair of fermionic SU(2)-doublets with opposite hypercharge and a fermionic SU(2)-triplet with zero hypercharge. We impose a discrete Z_2-symmetry that distinguishes the SM fermions from the new ones. Then, gauge invariance allows for two renormalizable Yukawa couplings between the new fermions and the SM Higgs field, as well as for direct masses for the doublet (M_D) and the triplet (M_T). After electroweak symmetry breaking, this model contains, in addition to SM particles, two charged Dirac fermions and a set of three neutral Majorana fermions, the lightest of which contributes to Dark Matter (DM). We consider a case where the lightest neutral fermion is an equal admixture of the two doublets with mass M_D close to the Z-boson mass. This state remains stable under radiative corrections thanks to a custodial SU(2)-symmetry and is consistent with the experimental data from oblique electroweak corrections. Moreover, the amplitudes relevant to spin-dependent or independent nucleus-DM particle scattering cross section both vanish at tree level. They arise at one loop at a level that may be observed in near future DM direct detection experiments. For Yukawa couplings comparable to the top-quark, the DM particle relic abundance is consistent with observation, not relying on co-annihilation or resonant effects and has a mass at the electroweak scale. Furthermore, the heavier fermions decay to the DM particle and to electroweak gauge bosons making this model easily testable at the LHC. In the regime of interest, the charged fermions suppress the Higgs decays to diphoton by 45-75 % relative to SM prediction.

Proton decay andμ→e+γconnection in a renormalizable SO(10) GUT for neutrinos

Supersymmetric SO(10) grand unified models with renormalizable Yukawa couplings involving {\bf 10}, {\bf 126} and {\bf 120} Higgs fields have been shown to give a very economical theory for understanding quark-lepton flavor in a unified framework. In previous papers, we showed how nucleon decay can be suppressed in these models without invoking cancellation, by choice of Yukawa flavor texture within a type II seesaw framework for neutrinos that explains all mixings and masses including the recently observed "large" $\theta_{13}$. In this follow-up paper, we extend our earlier work to the case of type I seesaw and show that the recently measured "large" $\theta_{13}$ can be accommodated in this case while suppressing proton decay. We then point out that the two cases (type I and II) lead to different testable predictions for $B(\mu\to e+\gamma)$ and $B(\tau\to \mu (e) +\gamma)$ as well as different flavor final states in nucleon decay. In particular, we find that for the type I seesaw case, $B(\tau\to \mu +\gamma)$ can be observable while at the same time suppressing $B(\mu\to e+\gamma)$, whereas in the type II seesaw case, $B(\tau\to \mu +\gamma)$ is always suppressed whereas $B(\mu\to e+\gamma)$ is observable.

Colored scalars and the CDF W+dijet excess

The recent data on $W+$dijet excess reported by CDF may be interpreted as the associated production of a $W$ and a new particle of mass about 150 GeV which subsequently decays into two hadron jets. We study the possibility of explaining the $W+$dijet excess by colored scalar bosons. There are several colored scalars which can have tree level renormalizable Yukawa couplings with two quarks, $({\bf 8}, {\bf 2},1/2)$, $(\bar{\bf 6}({\bf 3}), {\bf 3}({\bf 1}), -1/3)$, $(\bar {\bf 6}({\bf 3}), {\bf 1}, -4/3(2/3))$. If one of these scalars has a mass about 150 GeV, being colored it can naturally explain why the excess only shows up in the form of two hadron jets. Although the required production cross section and mass put constraints on model parameters and rule out some possible scenarios when confronted with other existing data, in particular FCNC data, we find that there are strong constraints on the Yukawa couplings of these scalars. Without forcing the couplings to be some special texture forms most of the scalars, except the $({\bf 3}, {\bf 3}, -1/3)$, are in trouble with FCNC data. We also study some features for search of these new particles at the RHIC and the LHC and find that related information can help further to distinguish different models.

Chiral U(1) flavor models and flavored Higgs doublets: the top FB asymmetry and the W jj

We present U(1) flavor models for leptophobic Z' with flavor dependent couplings to the right-handed up-type quarks in the Standard Model, which can accommodate the recent data on the top forward-backward (FB) asymmetry and the dijet resonance associated with a W boson reported by CDF Collaboration. Such flavor-dependent leptophobic charge assignments generally require extra chiral fermions for anomaly cancellation. Also the chiral nature of U(1)' flavor symmetry calls for new U(1)'-charged Higgs doublets in order for the SM fermions to have realistic renormalizable Yukawa couplings. The stringent constraints from the top FB asymmetry at the Tevatron and the same sign top pair production at the LHC can be evaded due to contributions of the extra Higgs doublets. We also show that the extension could realize cold dark matter candidates.

Erratum: Testing effective Yukawa couplings in Higgs boson searches at the Tevatron and LHC [Phys. Rev. D82, 113014 (2010)

We explore the possibility that, while the Higgs mechanism provides masses to the weak-gauge bosons at the electroweak scale as in the standard model, fermion masses are generated by an unknown mechanism at a higher energy scale. At low energies, the standard model can then be regarded as an effective field theory, where fermion masses explicitly break the electroweak SU(2)_L \times U(1)_Y gauge symmetry. If \Lambda is the renormalization scale where the renormalized Yukawa couplings vanish, then at energies lower than \Lambda, effective Yukawa couplings will be radiatively induced by nonzero fermion masses. In this scenario, Higgs-boson decays into photons and weak gauge-bosons pairs are in general quite enhanced for a light Higgs. However, depending on \Lambda, a substantial decay rate into b \bar{b} can arise, that can be of the same order as, or larger than, the enhanced H\to gamma gamma rate. A new framework for Higgs searches at hadron colliders is outlined, vector-boson fusion becoming the dominant production mechanism at the CERN LHC, with an important role also played by the WH/ZH associated production. A detailed analysis of the Higgs branching fractions and their implications in Higgs searches is provided, versus the energy scale \Lambda.

Testing effective Yukawa couplings in Higgs boson searches at the Tevatron and LHC

We explore the possibility that, while the Higgs mechanism provides masses to the weak-gauge bosons at the electroweak scale as in the standard model, fermion masses are generated by an unknown mechanism at a higher energy scale. At low energies, the standard model can then be regarded as an effective field theory, where fermion masses explicitly break the electroweak $\mathrm{SU}(2{)}_{\mathrm{L}}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1{)}_{\mathrm{Y}}$ gauge symmetry. If $\ensuremath{\Lambda}$ is the renormalization scale where the renormalized Yukawa couplings vanish, then at energies lower than $\ensuremath{\Lambda}$, effective Yukawa couplings will be radiatively induced by nonzero fermion masses. In this scenario, Higgs boson decays into photons and weak gauge-bosons pairs are in general quite enhanced for a light Higgs. However, depending on $\ensuremath{\Lambda}$, a substantial decay rate into $b\overline{b}$ can arise that can be of the same order as, or larger than, the enhanced $H\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ rate. A new framework for Higgs searches at hadron colliders is outlined, vector-boson fusion becoming the dominant production mechanism at the CERN LHC, with an important role also played by the $WH/ZH$ associated production. A detailed analysis of the Higgs branching fractions and their implications in Higgs searches is provided, versus the energy scale $\ensuremath{\Lambda}$.

An A4 model for neutrinos

The study of an extension of the standard model based on the flavor symmetry A4 is presented. Neutrino Majorana mass terms arise from dimension five operator and charged lepton masses from renormalizable Yukawa couplings. We introduce three Higgs doublets that belong to one triplet irreducible representation of A4. We study the most general A4-invariant scalar potential and the phenomenological consequences of the model. We find that the reactor angle could be as large as, sin2 θ13max ∼ 0.03, while the atmospheric mixing angle θ23 is close to maximal, sin2 θ23 = 1/2.

Scalar representations and minimal flavor violation

We discuss the representations that new scalar degrees of freedom (beyond those in the minimal standard model) can have if they couple to quarks in a way that is consistent with minimal flavor violation. If the new scalars are singlets under the flavor group then they must be color singlets or color octets. In this paper we discuss the allowed representations and renormalizable couplings when the new scalars also transform under the flavor group. We find that color $$ \bar 3 $$ and 6 representations are also allowed. We focus on the cases where the new scalars can have renormalizable Yukawa couplings to the quarks without factors of the quark Yukawa matrices. The renormalizable couplings in the models we introduce automatically conserve baryon number.

A4model for lepton masses and mixings

We study an extension of the standard model based on the flavor symmetry A4 only. Neutrino Majorana mass terms arise from dimension five operator and charged lepton masses from renormalizable Yukawa couplings. We introduce three Higgs doublets that belong to one triplet irreducible representation of A4. We study the most general A4-invariant scalar potential and the phenomenological consequences of the model. We find that the reactor angle could be as large as 0.03, while the atmospheric mixing angle is close to maximal.

Fermion masses and mixings in aμ-τsymmetricSO(10)

$\mu$-$\tau$ symmetry imposed on the neutrino mass matrix in the flavour basis is known to be quite predictive. We integrate this very specific neutrino symmetry into a more general framework based on the supersymmetric SO(10) grand unified theory. As in several other models, the fermion mass spectrum is determined by Hermitian mass matrices resulting from the renormalizable Yukawa couplings of the 16-plet of fermions with the Higgs fields transforming as $10, \bar{126},120$ representations of the SO(10) group. The $\mu$-$\tau$ symmetry is spontaneously broken through the 120-plet. Consequences of this scheme are considered for fermion masses using both type-I and type-II seesaw mechanism. This scenario is shown to lead to a generalized CP invariance of the mass matrices and vanishing CP violating phases if the Yukawa couplings are invariant under the $\mu$-$\tau$ symmetry. Small explicit breaking of the $\mu$-$\tau$ symmetry is then shown to provide a very good understanding of all the fermion masses and mixing. Detailed fits to the fermion spectrum are presented in several scenarios. One obtains a very good fit to all observables in the context of the type-I seesaw mechanism but type-II seesaw model also provides a good description except for the overall scale of the neutrino masses. Three major predictions on the leptonic mixing parameters in the type-I seesaw case are (1) the atmospheric mixing angle $\theta_{23}^{l}$ close to maximal, (2) $\theta_{13}^{l}$ close to the present upper bound and (3) negative but very small Dirac CP violating phase in the neutrino oscillations.

Fermion masses and mixings in a renormalizable SO(10)×Z2 GUT

We investigate a scenario in a supersymmetric SO(10) Grand Unified Theory in which the fermion mass matrices are generated by renormalizable Yukawa couplings of the 10⊕120⊕126¯ representation of scalars. We reduce the number of parameters by assuming spontaneous CP violation and a Z2 family symmetry, leading to nine real Yukawa coupling constants for three families. Since in the “minimal SUSY SO(10) GUT” an intermediate seesaw scale is ruled out and our scenario lives in the natural extension of this theory by the 120, we identify the vacuum expectation value (VEV) wR of (10,1,3)∈126¯ with the GUT scale of 2×1016GeV. In order to obtain sufficiently large neutrino masses, the coupling matrix of the scalar 126¯ is necessarily small and we neglect type II seesaw contributions to the light-neutrino mass matrix. We perform a numerical analysis of this 21-parameter scenario and find an excellent fit to experimentally known fermion masses and mixings. We discuss the properties of our numerical solution, including a consistency check for the VEVs of the Higgs-doublet components in the SO(10) scalar multiplets.

Charged-fermion masses in [formula omitted]: Analysis with scalars in [formula omitted

We consider the scenario in which the mass matrices of the charged fermions in the SO ( 10 ) grand unified theory are generated exclusively by renormalizable Yukawa couplings to one 10 ⊕ 120 representation of scalars. We analyze, partly analytically and partly numerically, this scenario in the three-generations case. We demonstrate that it leads to unification of the b and τ masses at the GUT scale. Testing this scenario against the mass values at the GUT scale, obtained from the renormalization-group evolution in the minimal SUSY extension of the Standard Model, we find that it is not viable: either the down-quark mass or the top-quark mass must be unrealistically low. If we include the CKM mixing angles in the test, then, in order that the mixing angles are well reproduced, either the top-quark mass or the strange-quark mass together with the down-quark mass must be very low. We conclude that, assuming a SUSY SO ( 10 ) scenario, charged-fermion mass generation based exclusively on one 10 ⊕ 120 representation of scalars is in contradiction with experiment.

Lepton masses and mixing angles from heterotic orbifold models

We systematically study the possibility for realizing realistic values of lepton mass ratios and mixing angles by using only renormalizable Yukawa couplings derived from heterotic $Z_6$-I orbifold. We assume one pair of up and down sector Higgs fields. We consider both the Dirac neutrino mass scenario and the seesaw scenario with degenerate right-handed majorana neutrino masses. It is found that realistic values of the charged lepton mass ratios, $m_e/m_\tau$ and $m_\mu/m_\tau$, the neutrino mass squared difference ratio, $\Delta m^2_{31}/\Delta m^2_{21}$, and the lepton mixing angles can be obtained in certain cases.

Searching for realistic 4d string models with a Pati–Salam symmetry. Orbifold grand unified theories from heterotic string compactification on a [formula omitted] orbifold

Motivated by orbifold grand unified theories, we construct a class of three-family Pati–Salam models in a Z 6 Abelian symmetric orbifold with two discrete Wilson lines. These models have marked differences from previously-constructed three-family models in prime-order orbifolds. In the limit where one of the six compactified dimensions (which lies in a Z 2 sub-orbifold) is large compared to the string length scale, our models reproduce the supersymmetry and gauge symmetry breaking pattern of 5d orbifold grand unified theories on an S 1 / Z 2 orbicircle. We find a horizontal 2 + 1 splitting in the chiral matter spectra—2 families of matter are localized on the Z 2 orbifold fixed points, and 1 family propagates in the 5d bulk—and identify them as the first-two and third families. Remarkably, the first two families enjoy a non-Abelian dihedral D 4 family symmetry, due to the geometric setup of the compactified space. In all our models there are always some color triplets, i.e., ( 6 , 1 , 1 ) representations of the Pati–Salam group, survive orbifold projections. They could be utilized to spontaneously break the Pati–Salam symmetry to that of the Standard Model. One model, with a 5d E 6 symmetry, may give rise to interesting low energy phenomenology. We study gauge coupling unification, allowed Yukawa couplings and some of their phenomenological consequences. The E 6 model has a renormalizable Yukawa coupling only for the third family. It predicts a gauge-Yukawa unification relation at the 5d compactification scale, and is capable of generating reasonable quark/lepton masses and mixings. Potential problems are also addressed, they may point to the direction for refining our models.

Quark masses and mixing angles in heterotic orbifold models

We study systematically the possibility for realizing realistic values of quark mass ratios mc/mt and ms/mb and the mixing angle Vcb by using only renormalizable Yukawa couplings derived from heterotic orbifold models. We assume one pair of up and down sector Higgs fields. We find many realistic Yukawa matrices.

Lepton flavor violation and the origin of the seesaw mechanism

The right-handed neutrino mass matrix that is central to the understanding of small neutrino masses via the seesaw mechanism can arise either (i) from renormalizable operators or (ii) from nonrenormalizable or super-renormalizable operators, depending on the symmetries and the Higgs content of the theory beyond the standard model. In this paper, we study lepton flavor violating (LFV) effects in the first class of seesaw models wherein the ${\ensuremath{\nu}}_{R}$ Majorana masses arise from renormalizable Yukawa couplings involving a $B\ensuremath{-}L=2$ Higgs field. We present detailed predictions for $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\tau}}\ensuremath{\mu}+\ensuremath{\gamma}$ and $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e+\ensuremath{\gamma}$ branching ratios in these models taking the current neutrino oscillation data into account. Focusing on minimal supergravity models, we find that for a large range of minimal supersymmetric standard model (MSSM) parameters, suggested by the relic abundance of neutralino dark matter which is consistent with Higgs boson mass and other constraints, these radiative decays are in the range accessible to planned experiments. We compare these predictions with lepton flavor violation in the second class of models arising entirely from the Dirac Yukawa couplings. We study the dependence of the ratio $r\ensuremath{\equiv}B(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e+\ensuremath{\gamma})/B(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\tau}}\ensuremath{\mu}+\ensuremath{\gamma})$ on the MSSM parameters and show that measurement of r can provide crucial insight into the origin of the seesaw mechanism.

Systematic study of horizontal gauge theories

We analyze all the possible continuous horizontal gauge groups GH in relation with their possibility to explain mb ⪡ mt. We assume that the only effective fermionic degrees of freedom correspond to the known fermions but allow the possibility of adding a right handed neutrino to each family. We assume that the Higgs fields which generate masses for these fermions, trough renormalizable Yukawa couplings, transform as an irreducible representation of SU(3)c ⊗ SU(2)L ⊗ U(1)Y ⊗ GH. Under these assumptions we find two U(1)H or U(1)H1 ⊗ U(1)H2 models free of anomalies and able to guarantee that only the top has a renormalizable mass-generating Yukawa coupling.