Right-handed Majorana Neutrino Mass Research Articles

Postinflationary Higgs relaxation and the origin of matter-antimatter asymmetry.

The recent measurement of the Higgs boson mass implies a relatively slow rise of the standard model Higgs potential at large scales, and a possible second minimum at even larger scales. Consequently, the Higgs field may develop a large vacuum expectation value during inflation. The relaxation of the Higgs field from its large postinflationary value to the minimum of the effective potential represents an important stage in the evolution of the Universe. During this epoch, the time-dependent Higgs condensate can create an effective chemical potential for the lepton number, leading to a generation of the lepton asymmetry in the presence of some large right-handed Majorana neutrino masses. The electroweak sphalerons redistribute this asymmetry between leptons and baryons. This Higgs relaxation leptogenesis can explain the observed matter-antimatter asymmetry of the Universe even if the standard model is valid up to the scale of inflation, and any new physics is suppressed by that high scale.

Strongly first-order electroweak phase transition and classical scale invariance

In this work, we examine the possibility of realizing a strongly first-order electroweak phase transition within the minimal classically scale invariant extension of the standard model (SM), previously proposed and analyzed as a potential solution to the hierarchy problem. By introducing one complex singlet scalar and three right-handed Majorana neutrinos, the scenario was successfully capable of achieving a radiative breaking of the electroweak symmetry (Coleman-Weinberg Mechanism), inducing non-zero masses for the SM neutrinos (seesaw mechanism), presenting a pseudoscalar dark matter candidate, and predicting the existence of a second $CP$-even boson in addition to the 125 GeV scalar. We construct the full finite-temperature one-loop effective potential of the model, including the resummed thermal daisy loops, and demonstrate that finite-temperature effects induce a first-order electroweak phase transition. Requiring the thermally-driven first-order phase transition to be sufficiently strong further constrains the model's parameter space; in particular, an $\mathcal O(0.01)$ fraction of the dark matter in the universe may be simultaneously accommodated with a strongly first-order electroweak phase transition. Moreover, such a phase transition disfavors right-handed Majorana neutrino masses above several hundreds of GeV, confines the pseudoscalar dark matter masses to $\sim 1$-2 TeV, predicts the mass of the second $CP$-even scalar to be $\sim 100$-300 GeV, and requires the mixing angle between the $CP$-even components of the SM doublet and the complex singlet to lie within the range $0.2 \lesssim \sin\omega \lesssim 0.4$. The obtained results are displayed in comprehensive exclusion plots, identifying the viable regions of the parameter space. Many of these predictions lie within the reach of the next LHC run.

Witten’s mechanism in the flippedSU(5)unification

We argue that Witten's loop mechanism for the right-handed Majorana neutrino mass generation identified originally in the $SO(10)$ grand unification context can be successfully adopted to the class of the simplest flipped $SU(5)$ models. In such a framework, the main drawback of the $SO(10)$ prototype---in particular, the generic tension among the gauge unification constraints and the absolute neutrino mass scale---is alleviated, and a simple yet potentially realistic and testable scenario emerges.

Hybrid textures of the right-handed Majorana neutrino mass matrix

We perform a systematic study of neutrino mass matrices having a vanishing cofactor and an equality between two cofactors of the mass matrix. Such texture structures of the effective neutrino mass matrix arise from type-I seesaw mechanism when the Dirac neutrino mass matrix is diagonal with equal elements and the right-handed Majorana neutrino mass matrix has hybrid textures with one equality of matrix elements and one zero matrix element. For three right-handed neutrinos there are sixty possible hybrid textures out of which only six are excluded by the present experimental data. We show that such textures can be derived using discrete symmetries. The predictions of experimentally allowed textures are examined for unknown parameters such as the effective Majorana mass of the electron neutrino and the Dirac-type CP-violating phase.

Higgs signatures in inverse seesaw model at the LHC

In the inverse seesaw mechanism where the spontaneously broken B-L symmetry induces tiny B-L violating Majorana masses for right-handed neutrinos, non-standard Higgs signatures can arise due to a possible Higgs doublet and singlet mixing and/or Higgs boson decays to a left- and right-handed neutrino. This leads to a remarkable feature of hadronically quiet di-lepton final states which can exhibit, in particular, lepton flavour violating signatures coming from flavour-dependent neutrino Yukawa couplings. In this process, one lepton coming from the right-handed decay could be soft enough can be missed by the trigger level cuts of CMS and ATLAS for the di-lepton plus missing energy signature. The prospects of such a signature are investigated for 8 TeV and 14 TeV center of mass energy of the LHC, taking the maximum value of the allowed neutrino Yukawa coupling and the right-handed neutrino mass of 100 GeV. A PYTHIA level simulation shows that the integrated luminosity of 10--20/fb and 1.6/fb for 8 TeV is required to observe the inclusive leptonic and lepton flavour violating signatures, respectively. For 14 TeV, the reach is more and a larger parameter space of the inverse seesaw model can be probed.

Anomalous Majorana neutrino masses from torsionful quantum gravity

The effect of quantum torsion in theories of quantum gravity is usually described by an axionlike field which couples to matter and to gravitation and radiation gauge fields. In perturbation theory, the couplings of this torsion-descent axion field are of derivative type and so preserve a shift symmetry. This shift symmetry may be broken, if the torsion-descent axion field mixes with other axions, which could be related to moduli fields in string-inspired effective theories. In particular, the shift symmetry may break explicitly via nonperturbative effects, when these axions couple to fermions via chirality-changing Yukawa couplings with appropriately suppressed coefficients. We show how in such theories an effective right-handed Majorana neutrino mass can be generated at two loops by gravitational interactions that involve global anomalies related to quantum torsion. We estimate the magnitude of the gravitationally induced Majorana mass and find that it is highly model dependent, ranging from the multi-TeV to keV scale.

Scaling ansatz, four zero Yukawa textures, and largeθ13

We investigate the ``scaling Ansatz'' in the neutrino sector within the framework of a type-I seesaw mechanism with diagonal charged lepton and right-handed Majorana neutrino mass matrices (${M}_{R}$). We also assume the four zero texture of Dirac neutrino mass matrices (${m}_{D}$) which severely constrain the phenomenological outcomes of such schemes. The scaling Ansatz and the present neutrino data allow only six such matrices out of 126 four zero Yukawa matrices. In this scheme, in order to generate large ${\ensuremath{\theta}}_{13}$, we break the scaling Ansatz in ${m}_{D}$ through a perturbation parameter and we also show that our breaking scheme is radiatively stable. We further investigate $CP$ violation and baryogenesis via leptogenesis in those surviving textures.

Vanishing minors in the neutrino mass matrix from abelian gauge symmetries

Augmenting the Standard Model by three right-handed neutrinos allows for an anomaly-free gauge group extension G_max = U(1)_(B-L) x U(1)_(L_e-L_mu) x U(1)_(L_mu-L_tau). While simple U(1) subgroups of G_max have already been discussed in the context of approximate flavor symmetries, we show how two-zero textures in the right-handed neutrino Majorana mass matrix can be enforced by the flavor symmetry, which is spontaneously broken very economically by singlet scalars. These zeros lead to two vanishing minors in the low-energy neutrino mass matrix after the seesaw mechanism. This study may provide a new testing ground for a zero-texture approach: the different classes of two-zero textures with almost identical neutrino oscillation phenomenology can in principle be distinguished by their different Z' interactions at colliders.

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.

Gauged baryon and lepton number inMSSM4brane worlds

A recent $D$-brane model designed to accommodate a phenomenologically acceptable fourth generation of chiral fermions was noted to produce an unexpected additional unbroken nonanomalous U(1) gauge group at the string scale. We show that the corresponding charges acting on minimal supersymmetric standard model fields count baryon and lepton numbers. If broken spontaneously at lower scales, these $\mathrm{U}(1{)}_{B}$ and $\mathrm{U}(1{)}_{L}$ symmetries provide potential avenues for preserving baryogenesis while nonetheless explaining the suppression of proton decay (without the need for $R$ parity), as well as the smallness of right-handed neutrino Majorana masses compared to the string scale.

Inverse seesaw in supersymmetry

We study a mechanism where tiny neutrino masses arise only from radiative contribution in a supersymmetric model. Without tuning the Higgs vev or Yukawa coupling, the tree-level light neutrino mass is rotated away by introducing a new singlet neutrino sL that forms a Dirac mass term with the right-handed neutrino nR in each generation. Even with the U(1)Lep lepton number violation term as non-zero right-handed Majorana neutrino mass MRnRc¯nR, the lightest neutrino remains massless at tree level due to an approximate symmetry. Consequently, tiny neutrino masses then only arise from radiative contributions and the right-handed neutrino Majorana mass MR can be at O(keV) to give rise to the correct light neutrino mass at O(eV). Supersymmetry ensures that the Majorana neutrino masses MRnRc¯nR and MR⁎sLc¯sL are not generated simultaneously.

TeV scale horizontal gauge symmetry and its implications inBphysics

We propose a gauged $U(1)_{H}$ horizontal symmetry around TeV scale that is a subgroup of a $SU(3)_{H}$ horizontal gauge symmetry broken at ${\cal O}(10^{14}) \GeV$. The breaking generates right-handed Majorana neutrino masses through a $SU(3)_H$ sextet scalar. A particular Majorana right-handed neutrino mass matrix explicitly determines the remnant $U(1)_{H}$ at low energy which only couples to $b-s$ and $\mu-\tau$ in the gauge eigenstate. The dangerous $K-\bar{K}$, $D-\bar{D}$ mixing and $B_s \rightarrow \mu^+ \mu^-$ are kept to be safe because the relevant couplings are suppressed through high powers of small mixing angles in the fermion rotation matrix. Our analysis which applies to the general case shows that the Tevatron di-muon anomaly can be explained through the $B_{s}$ and $B_{d}$ mixing while keeping all the other experimental constraints within 90 \% C. L. For the $B$ meson decay, the $B_{s}\to \mu^{\pm}\tau^{\mp}$ is the leading leptonic decay channel which is several orders of magnitude below current experimental bound.

Flavor structure in F-theory compactifications

F-theory is one of frameworks in string theory where supersymmetric grand unification is accommodated, and all the Yukawa couplings and Majorana masses of righthanded neutrinos are generated. Yukawa couplings of charged fermions are generated at codimension-3 singularities, and a contribution from a given singularity point is known to be approximately rank 1. Thus, the approximate rank of Yukawa matrices in low-energy effective theory of generic F-theory compactifications are minimum of either the number of generations N gen = 3 or the number of singularity points of certain types. If there is a geometry with only one E 6 type point and one D 6 type point over the entire 7-brane for SU(5) gauge fields, F-theory compactified on such a geometry would reproduce approximately rank-1 Yukawa matrices in the real world. We found, however, that there is no such geometry. Thus, it is a problem how to generate hierarchical Yukawa eigenvalues in F-theory compactifications. A solution in the literature so far is to take an appropriate factorization limit. In this article, we propose an alternative solution to the hierarchical structure problem (which requires to tune some parameters) by studying how zero mode wavefunctions depend on complex structure moduli. In this solution, the N gen × N gen CKM matrix is predicted to have only N gen entries of order unity without an extra tuning of parameters, and the lepton flavor anarchy is predicted for the lepton mixing matrix. The hierarchy among the Yukawa eigenvalues of the down-type and charged lepton sector is predicted to be smaller than that of the up-type sector, and the Majorana masses of left-handed neutrinos generated through the see-saw mechanism have small hierarchy. All of these predictions agree with what we observe in the real world. We also obtained a precise description of zero mode wavefunctions near the E 6 type singularity points, where the up-type Yukawa couplings are generated.

Low-scale leptogenesis and the domain wall problem in models with discrete flavor symmetries

We propose a new mechanism for leptogenesis, which is naturally realized in models with a flavor symmetry based on the discrete group A4, where the symmetry breaking parameter also controls the Majorana masses for the heavy right-handed (RH) neutrinos. During the early universe, for T≳TeV, part of the symmetry is restored, due to finite temperature contributions, and the RH neutrinos remain massless and can be produced in thermal equilibrium even at temperatures well below the most conservative gravitino bounds. Below this temperature the phase transition occurs and they become massive, decaying out of equilibrium and producing the necessary lepton asymmetry. Unless the symmetry is broken explicitly by Planck-suppressed terms, the domain walls generated by the symmetry breaking survive till the quark–hadron phase transition, where they disappear due to a small energy splitting between the A4 vacua caused by the QCD anomaly.

Leptogenesis in a perturbative SO(10) model

We consider a phenomenologically viable SO(10) grand unification model which allows perturbative calculations up to the Planck scale or the string scale. We use a set of Higgs superfields 10++16+45. In this framework, the data fitting of the charged fermion mass matrices is re-examined. This model can indeed reproduce the low-energy experimental data relating the charged fermion masses and mixings. As for the neutrino sector, we take the neutrino oscillation data as input data to construct right-handed Majorana neutrino mass matrix and get a prediction for the physics related to the right-handed neutrinos, e.g. the leptogenesis and for the proton decay. We propose two kinds of phenomenologically viable model, quoted as Model 1 and Model 2. We show that one of the models (Model 2) is consistent with all experimental constraints.

SU L (4)×U(1) model for electroweak unification and sterile neutrinos

Some of the basic problems in neutrino physics, such as new energy scales, the enormous gap between the neutrino masses and the lightest charged fermion mass, and the possible existence of sterile neutrinos in the eV mass range are studied in the local gauge group SU L (4)×U(1) for electroweak unification, which does not contain fermions with exotic electric charges. It is shown that the neutrino mass spectrum can be decoupled from that of the other fermions. The further normal seesaw mechanism for neutrinos, with right-handed neutrino Majorana masses of order M≫M weak as well a new eV-scale can be accommodated. The eV-scale seesaw may manifest itself in experiments like the Liquid Scintillation Neutrino Detector (LSND) and MiniBooNE (MB) experimental results and future neutrino experiments.

Heterotic road to the MSSM withRparity

In a previous paper, referred to as a ``Mini-Landscape'' search, we explored a ``fertile patch'' of the heterotic landscape based on a ${\mathbb{Z}}_{6}\ensuremath{-}\mathrm{II}$ orbifold with SO(10) and ${\mathrm{E}}_{6}$ local grand unified theory structures. In the present paper we extend this analysis. We find many models with the minimal supersymmetric standard model spectra and an exact $R$ parity. In all of these models, the vectorlike exotics decouple along $D$-flat directions. We present two benchmark models which satisfy many of the constraints of a realistic supersymmetric model, including nontrivial Yukawa matrices for three families of quarks and leptons and Majorana masses for right-handed neutrinos with nontrivial seesaw masses for the three light neutrinos. In an appendix we comment on the important issue of string selection rules and, in particular, the so-called ``gamma-rule''.

Neutrino masses and mixings from string theory instantons

We study possible patterns of neutrino masses and mixings in string models in which Majorana neutrino masses are generated by a certain class of string theory instantons recently considered in the literature. These instantons may generate either directly the dim = 5 Weinberg operator or right-handed neutrino Majorana masses, both with a certain flavour-factorised form. A hierarchy of neutrino masses naturally appears from the exponentially suppressed contributions of different instantons. The flavour structure is controlled by string amplitudes involving neutrino fields and charged instanton zero modes. For some simple choices for these amplitudes one finds neutrino mixing patterns consistent with experimental results. In particular, we find that a tri-bimaximal mixing pattern is obtained for simple symmetric values of the string correlators.

Neutrino Majorana masses from string theory instanton effects

Finding a plausible origin for right-handed neutrino Majorana masses in semirealistic compactifications of string theory remains one of the most difficult problems in string phenomenology. We argue that right-handed neutrino Majorana masses are induced by non-perturbative instanton effects in certain classes of string compactifications in which the $U(1)_{B-L}$ gauge boson has a St\"uckelberg mass. The induced operators are of the form $e^{-U}\nu_R\nu_R$ where $U$ is a closed string modulus whose imaginary part transforms appropriately under $B-L$. This mass term may be quite large since this is not a gauge instanton and $Re U$ is not directly related to SM gauge couplings. Thus the size of the induced right-handed neutrino masses could be a few orders of magnitude below the string scale, as phenomenologically required. It is also argued that this origin for neutrino masses would predict the existence of R-parity in SUSY versions of the SM. Finally we comment on other phenomenological applications of similar instanton effects, like the generation of a $\mu$-term, or of Yukawa couplings forbidden in perturbation theory.

Spacetime instanton corrections in 4D string vacua: The seesaw mechanism for D-brane models

We systematically investigate instanton corrections from wrapped Euclidean D-branes to the matter field superpotential of various classes of N = 1 supersymmetric D-brane models in four dimensions. Both gauge invariance and the counting of fermionic zero modes provide strong constraints on the allowed non-perturbative superpotential couplings. We outline how the complete instanton computation boils down to the computation of open string disc diagrams for boundary changing operators multiplied by a one-loop vacuum diagram. For concreteness we focus on E2-instanton effects in type IIA vacua with intersecting D6-branes, however the same structure emerges for type IIB and heterotic vacua. The instantons wrapping rigid cycles can potentially destabilise the vacuum or generate perturbatively absent matter couplings such as proton decay operators, μ-parameter or right-handed neutrino Majorana mass terms. The latter allow the realisation of the seesaw mechanism for MSSM like intersecting D-brane models.