Yukawa Coupling Matrix Research Articles

On the orthogonal matrix in the Casas-Ibarra parametrization for the Yukawa interactions of Majorana neutrinos

The Casas-Ibarra (CI) parametrization of the Yukawa coupling matrix of Majorana neutrinos is generalized by considering the exact seesaw relation and including non-unitarity of the 3×3 Pontecorvo-Maki-Nakagawa-Sakata (PMNS) flavor mixing matrix. With the help of a full 6×6 Euler-like block description of the flavor structure for the seesaw mechanism, we find that the orthogonal matrix O in the CI parametrization can be expressed as Oij=Mj/miFij with mi and Mj being the masses of light and heavy Majorana neutrinos and Fij consisting of the PMNS and active-sterile flavor mixing parameters (for i,j=1,2,3). Assuming a specific pattern of O is therefore equivalent to imposing some special conditions on the seesaw parameter space.

Threshold effects on the massless neutrino in the canonical seesaw mechanism

In this work, we revisit the one-loop renormalization group equations (RGEs) among non-degenerate seesaw scales, i.e., threshold effects in the canonical seesaw mechanism, which have been obtained for more than two decades. Different from the previous work only focusing on the Weinberg operator, we derive the complete one-loop RGEs of all three dimension-five operators in the Standard Model effective field theory with right-handed neutrinos (νSMEFT) and apply them to threshold effects in the canonical seesaw mechanism. We find some contributions from the Weinberg operator to its Wilson coefficient, the neutrino Yukawa coupling matrix, and the Higgs quartic coupling absent in the previous calculations. Based on the updated one-loop RGEs, we derive the RGE of the effective neutrino mass matrix’s determinant without any approximation. Then, for the first time, we provide a strict proof that the one-loop RG running effects among non-degenerate seesaw scales can not generate a non-zero mass for the initial massless neutrino in the minimal type-I seesaw mechanism or in the canonical one with a rank-degenerate neutrino Yukawa coupling matrix. One has to include two- or higher-loop corrections to achieve a non-zero mass for the massless neutrino.

Reheating process in the R 2 inflationary model with the baryogenesis scenario

Post-inflationary evolution and (re)heating of the viable inflationary model, the R 2 one, is made more realistic by including the leptogenesis scenario into it. For this purpose, right-handed Majorana neutrinos with a large mass are added to the matter sector of the Standard Model to explain the neutrino oscillation experiments and the baryon asymmetry of the Universe. We have found parameters that characterize this model: non-minimal coupling of the Higgs field ξ, the mass of the right-handed Majorana neutrino MN α and the Yukawa coupling matrix components for the right-handed Majorana neutrino. We have analyzed the effect of these parameters on the reheating process and leptogenesis in this model and how they affect the resultant physical quantities: spectral parameters of primordial perturbations and baryon asymmetry.

Low-scale leptogenesis with flavour and CP symmetries

We consider a type-I seesaw framework endowed with a flavour symmetry, belonging to the series of non-abelian groups ∆(3 n2) and ∆(6 n2), and a CP symmetry. Breaking these symmetries in a non-trivial way results in the right-handed neutrinos being degenerate in mass up to possible (further symmetry-breaking) splittings κ and λ, while the neutrino Yukawa coupling matrix encodes the entire flavour structure in the neutrino sector. For a fixed combination of flavour and CP symmetry and residual groups, this matrix contains five real free parameters. Four of them are determined by the light neutrino mass spectrum and by accommodating experimental data on lepton mixing well, while the angle θR is related to right-handed neutrinos. We scrutinise for all four lepton mixing patterns, grouped into Case 1) through Case 3 b.1), the potential to generate the baryon asymmetry of the Universe through low-scale leptogenesis numerically and analytically. The main results are: a) the possible correlation of the baryon asymmetry and the Majorana phases, encoded in the Pontecorvo-Maki-Nakagawa-Sakata mixing matrix, in certain instances; b) the possibility to generate the correct amount of baryon asymmetry for vanishing splittings κ and λ among the right-handed neutrinos as well as for large κ, depending on the case and the specific choice of group theory parameters; c) the chance to produce sufficient baryon asymmetry for large active-sterile mixing angles, enabling direct experimental tests at current and future facilities, if θR is close to a special value, potentially protected by an enhanced residual symmetry. We elucidate these results with representative examples of flavour and CP symmetries, which all lead to a good agreement with the measured values of the lepton mixing angles and, possibly, the current indication of the CP phase δ. We identify the CP-violating combinations relevant for low-scale leptogenesis, and show that the parametric dependence of the baryon asymmetry found in the numerical study can be understood well with their help.

Phenomenology of one zero texture Yukawa matrix in a flavor symmetric scotogenic model

The scotogenic model is well known for accommodating small neutrino mass and dark matter. Here, we have realized the scotogenic model with the help of discrete flavor symmetry [Formula: see text]. We have obtained three one zero textures of Yukawa coupling matrices from the model and have studied its impact on neutrino phenomenology and related aspects of cosmology. On the basis of [Formula: see text] symmetry, we have further discarded two structures of one zero texture Yukawa coupling matrix. We further analyze if the effective mass of active neutrinos obtained by the virtue of the Yukawa coupling matrix is consistent with the KamLAND-Zen limit for [Formula: see text]. Also different lepton flavor violating (LFV) processes such as [Formula: see text] and [Formula: see text] are implemented and their influence on neutrino phenomenology is studied corresponding to the Yukawa coupling matrix. The entire work is carried out considering the dark matter mass [Formula: see text] in the region 450–750 GeV. We have also obtained some significant results for baryon asymmetry of the Universe in agreement with the one zero texture of the coupling matrices. Furthermore, interesting results for relic abundance on the basis of distinct mass splittings are between the inert scalars.

Neutrino mass observables and non-Hermitian version of Type-I seesaw model

We study the non-Hermitian extension of the Lagrangian of the Standard Model extended by singlet right-handed heavy neutrinos. The Yukawa coupling matrices comprise of Hermitian and non-Hermitian components and the neutrino mass eigenvalues are calculated for three generation case. The increased number of unknown parameters in the theory due to non-Hermitian nature of coupling matrices impose problem for its productiveness. Hence we apply four zero texture for the Yukawa matrices. We consider the normal hierarchy of neutrino mass ordering and find the mixing angles and Dirac CP violating phase by diagonalizing the lepton mass matrices. The parameters of the mixing matrix are obtained in terms of the parameters of the non-Hermitian Yukawa coupling matrix. The values of the angles and phase are compatible with experimental data available for neutrino mass observables and using this the non-Hermitian parameters are constrained.

Bridging resonant leptogenesis and low-energy CP violation with an RGE-modified seesaw relation

We propose a special type-I seesaw scenario in which the Yukawa coupling matrix Yν can be fully reconstructed by using the light Majorana neutrino masses mi, the heavy Majorana neutrino masses Mi and the PMNS lepton flavor mixing matrix U. It is the RGE-induced correction to the seesaw relation that helps interpret the observed baryon-antibaryon asymmetry of the Universe via flavored resonant thermal leptogenesis with M1≃M2≪M3. We show that our idea works well in either the τ-flavored regime with equilibrium temperature T≃M1∈(109,1012] GeV or the (μ+τ)-flavored regime with T≃M1∈(105,109] GeV, provided the light neutrinos have a normal mass ordering. We find that the same idea is also viable for a minimal type-I seesaw model with two nearly degenerate heavy Majorana neutrinos.

Chasing after flavor symmetries of quarks from the bottom up

AbstractWe explore a flavor structure of quarks in the standard model under the assumption that flavor symmetries exist in a theory beyond the standard model, and seek their properties, using a bottom-up approach. We reacknowledge that a flavor-symmetric part of the Yukawa coupling matrix can be realized by a rank-one matrix, and a democratic-type matrix occupies a special position, based on Dirac’s naturalness.

Same-sign trilepton signal for stop quark in the presence of sneutrino dark matter

We have explored a minimal supersymmetric standard model scenario extended by one pair of gauge singlets per generation. In the model light neutrino masses and their mixings are generated via inverse seesaw mechanism. In such a scenario, a right-handed sneutrino can be the lightest supersymmetric particle and a cold Dark Matter (DM) candidate. If Casas-Ibarra parametrisation is imposed on the Dirac neutrino Yukawa coupling matrix ($Y_{\nu}$) to fit the neutrino oscillation data, the resulting $Y_{\nu}$ is highly constrained from the lepton flavor violating (LFV) decay constraints. The smallness of $Y_{\nu}$ requires the sneutrino DM to co-annihilate with other sparticle(s) in order to satisfy DM relic density constraint. We have studied sneutrino co-annihilation with wino and observed that this sneutrino-wino compressed parameter space gives rise to a novel same-sign trilepton signal for the stop quark, which is more effective than the conventional stop search channels in the present framework. We have shown that the choice of neutrino mass hierarchy strongly affects the signal event rate, making it easier to probe the scenario with inverted mass hierarchy.

Combining texture zeros with a remnant CP symmetry in the minimal type-I seesaw

In the framework of the two right-handed neutrino seesaw model, we consider maximally-restrictive texture-zero patterns for the lepton Yukawa coupling and mass matrices, together with the existence of a remnant CP symmetry. Under this premise, we find that several textures are compatible with the most recent data coming from neutrino oscillation and neutrinoless double beta decay experiments. It is shown that, the maximum number of allowed texture zeros in the Dirac Yukawa coupling matrix is two, for an inverted neutrino mass spectrum. In contrast, for Yukawa coupling matrices with just one texture zero, both normal and inverted orderings of neutrino masses are compatible with data. In all cases, the predictions for the low-energy Dirac and Majorana CP-violating phases, and for the effective mass parameter relevant in neutrinoless double-beta decay experiments, are presented and discussed. We also comment on the impact of future experimental improvements in scrutinising texture-zero patterns with a remnant CP symmetry, within the minimal version of the seesaw mechanism considered here.

One-loop effective LFV varvec{Zl_kl_m} vertex from heavy neutrinos within the mass insertion approximation

In this paper we study the effective lepton flavor violating vertex of an electroweak Z gauge boson and two charged leptons with different flavor, l_k and l_m, that is generated to one-loop in low scale seesaw models with right handed neutrinos whose masses are heavier than the electroweak scale. We first compute the form factor describing this vertex by using the mass insertion approximation, where the flavor non-diagonal entries of the neutrino Yukawa coupling matrix are the unique origin, to one-loop level, of lepton flavor changing processes with charged leptons in the external legs. Then, by considering the proper large right handed neutrino mass expansion of the form factor, we derive a formula for the Z l_k l_m effective vertex which is very simple and useful for fast phenomenological estimates. In the last part of this work we focus on the phenomenological applications of this vertex for simple and accurate estimates of the Z rightarrow l_k {bar{l}}_m decay rates. Concretely, this vertex will allow us to conclude easily on the maximum allowed decay rates by present data in the inverse seesaw model. The found rates are promising, at the reach of future lepton colliders.

Minimal type-I seesaw model with maximally restricted texture zeros

In the context of Standard Model (SM) extensions, the seesaw mechanism provides the most natural explanation for the smallness of neutrino masses. In this work we consider the most economical type-I seesaw realization in which two right-handed neutrinos are added to the SM field content. For the sake of predictability, we impose the maximum number of texture zeros in the lepton Yukawa and mass matrices. All possible patterns are analyzed in the light of the most recent neutrino oscillation data, and predictions for leptonic $CP$ violation are presented. We conclude that, in the charged-lepton mass basis, eight different texture combinations are compatible with neutrino data at $1\ensuremath{\sigma}$, all of them for an inverted-hierarchical neutrino mass spectrum. Four of these cases predict a $CP$-violating Dirac phase close to $3\ensuremath{\pi}/2$, which is around the current best-fit value from the global analysis of neutrino oscillation data. If one further reduces the number of free parameters by considering three equal elements in the Dirac neutrino Yukawa coupling matrix, several texture combinations are still compatible with data but only at $3\ensuremath{\sigma}$. For all viable textures, the baryon asymmetry of the Universe is computed in the context of thermal leptogenesis, assuming (mildly) hierarchical heavy Majorana neutrino masses ${M}_{1,2}$. It is shown that the flavored regime is ruled out, while the unflavored one requires ${M}_{1}\ensuremath{\sim}{10}^{14}\text{ }\text{ }\mathrm{GeV}$.

Comprehensive renormalization group analysis of the littlest seesaw model

We present a comprehensive renormalisation group analysis of the Littlest Seesaw model involving two right-handed neutrinos and a very constrained Dirac neutrino Yukawa coupling matrix. We perform the first $\chi^2$ analysis of the low energy masses and mixing angles, in the presence of renormalisation group corrections, for various right-handed neutrino masses and mass orderings, both with and without supersymmetry. We find that the atmospheric angle, which is predicted to be near maximal in the absence of renormalisation group corrections, may receive significant corrections for some non-supersymmetric cases, bringing it into close agreement with the current best fit value in the first octant. By contrast, in the presence of supersymmetry, the renormalisation group corrections are relatively small, and the prediction of a near maximal atmospheric mixing angle is maintained, for the studied cases. Forthcoming results from T2K and NOvA will decisively test these models at a precision comparable to the renormalisation group corrections we have calculated.

Effective lepton flavor violating Hℓiℓj vertex from right-handed neutrinos within the mass insertion approximation

In this work we present a new computation of the lepton flavor violating Higgs boson decays that are generated radiatively to one-loop from heavy right-handed neutrinos. We work within the context of the inverse seesaw model with three $\nu_R$ and three extra singlets $X$, but the results could be generalized to other low scale seesaw models. The novelty of our computation is that it uses a completely different method by means of the mass insertion approximation which works with the electroweak interaction states instead of the usual 9 physical neutrino mass eigenstates of the inverse seesaw model. This method also allows us to write the analytical results explicitly in terms of the most relevant model parameters, that are the neutrino Yukawa coupling matrix $Y_\nu$ and the right-handed mass matrix $M_R$, which is very convenient for a phenomenological analysis. This $Y_\nu$ matrix, being generically nondiagonal in flavor space, is the only responsible for the induced charged lepton flavor violating processes of our interest. We perform the calculation of the decay amplitude up to order ${\cal O}(Y_\nu^2+Y_\nu^4)$. We also study numerically the goodness of the mass insertion approximation results. In the last part we present the computation of the relevant one-loop effective vertex $H\ell_i\ell_j$ for the lepton flavor violating Higgs decay which is derived from a large $M_R$ mass expansion of the form factors. We believe that our simple formula found for this effective vertex can be of interest for other researchers who wish to estimate the $H \to \ell_i \bar \ell_j$ rates in a fast way in terms of their own preferred input values for the relevant model parameters $Y_\nu$ and $M_R$.

Conformal standard model with an extended scalar sector

We present an extended version of the Conformal Standard Model (characterized by the absence of any new intermediate scales between the electroweak scale and the Planck scale) with an enlarged scalar sector coupling to right-chiral neutrinos. The scalar potential and the Yukawa couplings involving only right-chiral neutrinos are invariant under a new global symmetry SU(3) N that complements the standard U(1) B−L symmetry, and is broken explicitly only by the Yukawa interaction, of order $$ \mathcal{O} $$ (10−6), coupling right-chiral neutrinos and the electroweak lepton doublets. We point out four main advantages of this enlargement, namely: (1) the economy of the (non-supersymmetric) Standard Model, and thus its observational success, is preserved; (2) thanks to the enlarged scalar sector the RG improved one-loop effective potential is everywhere positive with a stable global minimum, thereby avoiding the notorious instability of the Standard Model vacuum; (3) the pseudo-Goldstone bosons resulting from spontaneous breaking of the SU(3) N symmetry are natural Dark Matter candidates with calculable small masses and couplings; and (4) the Majorana Yukawa coupling matrix acquires a form naturally adapted to leptogenesis. The model is made perturbatively consistent up to the Planck scale by imposing the vanishing of quadratic divergences at the Planck scale (‘softly broken conformal symmetry’). Observable consequences of the model occur mainly via the mixing of the new scalars and the standard model Higgs boson.

Seesaw mechanism with four texture zeros in the neutrino Yukawa matrix

With the recent observation of nonzero \theta_{13}, five neutrino oscillation parameters are now known. By imposing four zeros in the Yukawa coupling matrix of the type I seesaw model, the number of parameters in the neutrino mass matrix is reduced to seven, and we are able to make predictions for the lightest neutrino mass, Dirac CP phase, and neutrinoless double beta decay. Four texture zeros in the Yukawa coupling matrix is equivalent to either a single texture zero or a single cofactor zero for an off-diagonal element of the light neutrino mass matrix. We find strong similarities between single texture zero models with one mass ordering and single cofactor zero models with the opposite mass ordering. In the context of a specific class of single-flavor leptogenesis models, we find additional constraints on the parameter space.

Neutrino Yukawa Textures within Type-I Seesaw

We review neutrino Yukawa textures with zeros within the framework of the type-I seesaw with three heavy right chiral neutrinos and in the basis where the latter and the charged leptons are mass diagonal. An assumed nonvanishing mass of every ultralight neutrino and the observed nondecoupling of any neutrino generation allow a maximum of four zeros in the Yukawa coupling matrix in family space. We show that the requirement of an exactμtsymmetry, coupled with the observational constraints, reduces seventy-two allowed such textures to only four corresponding to just two different forms of the light neutrino mass matrix: one with an inverted and the other with a normal mass ordering. The masses and Majorana phases of ultralight neutrinos are predicted within definite ranges with laboratory and cosmological observational inputs. Within the same framework, we also study Yukawa textures with a fewer number of zeros, but with exactμtsymmetry. We further formulate the detailed scheme of the explicit breaking ofμtsymmetry in terms of three small parameters for allowed four zero textures. The observed sizable mixing between the first and third generations of neutrinos is shown to follow for a suitable choice of these symmetry breaking parameters.

Radiatively Generated Leptogenesis inS4Flavor Symmetry Models

We study how leptogenesis can be implemented in the seesaw models withS4flavor symmetry, which lead to the tri-bimaximal neutrino mixing matrix. By considering renormalization group evolution from a high-energy scale of flavor symmetry breaking (the GUT scale is assumed) to the low-energy scale of relevant phenomena, the off-diagonal terms in a combination of Dirac Yukawa-coupling matrix can be generated and the degeneracy of heavy right-handed neutrino Majorana masses can be lifted. As a result, the flavored leptogenesis is successfully realized. We also investigate how the effective light neutrino mass|〈mee〉|associated with neutrinoless double beta decay can be predicted along with the neutrino mass hierarchies by imposing the experimental data on the low-energy observables. We find a link between the leptogenesis and the neutrinoless double beta decay characterized by|〈mee〉|through a high-energy CP phase ϕ, which is correlated with the low-energy Majorana CP phases. It is shown that the predictions of|〈mee〉|for some fixed parameters of the high-energy physics can be constrained by the current observation of baryon asymmetry.

Yukawa alignment in a multi Higgs doublet model: An effective approach

In the two Higgs doublet model, natural flavour conservation can be achieved through the use of a discrete Z2 symmetry. A less restrictive condition is the requirement of alignment in the Yukawa sector. So far, alignment has been an ansatz, not rooted in a specific model. In this Letter we present a model for alignment, which starts with 2+N Higgs doublets, with natural flavour conservation imposed by a discrete symmetry. Only two of these scalars couple to the fermions, the other N scalars are in a hidden sector. Assuming that the two scalar doublets coupled to fermions are heavy, their decoupling leads to an effective Yukawa interaction. The latter connects the fermions and the scalars of the hidden sector, and exhibits the same Yukawa coupling matrix for each of the N scalars.

Baryon asymmetry from leptogenesis with four zero neutrino Yukawa textures

The generation of the right amount of baryon asymmetry η of the Universe from supersymmetric leptogenesis is studied within the type-I seesaw framework with three heavy singlet Majorana neutrinos Ni (i = 1,2,3) and their superpartners. We assume the occurrence of four zeroes in the neutrino Yukawa coupling matrix Yν, taken to be μτ symmetric, in the weak basis where Ni (with real masses Mi > 0) and the charged leptons lα (α = e,μ,τ) are mass diagonal. The quadrant of the single nontrivial phase, allowed in the corresponding light neutrino mass matrix mν, gets fixed and additional constraints ensue from the requirement of matching η with its observed value. Special attention is paid to flavor effects in the washout of the lepton asymmetry. We also comment on the role of small departuresfrom high scale μτ symmetry due to RG evolution.