Heavy Charged Leptons Research Articles

Fatjet signatures of heavy neutrinos and heavy leptons in a left-right model with universal seesaw at the HL-LHC

We perform a collider search for fatjet signals originating from boosted heavy neutral and charged leptons with masses between a few hundred GeV to a TeV. These heavy leptons originate from the decay of heavy gauge bosons with masses above 4 TeV in a left-right symmetric extension of the Standard Model (SM), which considers a universal seesaw mechanism for the generation of all the SM fermion masses. The fatjet signals arise naturally in this model due to the presence of heavy seesaw partners of the SM fermions which decay to SM gauge bosons carrying large boosts. We employ substructure based variables lepton sub-jet fraction (LSF) and lepton mass drop (LMD) together with kinematic variables of fatjets to look for fatjet signals associated with non-isolated leptons. These variables help in reducing the SM backgrounds while retaining enough statistics for signal events, which leads to a robust discovery potential at the high-luminosity Large Hadron Collider (HL-LHC).

Charged Higgs induced 5 and 6 lepton signatures from heavy neutrinos at the LHC

We propose an anomaly free gauged U(1) extension of the SM where three right handed heavy neutrinos, being charged under the general U(1) gauge group, are introduced to explain the origin of the tiny neutrino mass through the seesaw mechanism after the general U(1) symmetry is broken. Due to the breaking of the general U(1) symmetry a neutral beyond the standard model gauge boson Z^prime acquires mass. There are two Higgs doublets in this model where one interacts with the SM fermions and the other one interacts with the right handed heavy neutrinos and charged leptons. The charged multiplet of the second Higgs can completely decay into the heavy neutrinos and charged lepton in the neutrinophilic limit of the model parameters. The charged Higgs pair production can be influenced due to presence of the Z^prime boson at the High Luminosity LHC (HL-LHC) in addition to the neutral SM gauge bosons. The pair produced charged Higgs bosons decay into SM charged leptons and heavy neutrinos. Following the leading decay modes of the heavy neutrinos into charged leptons and W boson we study the 5 and 6 lepton final states after the leptonic and hadronic decay of the W bosons considering solely muons and electrons in the final state. Combining the electron and muon final states we estimate the significance of the 5 and 6 charged lepton processes in the m_{H^pm }-m_N plane for different benchmark points of m_{Z^prime }. It is found that the 5 (6) charged lepton processes could be probed at the High Luminosity LHC (HL-LHC) with at least 5 (3) sigma significance and there are parameter regions where the significance could be larger.

Monopoles, exotic states and muon g-2 in TeV scale trinification

We study the low energy implications of a trinification model based on the gauge symmetry G= SU(3)_c times SU(3)_L times SU(3)_R, without imposing gauge coupling unification. A minimal model requires two Higgs multiplets that reside in the bi-fundamental representation of G, and this is shown to be adequate for accommodating the Standard Model (SM) fermion masses and generate, via loop corrections and seesaw mechanism, suitable masses for the heavy neutral leptons as well as the observed SM neutrinos. We estimate a lower bound of around 15 TeV for the masses of the new down- type quarks that are required by the SU(3)_L times SU(3)_R symmetry. We examine the resonant production at the LHC of the new gauge bosons, which leads to a lower bound of 16 TeV for the symmetry breaking scale of G. We also show how the muon g-2 anomaly can be resolved in the presence of these new gauge bosons and the heavy charged leptons present in the model. Finally, the model predicts the presence of a topologically stable monopole carrying three quanta (6 pi /e) of Dirac magnetic charge and mass gtrsim 160 TeV. If new matter fields lying in the fundamental representations of G are included, the model predicts the presence of exotic leptons, mesons and baryons carrying fractional electric charges such as pm e/3 and pm 2e/3, fully compatible with the Dirac quantization condition.

Leptonic CP violation from a vector-like lepton

Leptonic CP violation is yet to be confirmed as an additional source of CP violation in fundamental interactions. We study the case where leptonic CP violation is spontaneous and is induced by the mixing with a heavy charged vector-like lepton (VLL). We show that the non-decoupling of this VLL is linked with the presence of CP violation and its coupling with the SM leptons are partly fixed from the SM Yukawas. Due to the large leptonic mixing angles, these couplings are typically of the same order and there is no flavor preference. Strong but not definitive constraints come from charged lepton flavor violating processes because the VLL can decouple from one or two leptonic flavors in very special points of parameter space. These special points are very sensitive to the neutrino Majorana phases.

From Peccei Quinn symmetry to mass hierarchy problem

We propose a non-universal U(1) X gauge extension to the standard model (SM) and an additional Peccei–Quinn (PQ) global symmetry to study the mass hierarchy and strong CP problem. The scheme allows us to distinguish among fermion families and to generate the fermionic mass spectrum of particles of the SM. The symmetry breaking is performed by two scalar Higgs doublets and two scalar Higgs singlets, where one of these has the axion which turns out to be a candidate for cold dark matter. The exotic sector is composed by one up-like T and two down-like J 1,2 heavy quarks, two heavy charged leptons , one additional right-handed neutrino per family , and an invisible axion a. In addition, the large energy scale associated to the breaking of the PQ-symmetry gives masses to the right-handed neutrinos in such a way that the active neutrinos acquire eV-mass values due to the see-saw mechanism. On the other hand, from the non-linear effective Lagrangian, the flavour changing of the down quarks and charged leptons with the axion are considered.

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.

Parametrized leptogenesis from linear seesaw

We present a purely linear seesaw mechanism in a mirror left-right symmetric framework and then realize a novel leptogenesis scenario for parametrizing the cosmic baryon asymmetry by the charged lepton masses, the light Majorana neutrino mass matrix and the heavy mirror charged lepton masses. Through the same Yukawa couplings, the lepton-number-conserving decays of the mirror charged leptons can generate three individual lepton asymmetries stored in the ordinary lepton flavors, while the lepton-number-violating processes for the Majorana neutrino mass generation can wash out part of these lepton asymmetries. The remnant lepton asymmetries then can be partially converted to a baryon asymmetry by the sphaleron processes.

Search for Type-III SeeSaw heavy lepton with the ATLAS Detector at the LHC using TeV up to 140 fb–1

A search of heavy leptons N0 and L± pair production predicted by the Type-III SeeSaw mechanism is presented. This mechanism extends the Standard Model, introducing at least two new triplets of fermionic fields with zero hypercharge in the adjoint representation of SU(2)l, resulting in two heavy Dirac charged leptons and a heavy Majorana neutral lepton. This search uses data collected by the ATLAS detector at the Large Hadron Collider in protonproton collision at a centre-of-mass energy of 13 TeV, with an integrated luminosity up to 140 fb−1 corresponding to the full Run-2 dataset recorded between 2015-2018.The analysis focuses on all the possible production and boson decay channels of these heavy leptons, which are assumed to be degenerate in mass. The search is based on the separate optimization of each lepton multiplicity final state, considering 2, 3, 4 or more than 5 leptons. The power of the leptonic channels lies in the low expected background from Standard Model processes. Different control and signal regions are defined for each final state, designed to be orthogonal one each other. Good agreement between the number of events in data and Standard Model predictions is observed.The results are translated into exclusion limits on heavy lepton masses with a 95% confidence level using statistical uncertainties only. Assuming the branching ratios to all lepton flavours to be equal, a lower limit of 560 GeV is obtained.

A simple model to explain observed muon sector anomalies and small neutrino masses

Since its inception, no decisive departure from the predictions of the standard model (SM) has been reported. However, recently various experiments have observed a few hints of possible departure from SM predictions in lepton flavor universality observables such as , , muon (g–2), , etc. Many of these are observable where deviation from the SM in the range of (2–4)σ is observed related to the muon (μ) lepton. So these deviations may hint at a possible new physics (NP) in the muon sector. In this work we extend the SM by introducing two SM singlet heavy charged leptons (Fe, Fμ), whose left-handed components are charged under a new U(1)F gauge symmetry, one color triplet lepto-quark (ϕQ) doublet under SU(2)L, one inert Higgs doublet (ϕl), and three very heavy Majorana neutrinos (NiR), all of which are odd under a Z2 discrete symmetry. One more scalar (ϕ) charged only under the U(1)F, whose VEV gives masses to the U(1)F gauge boson as well as the heavy leptons. With these new particles, we show that the observed anomalies in the muon sector as well as small neutrino masses can be explained taking into account all the other experimental and theoretical constraints to date.

Four-body decays of B meson with lepton number violation

The existence of heavy meson lepton number violating (LNV) processes shows the Majorana nature of the neutrino. Much of this theoretical and experimental researche focuses on this type of decay. Four-body epton-number violation (LNV) processes of the B meson may have sizable branching ratios as they share the same vertex and mixing parameters with the three-body case. Mixing parameters between the heavy Majorana neutrino and charged leptons extracted from the three-body case can be used to constrain the branching ratios of four-body decays of the B meson. So we can update the upper limits of these mixing parameters with new experimental data of the three-body LNV decays. We also analyze using the updated parameters and estimate some channels’ reconstruction events using current experimental data from Belle.

LHC signals for singlet neutrinos from a natural warped seesaw mechanism. I

Recently, it was shown in arXiv:1512.06742 that a straightforward implementation of the type I seesaw mechanism in a warped extra dimensional framework is in reality a {\em natural} realization of "inverse" seesaw, i.e., the Standard Model (SM) neutrino mass is dominantly generated by exchange of pseudo-Dirac {\em TeV}-mass SM singlet neutrinos. By the AdS/CFT correspondence, this scenario is {\em dual} to these singlet particles being composites of some new strong dynamics, along with the SM Higgs boson, with the rest of the SM particles being mostly elementary. We study signals from production of these heavy neutrinos at the Large Hadron Collider (LHC). We focus on the scenario where the strong sector has a global $SU(2)_{\rm L} \times SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry; such a left-right (LR) structure being motivated by consistency with the electroweak (EW) precision tests. The singlet neutrinos are charged under $SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry, thus can be produced from $W^{ \pm }_R$ exchange, as in four-dimensional (4D) LR symmetric models. However, the direct coupling of light quarks to $W^{ \pm }_R$ is negligible, due to $W^{ \pm }_R$ also being composite; nonetheless, a sizable coupling can be induced by mixings among the various types of $W^{ \pm }$ bosons. Furthermore, $W^{ \pm }_R$ decays dominantly into the singlet and {\em composite} partner of charged lepton. This heavy charged lepton, in turn, decays into SM lepton, {\em plus} $Z$/Higgs, thus the latter can be used for extra identification of the signal. For a benchmark scenario with $W^{ \pm }_R$ of mass 2 TeV and singlet neutrino of mass 750 GeV, we find that, in both the di-lepton + di-jet + Higgs and tri-lepton + Higgs channels, significant evidence can be seen at the LHC14 for an integrated luminosity of 300/fb and that even discovery is possible with slightly more luminosity.

Vectorlike leptons: Muon g−2 anomaly, lepton flavor violation, Higgs boson decays, and lepton nonuniversality

In this paper, we consider the Standard Model (SM) with one family of vector-like (VL) leptons, which couple to all three families of the SM leptons. We study the constraints on this model coming from the heavy charged lepton mass bound, electroweak precision data, the muon anomalous magnetic moment, lepton flavor violation, Higgs decay constraints and a recently measured lepton non-universality observable, $R_{K^{*0}}$. We find that the strongest constraints are coming from the muon $g-2, \ R_{\mu\mu}=\Gamma(h\to\mu\mu)/\Gamma(h\to\mu\mu)_\text{SM}, \ R_{\gamma\gamma}$ and $\text{BR}(\mu\to e\gamma)$. Although VL leptons couple to all three families of the SM leptons, the ratio of electron-VL to muon-VL coupling is constrained to be $\langle\lambda_e/\lambda_\mu\rangle\lesssim10^{-4}$. We also find that our model cannot fit the observed value of $R_{K^{*0}}$.

Lepton portal limit of inert Higgs doublet dark matter with radiative neutrino mass

We study an extension of the Inert Higgs Doublet Model (IHDM) by three copies of right handed neutrinos and heavy charged leptons such that both the inert Higgs doublet and the heavy fermions are odd under the Z2 symmetry of the model. The neutrino masses are generated at one loop in the scotogenic fashion. Assuming the neutral scalar of the inert Higgs to be the dark matter candidate, we particularly look into the region of parameter space where dark matter relic abundance is primarily governed by the inert Higgs coupling with the leptons. This corresponds to tiny Higgs portal coupling of dark matter as well as large mass splitting within different components of the inert Higgs doublet suppressing the coannihilations. Such lepton portal couplings can still produce the correct relic abundance even if the Higgs portal couplings are arbitrarily small. Such tiny Higgs portal couplings may be responsible for suppressed dark matter nucleon cross section as well as tiny invisible branching ratio of the standard model Higgs, to be probed at ongoing and future experiments. We also briefly discuss the collider implications of such a scenario.

Electron and muon anomalous magnetic dipole moment in a 3\u20133\u20131 model

We calculate, in the context of a 3–3–1 model with heavy charged leptons, constraints on some parameters of the extra particles in the model by imposing that their contributions to both the electron and muon (g − 2) factors are in agreement with experimental data up to 1σ-3σ. In order to obtain realistic results we use some of the possible solutions of the left- and right-unitary matrices that diagonalize the lepton mass matrices, giving the observed lepton masses and at the same time allowing to accommodate the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix. We show that, at least up to 1-loop order, in the particular range of the space parameter that we have explored, it is not possible to fit the observed electron and muon (g − 2) factors at the same time unless one of the extra leptons has a mass of the order of 20–40 GeVs and the energy scale of the 331 symmetry to be of around 60–80 TeVs.

Threshold enhancement of diphoton resonances

The data collected by the LHC collaborations at an energy of 13 TeV indicates the presence of an excess in the diphoton spectrum that would correspond to a resonance of a 750 GeV mass. The apparently large production cross section is nevertheless very difficult to explain in minimal models. We consider the possibility that the resonance is a pseudoscalar boson $A$ with a two--photon decay mediated by a charged and uncolored fermion having a mass at the $\frac12 M_A$ threshold and a very small decay width, $\ll 1$ MeV; one can then generate a large enhancement of the $A\gamma\gamma$ amplitude which explains the excess without invoking a large multiplicity of particles propagating in the loop, large electric charges and/or very strong Yukawa couplings. The implications of such a threshold enhancement are discussed in two explicit scenarios: i) the Minimal Supersymmetric Standard Model in which the $A$ state is produced via the top quark mediated gluon fusion process and decays into photons predominantly through loops of charginos with masses close to $\frac12 M_A$ and ii) a two Higgs doublet model in which $A$ is again produced by gluon fusion but decays into photons through loops of vector--like charged heavy leptons. We also comment on a minimal scenario in which the $A$ state couples only to photons through a heavy lepton loop and is both produced and decays through this coupling. In all these scenarios, while the mass of the charged fermion has to be adjusted to be extremely close to half of the $A$ resonance mass, the small total widths are naturally obtained if only suppressed three-body decay channels occur. Finally, the implications of some of these scenarios for dark matter are discussed.

Searches for New Physics with Lepton Flavours and Multi-lepton Final States in ATLAS

Lepton flavours and multi-lepton signatures are powerful probes to search for new physics. Also, the lepton sector of the Standard Model has to be extended to describe the observed neutrino oscillations. ATLAS experiment searches for lepton flavour violation, new gauge boson allowing to unify quark and lepton fields, heavy charged and neutral leptons as well as for new phenomena in multiphoton final states are summarized in this document. The most recent results will be discussed and compared with existing results.

Exotic leptons: Collider and muon magnetic moment constraints

In light of the ongoing effort on reducing the theoretical uncertainties and an upcoming experiment concerning muon magnetic moment, we perform a detailed study of an 3-4-1 electroweak gauge extension of the standard model that contains exotic charged leptons in its spectrum. We discuss flavor changing neutral current, collider and electroweak bounds on the model and derive [Formula: see text] limits using current and projected limits on the muon magnetic moment. In summary, we exclude the masses of new gauge bosons that couple to muons and heavy charged leptons up to 700 GeV. Moreover, we find a projected lower bound on the scale of symmetry breaking to be 2 TeV.

Electron and neutron electric dipole moment in the 3-3-1 model with heavy leptons

We calculate the electric dipole moment for the electron and neutron in the framework of the 3-3-1 model with heavy charged leptons. We assume that the only source of $CP$ violation arises from a complex trilinear coupling constant and the three complex vacuum expectation values. However, two of the vacua phases are absorbed and the other two are equal up to a minus sign. Hence only one physical phase survives. In order to be compatible with the experimental data this phase has to be smaller than $10^{-6}$.

Neutrinos with a linear seesaw mechanism in a scenario of gaugedB−Lsymmetry

We consider a mechanism for neutrino mass generation, based on a local B-L extension of the standard model, which becomes a linear seesaw regime for light neutrinos after spontaneous symmetry breaking. The spectrum of extra particles includes heavy neutrinos with masses near the TeV scale and a heavy Z' boson, as well as three extra neutral scalars and a charged scalar pair. We study the production and decays of these heavy particles at the LHC. Z' will decay mainly into heavy neutrino pairs or charged lepton pairs, similar to other low scale seesaw scenarios with local B-L, while the phenomenology of the extra scalars is what distinguishes the linear seesaw from the previous models. One of the neutral scalars is produced by Z' Z' fusion and decays mainly into vector boson pairs, the other two neutral scalars are less visible as they decay only into heavy or light neutrino pairs, and finally the charged scalars will decay mainly into charged leptons and missing energy.

Doubly charged heavy leptons at LHC via contact interactions

We study the production of doubly charged excited leptons at the LHC. These exotic states are predicted in extended weak-isospin composite models. A recent analysis of such exotic states was based on a pure gauge model with magnetic type interactions. We include here the mechanism of contact interactions and show that this turns out to dominate the single production of the doubly charged leptons. We perform a feasibility analysis of the observation of the tri-lepton signature associated with the production of the exotic doubly charged lepton simulating the response of a generic detector. We give exclusion plots in the parameter space, within statistical uncertainties, at different luminosities.