Singlet Leptons Research Articles

Type- 3/2 seesaw mechanism

The type-I seesaw mechanism provides a natural explanation for tiny neutrino masses. The right-handed neutrino masses it requires are, however, too large to keep the Higgs boson mass at its measured value. We show that vector spinors, singlet leptons that are like right-handed neutrinos, generate tiny neutrino masses naturally through the exchange of spin-$1/2$ and spin-$3/2$ components. This one-step seesaw mechanism, which we call the type-$3/2$ seesaw, keeps the Higgs boson mass unchanged at one loop and gives cause therefore to no fine-tuning problem. If the on-shell vector spinor is a pure spin-$3/2$ particle, then it becomes a potential candidate for hidden dark matter which gets diluted due only to the expansion of the Universe. The type-$3/2$ seesaw provides a natural framework for the neutrino, Higgs boson, and dark matter sectors, with overall agreement with current experiments and observations.

Naturalness and the muon magnetic moment

We study a predictive model for explaining the apparent deviation of the muon anomalous magnetic moment from the Standard Model expectation. There are no new scalars and hence no new hierarchy puzzles beyond those associated with the Higgs; the only new particles at the TeV scale are vector-like singlet and doublet leptons. Interestingly, this simple model provides a calculable example violating the Wilsonian notion of naturalness: despite the absence of any symmetries prohibiting its generation, the coefficient of the naively leading dimension-six operator for (g − 2) vanishes at one-loop. While effective field theorists interpret this either as a surprising UV cancellation of power divergences, or as a delicate cancellation between matching UV and calculable IR corrections to (g − 2) from parametrically separated scales, there is a simple explanation in the full theory: the loop integrand is a total derivative of a function vanishing in both the deep UV and IR. The leading contribution to (g − 2) arises from dimension-eight operators, and thus the required masses of new fermions are lower than naively expected, with a sizeable portion of parameter space already covered by direct searches at the LHC. The viable parameter space free of fine-tuning for the muon mass will be fully covered by future direct LHC searches, and all of the parameter space can be probed by precision measurements at planned future lepton colliders.

Minimal flavor violation in the see-saw portal

We consider an extension of the Standard Model with two singlet leptons, with masses in the electroweak range, that induce neutrino masses via the see-saw mechanism, plus a generic new physics sector at a higher scale, Λ. We apply the minimal flavor violation (MFV) principle to the corresponding Effective Field Theory (νSMEFT) valid at energy scales E ≪ Λ. We identify the irreducible sources of lepton flavor and lepton number violation at the renormalizable level, and apply the MFV ansätz to derive the scaling of the Wilson coefficients of the νSMEFT operators up to dimension six. We highlight the most important phenomenological consequences of this hypothesis in the rates for exotic Higgs decays, the decay length of the heavy neutrinos, and their production modes at present and future colliders. We also comment on possible astrophysical implications.

Minimal vectorlike leptonic dark matter and signatures at the LHC

We propose a minimal vector-like leptonic dark matter (DM) with renormalisable interaction in a beyond the Standard Model (SM) scenario, where the SM is augmented with a vector-like doublet and a singlet leptons. The additional fermions are odd under a discrete $Z_2$ symmetry, while the rest of the SM particles are singlets, and thus providing stability to the DM. In this scenario, we show that, the DM emerges as an admixture of the neutral component of the vector-like doublet and the singlet leptons. The singlet-doublet mixing ($\sin \theta$) plays a crucial role in yielding the correct relic density as well as in obtaining null direct DM search results through an interplay of interactions via $Z$ and Higgs mediation. The mixing is also strongly constrained from the invisible Z and Higgs decay width. We found that the correct relic abundance of DM can be obtained in a large region of parameter space for DM-mass larger than $ M_Z/2$ and $\sin \theta \le 0.1$. The details of model phenomenology with collider signatures at the Large hadron Collider (LHC) are discussed. In particular, we show that for $\sin \theta \le 0.01$, the charged companion of the DM can give rise to an observable displaced vertex signature, marking a significant departure from other fermionic DM scenarios, while keeping the relic abundance intact.

Leptogenesis in SO(10)

We consider $SO(10)$ Grand Unified Theories (GUTs) with vacuum expectation values (vevs) for fermion masses in the $\mathbf{10} + \mathbf{\overline{126}}$ representation. We show that the baryon asymmetry generated via leptogenesis is completely determined in terms of measured low energy observables and of one single high energy parameter related to the ratio of the $\mathbf{10}$ and $\mathbf{\overline{126}}$ $SU(2)$ doublet vevs. We identify new decay channels for the heavy Majorana neutrinos into $SU(2)$ singlet leptons $e^c$ which can sizeably affect the size of the resulting baryon asymmetry. We describe how to equip $SO(10)$ fits to low energy data with the additional constraint of successful leptogenesis, and we apply this procedure to the fits carried out in ref.~\cite{Dueck:2013gca}. We show that a baryon asymmetry in perfect agreement with observations is obtained.

Leptogluons in Dilepton Production at the LHC

In the composite models with colored substructure of the fermions the color singlet leptons are accompanied by a composite color octet partners, which are known as leptogluons. We consider the effect of leptogluons in the dilepton production at the LHC and show that in the reachable parameter range this effect is typically dominated by t-channel leptogluon exchange (indirect channel). We show that this channel alone can give a sizable contribution to the dimuon production at the LHC for TeV scale values of the invariant mass of the muon-antimuon pairs.

Multi-component dark matter with magnetic moments for Fermi-LAT gamma-ray line

Abstract We propose a model of multi-component dark matter with magnetic moments to explain the 130 GeV gamma-ray line hinted by the Fermi-LAT data. Specifically, we consider a U (1) X dark sector which contains two vector-like fermions besides the related gauge and Higgs fields. A very heavy messenger scalar is further introduced to construct the Yukawa couplings of the dark fermions to the heavy [ SU (2)]-singlet leptons in the SU (3) c × SU (2) L × SU (2) R × U (1) B−L left–right symmetric models for universal seesaw. A heavier dark fermion with a very long lifetime can mostly decay into a lighter dark fermion and a photon at one-loop level. The dark fermions can serve as the dark matter particles benefited from their annihilations into the dark gauge and Higgs fields. In the presence of a U (1) kinetic mixing, the dark matter fermions can be verified by the ongoing and forthcoming dark matter direct detection experiments.

Dirac gauginos and unification in F-theory

Supersymmetric models in which the gauginos acquire Dirac masses, rather than Majorana masses, offer an appealing alternative to the minimal supersymmetric standard model, especially in the light of the bounds set on superpartner masses by the 2011 LHC data. Dirac gauginos require the presence of chiral multiplets in the adjoint representation of the gauge group, and the realisation of such scenarios in F-theory is the subject of this paper. The chiral adjoints drastically alter the usual picture of gauge coupling unification, but this is disturbed anyway in F-theory models with non-trivial hypercharge flux. The interplay between these two factors is explored, and it is found for example that viable F-theory unification can be achieved at around the reduced Planck scale, if there is an extra vector-like pair of singlet leptons with TeV-scale mass. I then discuss the conditions which must be satisfied by the geometry and hypercharge flux of an F-theory model with Dirac gauginos. One nice possibility is for the visible sector to be localised on a K3 surface, and this is discussed in some detail. Finally, I describe how to achieve an unbroken discrete R-symmetry in such compactifications, which is an important ingredient in many models with Dirac gauginos, and write down a simple example which has adjoint chiral multiplets, an appropriate R-symmetry, and allows for viable breaking of SU(5) by hypercharge flux.

White dwarf axions, PAMELA data, and flipped-SU(5)

Recently, there are two hints arising from physics beyond the standard model. One is a possible energy loss mechanism due to emission of very weakly interacting light particles from white dwarf stars, with a coupling strength ∼ 0.7 × 10 − 13 , and another is the high energy positrons observed by the PAMELA satellite experiment. We construct a supersymmetric flipped-SU(5) model, SU ( 5 ) × U ( 1 ) X with appropriate additional symmetries, [ U ( 1 ) H ] gauge × [ U ( 1 ) R × U ( 1 ) Γ ] global × Z 2 , such that these are explained by a very light electrophilic axion of mass 0.5 meV from the spontaneously broken U ( 1 ) Γ and two component cold dark matters from Z 2 parity. We show that in the flipped-SU(5) there exists a basic mechanism for allowing excess positrons through the charged SU(5) singlet leptons, but not allowing antiproton excess due to the absence of the SU(5) singlet quarks. We show the discovery potential of the charged SU(5) singlet E at the LHC experiments by observing the electron and positron spectrum. With these symmetries, we also comment on the mass hierarchy between the top and bottom quarks.

The νMSM, leptonic asymmetries, and properties of singlet fermions

We study in detail the mechanism of baryon and lepton asymmetry generation in the framework of the $\nu$MSM (an extension of the Standard Model by three singlet fermions with masses smaller than the electroweak scale). We elucidate the issue of CP-violation in the model and define the phase relevant for baryogenesis. We clarify the question of quantum-mechanical coherence, essential for the lepton asymmetry generation in singlet fermion oscillations and compute the relevant damping rates. The range of masses and couplings of singlet leptons which can lead to successful baryogenesis is determined. The conditions which ensure survival of primordial (existing above the electroweak temperatures) asymmetries in different leptonic numbers are analysed. We address the question whether CP-violating reactions with lepton number non-conservation can produce leptonic asymmetry {\em below} the sphaleron freeze-out temperature. This asymmetry, if created, leads to resonant production of dark matter sterile neutrinos. We show that the requirement that a significant lepton asymmetry be produced puts stringent constraints on the properties of a pair of nearly degenerate singlet fermions, which can be tested in accelerator experiments. In this region of parameters the $\nu$MSM provides a common mechanism for production of baryonic matter and dark matter in the universe. We analyse different fine-tunings of the model and discuss possible symmetries of the $\nu$MSM Lagrangian that can lead to them.

SpontaneousR-parity violation: Lightest neutralino decays and neutrino mixing angles at future colliders

We study the decays of the lightest supersymmetric particle (LSP) in models with spontaneously broken R-parity. We focus on the two cases that the LSP is either a bino or a neutral singlet lepton. We work out the most important phenomenological differences between these two scenarios and discuss also how they might be distinguished from explicit R-parity breaking models. In both cases we find that certain ratios of decay branching ratios are correlated with either the solar or the atmospheric (and reactor) neutrino angle. The hypothesis that spontaneous R-parity violation is the source of the observed neutrino masses is therefore potentially testable at the LHC.

How to find neutral leptons of the νMSM?

An extension of the Standard Model by three singlet fermions with masses smaller than the electroweak scale allows to explain simultaneously neutrino oscillations, dark matter and baryon asymmetry of the Universe. We discuss the properties of neutral leptons in this model and the ways they can be searched for in particle physics experiments. We establish, in particular, a lower and an upper bound on the strength of interaction of neutral leptons coming from cosmological considerations and from the data on neutrino oscillations. We analyse the production of neutral leptons in the decays of different mesons and in pp collisions. We study in detail decays of neutral leptons and establish a lower bound on their mass coming from existing experimental data and Big Bang Nucleosynthesis. We argue that the search for a specific missing energy signal in kaon decays would allow to strengthen considerably the bounds on neutral fermion couplings and to find or definitely exclude them below the kaon threshold. To enter into cosmologically interesting parameter range for masses above kaon mass the dedicated searches similar to CERN PS191 experiment would be needed with the use of intensive proton beams. We argue that the use of CNGS, NuMI, T2K or NuTeV beams could allow to search for singlet leptons below charm in a large portion of the parameter space of the νMSM. The search of singlet fermions in the mass interval 2-5 GeV would require a considerable increase of the intensity of proton accelerators or the detailed analysis of kinematics of more than 1010 B-meson decays. © SISSA 2007.

Higgs boson production from a singlet charged lepton in electron-positron collisions

A vector-like charged singlet lepton in the mass range m H+ m τ < m L< m W is a good source of the standard model Higgs boson, via its decay mode L→ τH. In the e +e − collisions at LEP II, Higgs boson production from this source is about an order of magnitude larger than that from the standard Bjorken process. This is also a very clean signal for the singlet lepton, as the associated backgrounds are several orders of magnitude smaller. We also discuss how the singlet signals can be distinguished from those of a sequential doublet lepton.

Singlet fermions and possible new signals for the intermediate Higgs boson

If SU (2) singlet fermions exist, we get new channels for the decays of the intermediate Higgs boson, namely H→tc̄, H→τL (L being a singlet lepton). For a large region of the parameter space, these modes dominate over the usual standard model decay modes. The associated signals give a much better chance of discovering an intermediate mass Higgs boson at the Superconducting Supercollider, compared to what is expected in the standard model.

Asymmetric left-right model for generating radiative quark and lepton masses.

The conventional SU(2){sub {ital L}}{times}SU(2){sub {ital R}}{times}U(1) electroweak gauge model is extended to include two heavy singlet quarks and one heavy singlet lepton. The scalar sector consists of two (2,1,1/2) doublets, two (1,2,1/2) doublets, one (2,1,3/2) doublet, one (1,2,3/2) doublet, as well as one (1,3,1) triplet. Realistic radiative masses for all light quarks and leptons are obtained. A {ital Z}{sub 2} discrete symmetry serves to distinguish the electron (or muon) and its neutrino from the other leptons.

Radiative quark and lepton masses in a left-right gauge model.

The conventional left-right gauge model based on the group SU(3){times}SU(2){sub {ital L}}{times}SU(2){sub {ital R}}{times}U(1) is extended to include singlet quarks and leptons but the usual Higgs-boson doublet-doublet is replaced by two doublet-singlets and two singlet-doublets. As a result, realistic radiative masses for light quarks and leptons can be obtained. Neutrino masses are further suppressed by the seesaw mechanism. There is no need for any assumed discrete symmetry.

CP violation in the lepton sector with small neutrino masses

Small, even vanishing neutrino masses are consistent with large CP violation effects in the leptonic sector. As an example, and a proof of existence of this situation, we present a model based on the SU(2) L × SU(2) R × U(1) gauge symmetry, where the fermion content has been Extended to include a gauge singlet lepton S L. Interesting features of this model (which should be seen as a mere example) are: (1) neutrino masses can either vanish or be in the range where the Mikheyev-Smirnov-Wolfenstein (MSW) mechanism for the solar neutrino puzzle is operative, and (2) CP violation effects in the lepton sector are manifestly large and therefore accessible to current experiments. In particular, it is found that the electron electric dipole moment, d rme , can be of order 10 −24 e·cm; if generation mixing is negligible, the μ longitudinal polarization, P rmL , in the decay K L → μ μ can be in the range 10 −10−10 −3.

High energy behavior of the electromagnetic singlet current in the Glashow-Weinberg-Salam model

The electromagnetic singlet lepton form factors are shown to exhibit “anomalous” short distance behavior which spoils perturbative renormalizability of the physical electromagnetic current. Implications for formulations of gauge theories in terms of gauge invariant quantities are pointed out.