Exotic Fermions Research Articles

Sharp gamma-ray spectral features from scalar dark matter annihilations

The search for sharp features in the gamma-ray spectrum is a promising approach to identify a signal from dark matter annihilations over the astrophysical backgrounds. In this paper we investigate the generation of gamma-ray lines and internal bremsstrahlung signals in a toy model where the dark matter particle is a real scalar that couples to a lepton and an exotic fermion via a Yukawa coupling. We show that the Fermi-LAT and H.E.S.S. searches for line-like spectral features severely constrain regions of the parameter space where the scalar dark matter is thermally produced. Finally, we also discuss the complementarity of the searches for sharp spectral features with other indirect dark matter searches, as well as with direct and collider searches.

Three sterile neutrinos inE6

Candidates for unification of the electroweak and strong interactions include the grand unified groups SU(5), SO(10), and the exceptional group ${\mathrm{E}}_{6}$. The 27-dimensional fundamental representation of ${\mathrm{E}}_{6}$ contains exotic fermions, including weak isosinglet quarks of charge $\ensuremath{-}1/3$, vectorlike weak isodoublet leptons, and neutral leptons which are singlets under both left-handed and right-handed SU(2). These last are candidates for light ``sterile'' neutrinos, hinted at by some recent short-baseline neutrino experiments.In order to accommodate three families of quarks and charged leptons, an ${\mathrm{E}}_{6}$ model must contain three 27-plets, each of which contains a sterile neutrino candidate $n$. The mixing pattern within a 27-plet is described, and experimental consequences are discussed.

Majorana dark matter in warped extra dimensions

Generic extensions of the Standard Model that respect baryon and lepton numbers have accidentally stable particles. Typical examples are the lightest exotic neutral fermion, or "neutralino", and fields with non-trivial lepton and baryon charges. In this paper we show that an accidentally stable neutralino is a natural dark matter candidate in models with warped extra dimensions. We find that annihilation into other Kaluza-Klein resonances is often allowed and very efficient. The observed dark matter abundance may then be obtained with couplings of order unity and a compactification scale above the TeV. Light dark matter is also possible in the presence of unsuppressed couplings to the Higgs boson. In this latter case direct detection experiments will soon be able to probe a significant portion of the parameter space.

Oblique parameters in gauged baryon and lepton numbers with a 125 GeV Higgs

In an extension of the standard model, where baryon number and lepton number are local gauge symmetries, we analyze the effect of corrections from exotic fermions and scalars on the oblique parameters S, T, U. Because a light neutral Higgs h0 with mass around 124–126 GeV strongly constrains the corresponding parameter space of this model, we also investigate the gluon fusion process gg→h0 and two photon decay of the lightest neutral Higgs h0→γγ at the Large Hadron Collider.

Boosting top partner searches in composite Higgs models

Fermionic third generation top partners are generic in composite Higgs models. They are likely to decay into third generation quarks and electroweak bosons. We propose a novel cut-and-count-style analysis in which we cross correlate the model-dependent single and model-independent pair production processes for the top partners ${X}_{5/3}$ and $B$. In the class of composite Higgs models we study, ${X}_{5/3}$ is very special as it is the lightest exotic fermion. A constraint on the mass of ${X}_{5/3}$ directly extends to constrains on all top partner masses. By combining jet substructure methods with conventional reconstruction techniques we show that in this kind of final state a smooth interpolation between the boosted and unboosted regime is possible. We find that a reinterpretation of existing searches can improve bounds on the parameter space of composite Higgs models. Further, at 8 TeV a combined search for ${X}_{5/3}$ and $B$ in the $l+\text{jets}$ final state can be more sensitive than a search involving same-sign dileptons.

Some phenomenological aspects of a newU(1)′model

We propose a new nonuniversal $U(1{)}^{\ensuremath{'}}$ extension of the standard model with the addition of three exotic quark singlets, two scalar singlets and one additional scalar doublet. We obtain family-dependent couplings with a new ${Z}^{\ensuremath{'}}$ boson in the quark sector and universal couplings in the lepton sector. From experimental data on ${Z}^{\ensuremath{'}}$ research at the CERN LHC, we find limit regions in the $U(1{)}^{\ensuremath{'}}$ free parameters (${Z}^{\ensuremath{'}}$ mass and coupling constant), which we use to obtain the total decay width and invariant-mass distributions. By introducing discrete symmetries and mixing couplings between ordinary and exotic fermions, we obtain predictable mass relations in the quark sector compatible with the phenomenological values without large fine-tuning of the Yukawa couplings and with few free parameters, where hierarchies between quark families can be understood from the existence of heavy beyond-standard-model particles.

Radiatively induced type II seesaw models and vectorlike5/3charge quarks

Understanding small neutrino masses in type II seesaw models with TeV scale SM triplet Higgs bosons requires that its coupling with the standard model Higgs doublet $H$ be ``dialed'' down to be order eV to KeV, which is a hierarchy of a factor of ${10}^{\ensuremath{-}11}--{10}^{\ensuremath{-}8}$ with respect to the weak scale. We present a class of SUSY extensions of the type II seesaw model where this dimensionful small coupling is radiatively induced, thus making its smallness natural. Remarkably, in our model, this coupling is vanishing if SUSY is unbroken. Furthermore, in order to radiatively generate the coupling between the Higgs doublets and the triplet in this class of models, exotic matter fields need to be introduced. We discuss in detail one model where neutrino mass is triggered by a finite loop of an exotic vectorlike quark doublet which contains a quark $X$ with electric charge $5/3$ and a top partner ${t}^{\ensuremath{'}}$. We then discuss in detail the phenomenology of the model, paying special attention to the consequences of the interactions of the the exotic heavy quarks and the scalars of the model. Implications for neutrinoless double beta decay and for the LHC experiments are discussed in detail. Remarkably, in this model, both the seesaw triplet and the heavy quarks can manifest at colliders in a host of different signatures, including some that significantly differ from those of the minimal models. Depending on the choice of the hierarchy of couplings, the decay of the heavy quarks and of the seesaw triplet may be subject to bounds that can be tighter or looser than the bounds from standard LHC searches. Furthermore, we point out a new short-distance contribution to neutrinoless double beta decay mediated by the simultaneous propagation of the type II triplet and exotic fermions. Remarkably, this contribution to the neutrinoless double beta decay is parametrically quite independent from the scale of the generated neutrino mass.

Probing exotic fermions from a seesaw/radiative model at the LHC

There exist tree-level generalizations of the Type-I and Type-III seesaw mechanisms that realize neutrino mass via low-energy effective operators with d>5. However, these generalizations also give radiative masses that can dominate the seesaw masses in regions of parameter space --- i.e. they are not purely seesaw models, nor are they purely radiative models, but instead they are something in between. A recent work detailed the remaining minimal models of this type. Here we study the remaining model with d=9 and investigate the collider phenomenology of the exotic quadruplet fermions it predicts. These exotics can be pair produced at the LHC via electroweak interactions and their subsequent decays produce a host of multi-lepton signals. Furthermore, the branching fractions for events with distinct charged-leptons encode information about both the neutrino mass hierarchy and the leptonic mixing phases. In large regions of parameter-space discovery at the LHC with a 5 sigma significance is viable for masses approaching the TeV scale.

A model with chiral quarks of electric charges −4/3 and 5/3

We present a new model based on the SU(3)⨂SU(2)⨂U(1) symmetry, in which there is a new consistent set of chiral fermion fields that renders the model free from anomalies. The new fermions do not share the usual family structure of the Standard Model and some of them have exotic electric charges, as the quarks X and Y with electric charge 5/3 and −4/3, respectively. Interestingly, the model contains a new heavy neutral lepton which may be a dark matter candidate. Two Higgs doublets are present in our construction, so that two CP even scalars are present in the model particle spectrum. One of them is similar to the Standard Model Higgs boson, while the other one couples mainly with the new exotic fermions. We performed a discovery analysis showing that the 8 TeV LHC can find the Y quark from single and pair production with masses from 300 GeV up to ~ 750 GeV. We also show that the new spectrum does not contribute significantly to the oblique EW parameters, and that dangerous flavor changing neutral currents are suppressed. Characteristic signatures from the other new fermions in the model are also commented.

Topological Transitions of Gapless Paired States in Mixed-Geometry Lattices

We propose a mixed-geometry system of fermionic species selectively confined in lattices of different geometry. We investigate how such asymmetry can lead to exotic multiband fermion pairing in an example system of honeycomb and triangular lattices. A rich phase diagram of interband pairing with gapped and gapless excitations is found at zero temperature. We find that the two-band contribution of the honeycomb lattices to the paired state helps to stabilize the gapless phase with one or two Fermi surfaces. We also show that the Fermi surface topology further divides the gapless phase into subclasses between which the system undergoes density-driven Lifshitz transitions.

Implications of gauge-mediated supersymmetry breaking with vectorlike quarks and a∼125 GeVHiggs boson

We investigate the implications of models that achieve a Standard Model-like Higgs boson of mass near 125 GeV by introducing additional TeV-scale supermultiplets in the vectorlike $\mathbf{10}+\overline{\mathbf{10}}$ representation of $SU(5)$, within the context of gauge-mediated supersymmetry breaking. We study the resulting mass spectrum of superpartners, comparing and contrasting to the usual gauge-mediated and constrained minimal supersymmetric Standard Model scenarios, and discuss implications for LHC supersymmetry searches. This approach implies that exotic vectorlike fermions ${t}_{1,2}^{\ensuremath{'}}$, ${b}^{\ensuremath{'}}$, and ${\ensuremath{\tau}}^{\ensuremath{'}}$ should be within the reach of the LHC. We discuss the masses, the couplings to electroweak bosons, and the decay branching ratios of the exotic fermions, with and without various unification assumptions for the mass and mixing parameters. We comment on LHC prospects for discovery of the exotic fermion states, both for decays that are prompt and for those that are nonprompt on detector-crossing time scales.

Exotic fermion multiplets as a solution to baryon asymmetry, dark matter, and neutrino masses

We propose an extension to the standard model where three exotic fermion 5-plets and one scalar 6-plet are added to the particle content. By demanding that all interactions are renormalizable and standard model gauge invariant, we show that the lightest exotic particle in this model can be a dark matter candidate as long as the new 6-plet scalar does not develop a nonzero vacuum expectation value. Furthermore, light neutrino masses are generated radiatively at one-loop while the baryon asymmetry is produced by the CP-violating decays of the second lightest exotic particle. We have demonstrated using concrete examples that there is a parameter space where a consistent solution to the problems of baryon asymmetry, dark matter and neutrino masses can be obtained.

FLAVOR VIOLATION IN A MINIMAL SO(10)×A4 SUSY GUT

Flavor violating processes in the quark and lepton sectors are investigated within a realistic supersymmetric SO(10)×A4 grand unification model. By employing exotic heavy fermion fields, this model successfully describes various features of the fermion masses and mixings including large neutrino mixings accompanied by small quark mixings. In this model the flavor violation is induced at GUT scale, at which A4 flavor symmetry is broken, as a consequence of the large mixings of the light fermion fields with these exotic heavy fields. The stringent experimental constraint from μ→eγ decay rate necessitates a high degree of degeneracy of the supersymmetry breaking soft scalar masses of the exotic heavy fields and supersymmetric scalar partners of the light fermion fields. The choice of slepton masses of order 1 TeV is found to be consistent with the constraints from branching ratio of μ→eγ and with all other flavor changing neutral current processes being sufficiently suppressed.

Interaction of Phonons and Dirac Fermions on the Surface ofBi2Se3: A Strong Kohn Anomaly

We report the first measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi2Se3. The surface phonon measurements were carried out with the aid of coherent helium beam surface scattering techniques. The results reveal a prominent signature of the exotic metallic Dirac fermion quasiparticles, including a strong Kohn anomaly. The signature is manifest in a low energy isotropic convex dispersive surface phonon branch with a frequency maximum of 1.8 THz and having a V-shaped minimum at approximately 2kF that defines the Kohn anomaly. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model.

New Limit on Time-Reversal Violation in Beta Decay

New Limit on Time-Reversal Violation in Beta Decay

SU(5) × SU(5) unification revisited

The idea of grand unification in a minimal supersymmetric SU(5)xSU(5) framework is revisited. It is shown that the unification of gauge couplings into a unique coupling constant can be achieved at a high-energy scale compatible with proton decay constraints. This requires the addition of a minimal particle content at intermediate energy scales. In particular, the introduction of the SU(2)_L triplets belonging to the (15,1)+(\bar{15},1) representations, as well as of the scalar triplet \Sigma_3 and octet \Sigma_8 in the (24,1) representation, turns out to be crucial for unification. The masses of these intermediate particles can vary over a wide range, and even lie in the TeV region. In contrast, the exotic vector-like fermions must be heavy enough and have masses above 10^10 GeV. We also show that, if the SU(5)xSU(5) theory is embedded into a heterotic string scenario, it is not possible to achieve gauge coupling unification with gravity at the perturbative string scale.

Fermions with cubic and quartic spectrum

We study exotic fermions with spectrum E^2 ~ p^{2N}. Such spectrum emerges in the vicinity of the Fermi point with multiple topological charge N, if special symmetry is obeyed. When this symmetry is violated, the multiple Fermi point typically splits into N elementary Fermi points -- Dirac points with N=1 and spectrum E^2 ~ p^2 .

Non-universal minimal Z′ models: present bounds and early LHC reach

We consider non-universal 'minimal' Z' models, whose additional U(1) charge is a non-anomalous linear combination of the weak hypercharge Y, the baryon number B and the partial lepton numbers (L_e, L_mu, L_tau), with no exotic fermions beyond three standard families with right-handed neutrinos. We show that the observed pattern of neutrino masses and mixing can be fully reproduced by a gauge-invariant renormalizable Lagrangian, and flavor-changing neutral currents in the charged lepton sector are suppressed by a GIM mechanism. We then discuss the phenomenology of some benchmark models. The electrophilic B-3L_e model is significantly constrained by electroweak precision tests, but still allows to fit the hint of an excess observed by CDF in dielectrons but not in dimuons. The muonphilic B-3L_mu model is very mildly constrained by electroweak precision tests, so that even the very early phase of the LHC can explore significant areas of parameter space. We also discuss the hadrophobic L_mu-L_tau model, which has recently attracted interest in connection with some puzzling features of cosmic ray spectra.

Minimal matter at the large hadron collider

We classify all possible new U ( 1 ) Y ⊗ SU ( 2 ) L ⊗ SU ( 3 ) c multiplets that can couple to pairs of SM particles. Assuming that production of such new particles is dominated by their gauge interactions we study their signals at LHC, finding the following five main classes: (i) lepto-quark 2 ℓ 2 q signals; (ii) di-lepton 4 ℓ signals; (iii) di-quarks 4 j signals, (iv) heavy-lepton 2 ℓ 2 V signals and (v) heavy quarks 2 j 2 V signals, where V denotes heavy SM vectors (with W being associated to exotic fermions). In each case we outline the most promising final states, the SM backgrounds and propose the needed searches.

Two Higgs doublet type III seesaw with μ-τ symmetry at LHC

We propose a two Higgs doublet Type III seesaw model with μ-τ flavor symmetry. We add an additional SU(2) Higgs doublet and three SU(2) fermion triplets in our model. The presence of two Higgs doublets allows for an explanation of small neutrino masses with triplet fermions in the 100 GeV mass range, without reducing the Yukawa couplings to extremely small values. The triplet fermions couple to the gauge bosons and can be thus produced at the LHC. We study in detail the effective cross-sections for the production and subsequent decays of these heavy exotic fermions. We show for the first time that the μ-τ flavor symmetry in the low energy neutrino mass matrix results in mixing matrices for the neutral and charged heavy fermions that are not unity and which carry the flavor symmetry pattern. This flavor structure can be observed in the decays of the heavy fermions at LHC. The large Yukawa couplings in our model result in the decay of the heavy fermions into lighter leptons and Higgs with a decay rate which is about 1011 times larger than what is expected for the one Higgs Type III seesaw model with 100 GeV triplet fermions. The smallness of neutrino masses constrains the neutral Higgs mixing angle sin α in our model in such a way that the heavy fermions decay into the lighter neutral CP even Higgs h0, CP odd Higgs A0 and the charged Higgs H±, but almost never to the heavier neutral CP even Higgs H0. The small value for sin α also results in a very long lifetime for h0. This displaced decay vertex should be visible at LHC. We provide an exhaustive list of collider signature channels for our model and identify those that have very large effective cross-sections at LHC and almost no standard model background.