Vector-like Leptons Research Articles

Electron and muon g − 2, radiative neutrino mass, and ℓ′ → ℓγ in a U(1)e−μ model

A nonconventional U(1)e−μ gauge model is proposed to explain the observed neutrino masses and the unexpected anomalous magnetic moments of the electron and muon (lepton g−2), where for suppressing the neutrino coupling to Z′ gauge boson, only the right-handed electron and muon in the standard model carry the U(1)e−μ charge. Although the light lepton masses are suppressed when the gauge symmetry is spontaneously broken, they can be generated through the Yukawa couplings to newly introduced particles, such as vector-like lepton doublets and singlets, and scalar singlets. It is found that the same Yukawa couplings combined with the new scalar couplings to the Higgs can induce the radiative lepton-flavor violation processes ℓ′→ℓγ and lepton g−2, where the lepton g−2 is proportional to mℓ. When Majorana fermions and a scalar singlet are further added into the model, the active neutrinos can obtain masses via the radiative seesaw mechanism. When the bounds from the me and mμ and the neutrino data are satisfied, we find that the electron g−2 can reach an order of −10−12, and the muon g−2 can be an order of 10−9. In addition, when the μ→eγ decay is suppressed, the resulting branching ratio for τ→eγ can be of O(10−8), and that for τ→μγ can be as large as the current upper limit.

Probing the Lμ−Lτ gauge boson at electron colliders

We investigate the minimal $U(1)_{L_\mu-L_\tau}$ model with extra heavy vector-like leptons or charged scalars. By studying the kinetic mixing between $U(1)_{L_\mu-L_\tau}$ gauge boson $Z^\prime$ and standard model photon, which is absent at tree level and will arise at one loop level due to $\mu$, $\tau$ and new heavy charged leptons or scalars, the interesting behavior is shown. It can provide possibility for visible signatures of new heavy particles. We propose to search for $Z^\prime$ at electron collider experiments, such as Belle II, BESIII and future Super Tau Charm Factory (STCF), using the monophoton final state. The parameter space of $Z^\prime$ is probed, and scanned by its gauge coupling constant $g_{Z^\prime}$ and mass $m_{Z^\prime}$. We find that electron colliders have sensitivity to the previously unexplored parameter space for $Z^\prime$ with MeV-GeV mass. Future STCF experiments with $\sqrt s=2-7$ GeV can exclude the anomalous muon magnetic moment favored area when $m_{Z^\prime}<5$ GeV with the luminosity of 30 ab$^{-1}$. For $m_{Z^\prime} < 2m_\mu$, $g_{Z^\prime}$ can be down to $4.2\times 10^{-5}$ at 2 GeV STCF.

Phenomenology of vector-like leptons with Deep Learning at the Large Hadron Collider

In this paper, a model inspired by Grand Unification principles featuring three generations of vector-like fermions, new Higgs doublets and a rich neutrino sector at the low scale is presented. Using the state-of-the-art Deep Learning techniques we perform the first phenomenological analysis of this model focusing on the study of new charged vector-like leptons (VLLs) and their possible signatures at CERN’s Large Hadron Collider (LHC). In our numerical analysis we consider signal events for vector-boson fusion and VLL pair production topologies, both involving a final state containing a pair of charged leptons of different flavor and two sterile neutrinos that provide a missing energy. We also consider the case of VLL single production where, in addition to a pair of sterile neutrinos, the final state contains only one charged lepton. We propose a novel method to identify missing transverse energy vectors by comparing the detector response with Monte-Carlo simulated data. All calculated observables are provided as data sets for Deep Learning analysis, where a neural network is constructed, based on results obtained via an evolutive algorithm, whose objective is to maximise either the accuracy metric or the Asimov significance for different masses of the VLL. Taking into account the effect of the three analysed topologies, we have found that the combined significance for the observation of new VLLs at the high-luminosity LHC can range from 5.7σ, for a mass of 1.25 TeV, all the way up to 28σ if the VLL mass is 200 GeV. We have also shown that by the end of the LHC Run-III a 200 GeV VLL can be excluded with a confidence of 8.8 standard deviations. The results obtained show that our model can be probed well before the end of the LHC operations and, in particular, providing important phenomenological information to constrain the energy scale at which new gauge symmetries emergent from the considered Grand Unification picture can be manifest.

Muon anomalous magnetic moment in two-Higgs-doublet models with vectorlike leptons

We show that inclusion of a single generation of vector-like leptons in the Two-Higgs Doublet Models significantly enlarges the allowed parameter space consistent with the muon anomalous magnetic moment, as well as with other theoretical and experimental constraints. While previously $(g -2)_\mu$ could only be resolved in Type-X scenario by requiring a light pseudoscalar Higgs boson and large $\tan \beta$, contributions of vector-like leptons via two-loop Barr Zee diagrams broaden the allowed parameter space, allowing $\tan \beta$ as low as 10 and pseudoscalar masses as large as $\cal{O}$(1 TeV) while fulfilling the stringent constraints from precision and flavor observable. Similar results are obtained for Type-II scenarios, but there the parameter space is more restricted by flavor observables.

Global electroweak fit and vector-like leptons in light of the Cabibbo angle anomaly

The “Cabibbo Angle Anomaly” (CAA) originates from the disagreement between the CKM elements Vud and Vus extracted from superallowed beta and kaon decays, respectively, once compared via CKM unitarity. It points towards new physics with a significance of up to 4 σ, depending on the theoretical input used, and can be explained through modified W couplings to leptons. In this context, vector-like leptons (VLLs) are prime candidates for a corresponding UV completion since they can affect Wℓν couplings at tree-level, such that this modification can have the dominant phenomenological impact. In order to consistently assess agreement data, a global fit is necessary which we perform for gauge-invariant dimension-6 operators and all patterns obtained for the six possible representations (under the SM gauge group) of VLLs. We find that even in the lepton flavour universal case, including the measurements of the CKM elements Vus and Vud into the electroweak fit has a relevant impact, shifting the best fit point significantly. Concerning the VLLs we discuss the bounds from charged lepton flavour violating processes and observe that a single representation cannot describe experimental data significantly better than the SM hypothesis. However, allowing for several representations of VLLs at the same time, we find that the simple scenario in which N couples to electrons via the Higgs and Σ1 couples to muons not only explains the CAA but also improves the rest of the electroweak fit in such a way that its best fit point is preferred by more than 4 σ with respect to the begin.

Model building from asymptotic safety with Higgs and flavor portals

We perform a comprehensive search for Standard Model extensions inspired by asymptotic safety. Our models feature a singlet matrix scalar field, three generations of vector-like leptons, and direct links to the Higgs and flavor sectors via new Yukawa and portal couplings. A novel feature is that the enlarged scalar sector may spontaneously break lepton flavor universality. We provide a complete two-loop renormalization group analysis of the running gauge, Yukawa, and quartic couplings to find ultraviolet fixed points and the BSM critical surface of parameters, $i.e.$ the set of boundary conditions at the TeV scale for which models remain well-behaved and predictive up to the Planck scale without encountering Landau poles or instabilities. This includes templates for asymptotically safe Standard Model extensions which match the measured values of gauge couplings and the Higgs, top, and bottom masses. We further detail the phenomenology of our models covering production, decay, fermion mixing, anomalous magnetic moments, effects from scalar mixing and chiral enhancement, and constraints on model parameters from data. Signatures at proton-proton and lepton colliders such as lepton flavor violation and displaced vertices, and the prospect for electric dipole moments or charged lepton-flavor-violating type processes, are also indicated.

Explaining g \u2212 2 anomalies in two Higgs doublet model with vector-like leptons

We consider the two Higgs doublet model (2HDM) along with a generation of vector-like lepton doublet and singlet to explain the observed discrepancies in the electron and muon anomalous magnetic moments. The type-X (lepton-specific) 2HDM can allow a light pseudo-scalar which is known to explain the muon anomalous magnetic moment at two-loop. Such a light particle induces a sizable negative contribution to the electron anomalous magnetic moment at one-loop in the presence of vector-like leptons evading all the experimental constraints.

Simple model for large CP violation in charm decays, B-physics anomalies, muon g \u2212 2 and dark matter

We present a minimal extension of the Standard Model that can simultaneously account for the anomalies in semi-leptonic B meson decays and the muon g − 2, give large CP violation in charm decays (up to the value recently measured by LHCb), and provide thermal-relic dark matter, while evading all constraints set by other flavour observables, LHC searches, and dark matter experiments. This is achieved by introducing only four new fields: a vectorlike quark, a vectorlike lepton, and two scalar fields (a singlet and a doublet) that mix due to the electroweak symmetry breaking and provide the dark matter candidate. The singlet-doublet mixing induces chirally-enhanced dipole transitions, which are crucial for the explanation of the muon g − 2 discrepancy and the large charm CP violation, and allows to achieve the observed dark matter density in wide regions of the parameter space.

Effective field theory for vector-like leptons and its collider signals

We argue that in models with several high scales; e.g. in composite Higgs models or in gauge extensions of the Standard Model (SM), vector-like leptons can be likely produced in a relatively large s region of the phase space. Likewise, they can easily decay into final states not containing SM gauge bosons. This contrasts with the topology in which these new particles are being searched for at the LHC. Adopting an effective field theory approach, we show that searches for excited leptons must be used instead to test this scenario. We derive bounds on all the relevant interactions of dimension six; the most constrained ones being of about 0.05 TeV−2. We build new observables to improve current analyses and study the impact on all single-field UV completions of the SM extended with a vector-like lepton that can be captured by the effective field theory at tree level, in the current and in the high-luminosity phase of the LHC.

An explanation for the muon and electron g \u2212 2 anomalies and dark matter

We propose simple models with a flavor-dependent global U(1)ℓ and a discrete ℤ2 symmetries to explain the anomalies in the measured anomalous magnetic dipole moments of muon and electron, (g − 2)μ,e, while simultaneously accommodating a dark matter candidate. These new symmetries are introduced not only to avoid the dangerous lepton flavor-violating decays of charged leptons, but also to ensure the stability of the dark matter. Our models can realize the opposite-sign contributions to the muon and electron g − 2 via one-loop diagrams involving new vector-like leptons. Under the vacuum stability and perturbative unitarity bounds as well as the constraints from the dark matter direct searches and related LHC data, we find suitable parameter space to simultaneously explain (g − 2)μ,e and the relic density. In this parameter space, the coupling of the Higgs boson with muons can be enhanced by up to ∼ 38% from its Standard Model value, which can be tested in future collider experiments.

Vectorlike leptons and inert scalar triplet: Lepton flavor violation, g−2 , and collider searches

We investigate simplified models involving an inert scalar triplet and vector-like leptons that can account for the muon $g-2$ anomaly. These simplified scenarios are embedded in a model that features W' and Z' bosons, which are subject to stringent collider bounds. The constraints coming from the muon $g-2$ anomaly are put into perspective with collider bounds, as well as bounds coming from lepton flavor violation searches. The region of parameter space that explains the $g-2$ anomaly is shown to be within reach of lepton flavor violation probes and future colliders such as HL-LHC and HE-LHC.

On a Standard Model Extension with Vector-like Fermions and Abelian Symmetry

We investigate an extension of the standard model with vector-like fermions and an extra Abelian gauge symmetry. The particle mass spectrum is calculated explicitly. The Lagrangian terms for all the gauge interactions of leptons and quarks in the model are derived. We observe that while there is no new mixing in the lepton sector, the quark mixing plays an important role in the magnitudes of gauge interactions for quarks, particularly the interactions with massive \(W\), \(Z\) and \(Z'\) bosons. We calculate the contributions of the new vector-like leptons and quarks to the Peskin-Takeuchi parameters as well as the \(\rho\) parameter of the electroweak precision tests, and show that the model is realistic.

Generalised \u03bc-\u03c4 symmetries and calculable gauge kinetic and mass mixing in mathrm{U}{(1)}_{L_{mu }-{L}_{tau }} models

Extensions of the standard model with a U(1) gauge symmetry contain gauge invariant kinetic mixing, sin χ, and gauge non-invariant mass mixing, δM2, between the hypercharge and the new gauge boson Z′. These represent a priori incalculable but phe- nomenologically important parameters of the theory. They become calculable if there exist spontaneously or softly broken symmetries which forbid them at tree level but allow their generation at the loop level. We discuss various symmetries falling in this category in the context of the gauged Lμ − Lτ models and their interplay with lepton mixing. It is shown that one gets phenomenologically inconsistent lepton mixing parameters if these symmetries are exact. Spontaneous breaking of these symmetries can lead to consistent lepton mixing and also generates finite and calculable values of these parameters at one or two loop order depending on the underlying symmetry. We calculate these parameters in two specific cases: (i) the standard seesaw model with μ-τ symmetry broken by the masses of the right-handed neutrinos and (ii) in a model containing a pair of vectorlike charged leptons which break μ-τ symmetry. In case (i), the right-handed neutrinos are the only source of gauge mixing. The kinetic mixing parameters are suppressed and vanish if the right-handed neutrinos decouple from the theory. In contrast, there exists a finite gauge mixing in case (ii) which survives even when the masses of vectorlike leptons are taken to infinity, exhibiting non-decoupling behaviour. The seesaw model discussed here represents a complete framework with practically no kinetic mixing and hence can survive a large number of experimental probes used to rule out specific ranges in the coupling g′ and mass {M}_{Z^{prime }} . The model can generate non-universality in tau decays, which can be tested in future experiments.

Complete vectorlike fourth family with U(1)′ : A global analysis

In this paper we present an in-depth analysis of a recently proposed Standard Model extension with a complete fourth generation of quarks and leptons, which are vector-like with respect to the Standard Model gauge group and charged under a new spontaneously broken vector-like $\mathrm{U(1)^\prime}$ gauge symmetry. The model is designed to explain the known muon anomalies, i.e. the observed deviations from Standard Model predictions in the anomalous magnetic moment of the muon, $\Delta a_\mu$, and in $b \rightarrow s \ell^+ \ell^-$ processes. We perform a global $\chi^2$ analysis of the data with $65$ model parameters and including $98$ observables. We find many points with $\chi^2$ per degree of freedom $\leq 1$. The vector-like leptons and the new heavy $Z^\prime$ are typically much lighter than a TeV and would, thus, be eminently visible at the HL-LHC.

Spontaneous twin symmetry breaking

We consider a twin Higgs scenario where the $Z_2$ twin symmetry is broken spontaneously, not explicitly. This scenario provides an interesting interpretation of the Higgs metastability in the standard model; the $SU(4)$ breaking scale $f$ is determined by the scale where the Higgs self quartic coupling flips its sign. However, for the misalignment of nonzero vacuum expectation values of the twin Higgs fields, parameter tuning of ${\cal O}(m_h^2/f^2)$ is required like explicit twin symmetry breaking scenarios. For the minimal model with the exact twin symmetry, $f$ is ${\cal O}$($10^{10}$ GeV), so the model is very unnatural. We point out that the tuning can be significantly reduced ($f\gtrsim 2.7$ TeV) if there are twin vector-like leptons with large Yukawa coupling to twin Higgs fields.

A nonunitary interpretation for a single vector leptoquark combined explanation to the B-decay anomalies

In order to simultaneously account for both RD(∗) and RK (∗) anomalies in B-decays, we consider an extension of the Standard Model by a single vector leptoquark field, and study how one can achieve the required lepton flavour non-universality, starting from a priori universal gauge couplings. While the unitary quark-lepton mixing induced by SU(2)L breaking is insufficient, we find that effectively nonunitary mixings hold the key to simultaneously address the RK (∗) and RD(∗) anomalies. As an intermediate step towards various UV-complete models, we show that the mixings of charged leptons with additional vector-like heavy leptons successfully provide a nonunitary framework to explain RK (∗) and RD(∗) . These realisations have a strong impact for electroweak precision observables and for flavour violating ones: isosinglet heavy lepton realisations are already excluded due to excessive contributions to lepton flavour violating Z -decays. Furthermore, in the near future, the expected progress in the sensitivity of charged lepton flavour violation experiments should allow to fully probe this class of vector leptoquark models.

Radiatively scotogenic type-II seesaw and a relevant phenomenological analysis

When a small vacuum expectation value of Higgs triplet (\U0001d710∆) in the type-II seesaw model is required to explain neutrino oscillation data, a fine-tuning issue occurs on the mass-dimension lepton-number-violation (LNV) scalar coupling. Using the scotogenic approach, we investigate how a small LNV term is arisen through a radiative correction when an Z2-odd vector-like lepton (X) and an Z2-odd right-handed Majorana lepton (N) are introduced to the type-II seesaw model. Due to the dark matter (DM) direct detection constraints, the available DM candidate is the right-handed Majorana particle, whose mass depends on and is close to the mX parameter. Combing the constraints from the DM measurements, the h → γγ decay, and the oblique T -parameter, it is found that the preferred range of v∆ is approximately in the region of 10−5−10−4 GeV; the mass difference between the doubly and the singly charged Higgs is less than 50 GeV, and the influence on the h → Z γ decay is not significant. Using the constrained parameters, we analyze the decays of each Higgs triplet scalar in detail, including the possible three-body decays when the kinematic condition is allowed. It is found that with the exception of doubly charged Higgs, scalar mixing effects play an important role in the Higgs triplet two-body decays when the scalar masses are near-degenerate. In the non-degenerate mass region, the branching ratios of the Higgs triplet decays are dominated by the three-body decays.

Complete vectorlike fourth family and new U(1)′ for muon anomalies

We consider the Standard Model (SM) with the addition of a $\mathrm{U(1)^\prime}$ gauge symmetry and a complete fourth family of quarks and leptons which are vector-like with respect to the full $\mathrm{SU(3)_C}\times \mathrm{SU(2)_L} \times \mathrm{U(1)_Y}\times \mathrm{U(1)^\prime}$ gauge symmetry. The model provides a unified explanation of experimental anomalies in $(g - 2)_\mu$ as well as $b \rightarrow s \ell^+ \ell^-$ decays. We find good fits to the deviations from the SM, while at the same time fitting all other SM observables. The model includes a new $Z^\prime$ gauge boson, a $\mathrm{U(1)^\prime}$-breaking scalar, and vector-like leptons all with mass of order a few $100$ GeV. It is consistent with all currently released high energy experimental data, however, it appears imminently testable with well designed future searches. Also precision flavor experiments, especially more accurate direct determinations of CKM matrix elements, would allow to probe the best fit points.

Search for vectorlike leptons in multilepton final states in proton-proton collisions at s=13 TeV

A search for vectorlike leptons in multilepton final states is presented. The data sample corresponds to an integrated luminosity of 77.4 fb−1 of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC in 2016 and 2017. Events are categorized by the multiplicity of electrons, muons, and hadronically decaying τ leptons. The missing transverse momentum and the scalar sum of the lepton transverse momenta are used to distinguish the signal from background. The observed results are consistent with the expectations from the standard model hypothesis. The existence of a vectorlike lepton doublet, coupling to the third-generation standard model leptons in the mass range of 120–790 GeV, is excluded at 95% confidence level. These are the most stringent limits yet on the production of a vectorlike lepton doublet, coupling to the third-generation standard model leptons.Received 26 May 2019DOI:https://doi.org/10.1103/PhysRevD.100.052003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.© 2019 CERN, for the CMS CollaborationPhysics Subject Headings (PhySH)Physical SystemsHypothetical fermionsTechniquesHadron collidersParticles & Fields

Electroweak baryogenesis with vector-like leptons and scalar singlets

We investigate the viability of electroweak baryogenesis in a model with a first order electroweak phase transition induced by the addition of two gauge singlet scalars. A vector-like lepton doublet is introduced in order to provide CP violating interactions with the singlets and Standard Model leptons, and the asymmetry generation dynamics are examined using the vacuum expectation value insertion approximation. We find that such a model is readily capable of generating sufficient baryon asymmetry while satisfying electron electric dipole moment and collider phenomenology constraints.