Lepton Triplet Research Articles

Interplay of inert doublet and vector-like lepton triplet with displaced vertices at the LHC/FCC and MATHUSLA

We study the interaction between the inert Higgs doublet (IDM) dark matter and a vector-like SU(2) triplet lepton (VLL), both of which are Z2-odd. The vector current of the VLL with the Z-boson rules out a fermionic or two-component dark matter scenario. However, a compressed mass spectrum and a sufficiently small Yukawa coupling allows co-annihilation and late decay of the VLL into the IDM sector, affecting the relic density of the pseudoscalar dark matter. The same two factors enable displaced decay of the VLL states, providing novel signatures involving hadronically quiet displaced multi-lepton final states. Such signatures to probe the model are studied at the 14 and 27 TeV LHC, as well as the 100 TeV FCC-hh. In addition to being detectable at the CMS/ATLAS experiments, if the new particles have sub-100 GeV masses, signals can also be seen at the proposed MATHUSLA detector.

Muon (g − 2) in U(1) L μ − L τ scotogenic model extended with vector like fermion

The latest results of anomalous muon magnetic moment at Fermilab show a discrepancy of 4.2 σ between the Standard Model (SM) prediction and experimental value. In this work, we revisit symmetry with in the paradigm of scotogenic model which explains muon (g − 2) and neutrino mass generation, simultaneously. The mass of new gauge boson generated after the spontaneous symmetry breaking of is constrained, solely, in light of the current neutrino oscillation data to explain muon (g − 2). In particular, we have obtained two regions I and II, around 150 MeV and 500 MeV, respectively, in plane which explain the neutrino phenomenology. Region I is found to be consistent with muon neutrino trident (MNT) bound (g μ τ ≤ 10−3) to explain muon (g − 2), however, region II violates it for mass range . We, then, extend the minimal gauged scotogenic model by a vector like lepton (VLL) triplet ψ T . The mixing of ψ T with inert scalar doublet η leads to chirally enhanced positive contribution to muon anomalous magnetic moment independent of Z μ τ mass. Furthermore, we have, also, investigated the implication of the model for 0ν β β decay and CP violation. The non-observation of 0ν β β decay down to the sensitivity of 0.01 eV shall refute the model. The model, in general, is found to be consistent with both CP conserving and CP violating solutions.

Probing heavy triplet leptons of the type-III seesaw mechanism at future muon colliders

We study the search potential of heavy triplet leptons in a type-III seesaw mechanism at future high-energy and high-luminosity muon colliders. The impact of up-to-date neutrino oscillation results is taken into account on neutrino parameters and the consequent decay patterns of heavy leptons in a type-III seesaw. The three heavy leptons in the Casas-Ibarra parametrization with diagonal unity matrix are distinguishable in their decay modes. We consider the pair production of charged triplet leptons ${E}^{+}{E}^{\ensuremath{-}}$ through both ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ annihilation and vector boson fusion (VBF) processes. The leading-order framework of electroweak parton distribution functions is utilized to calculate the VBF cross sections. The charged triplet leptons can be probed for masses above $\ensuremath{\sim}1\text{ }\text{ }\mathrm{TeV}$. The ${E}^{+}{E}^{\ensuremath{-}}\ensuremath{\rightarrow}ZZ{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ channel can be further utilized to fully reconstruct the three triplet leptons and distinguish neutrino mass patterns. The pair production of heavy neutrinos $N$ and the associated production ${E}^{\ifmmode\pm\else\textpm\fi{}}N$ are only induced by VBF processes and lead to a lepton-number-violating (LNV) signature. We also study the search potential of LNV processes at future high-energy muon colliders with $\sqrt{s}=30\text{ }\text{ }\mathrm{TeV}$.

CP -violating dark photon kinetic mixing and type-III seesaw model

The hypothetical dark photon portal connecting the visible and dark sectors of the Universe has received considerable attention in recent years, with a focus on CP-conserving kinetic mixing between the Standard Model (SM) hypercharge gauge boson and a new U(1)$_X$ gauge boson. In the effective field theory context, one may write down non-renormalizable CP-violating kinetic mixing interactions involving the $X$ and SU(2)$_L$ gauge bosons. We construct for the first time a renormalizable model for CP-violating kinetic mixing that induces CP-violating non-Abelian kinetic mixing at mass dimension five. The model grows out of the type-III seesaw model, with the lepton triplets containing right-handed neutrinos playing a crucial role in making the model renormalizable and providing a bridge to the origin of neutrino mass. This scenario also accommodates electron electric dipole moments (EDM) as large as current experimental bound, making future EDM searches an important probe of this scenario.

Excited lepton triplet contribution to electroweak observables at one loop level

In this paper, we present the one-loop radiative corrections to the electroweak precision observable Delta rho coming from the I_W=1 multiplet excited leptons. We have calculated the couplings of the exotic lepton triplet to the vector bosons and ordinary leptons using the effective Lagrangian approach. These couplings are then used to estimate the excited lepton triplet contribution to the Delta rho parameter. The mass degenerate excited lepton contribution to Delta rho is small and can be neglected. However, if the excited leptons are non-degenerate, their contribution can be large which can result in more stringent constraints on the excited fermion parameter space compared to the constraints from present experimental searches and perturbative unitarity condition.

Hierarchical quark and lepton masses with sequential U(1) gauge symmetry

Instead of right-handed neutrino singlets, the standard model is extended to include lepton triplets (Σ+,Σ0,Σ−). Each quark and lepton family may now transform under an anomaly-free U(1)X gauge symmetry, known already for many years. A new sequential application is presented, using just the one Higgs doublet of the standard model, together with two U(1)X Higgs singlets. The resulting structure has hierarchical quark and lepton masses, as well as a viable seesaw neutrino mass matrix.

Heavy stable charged tracks as signatures of non-thermal dark matter at the LHC: a study in some non-supersymmetric scenarios

We consider two theoretical scenarios, each including a ℤ2-odd sector and leading to an elementary dark matter candidate. The first one is a variant of the Type-III seesaw model where one lepton triplet is ℤ2-odd, together with a heavy sterile neutrino. It leads to a fermionic dark matter, together with the charged component of the triplet being a quasi-stable particle which decays only via a higher-dimensional operator suppressed by a high scale. The second model consists of an inert scalar doublet together with a ℤ2-odd right-handed Majorana neutrino dark matter. A tiny Yukawa coupling delays the decay of the charged component of the inert doublet into the dark matter candidate, making the former long-lived on the scale of collider detectors. The parameter space of each model has been constrained by big-bang nucleosynthesis constraints, and also by estimating the contribution to the relic density through freeze-out of the long-lived charged particle as well the freeze-in production of the dark matter candidate. We consider two kinds of signals at the Large Hadron Collider for each case. For the first kind of models, namely two charged tracks and single track and for the second kind, the characteristic signals are opposite as well as same-sign charged track pairs. We perform a detailed analysis using event selection criteria consistent with the current experimental programmes. It is found that the scenario with a lepton triplet can be probed upto 960 (1190) GeV with an integrated luminosity of 300 (3000) fb−1, while the corresponding numbers for the inert doublet scenario are 630 (800) GeV. Furthermore, the second kind of signal mentioned in each case allows us to differentiate different dark matter scenarios from each other.

Inert Higgs Doublet Dark Matter in Type-II Seesaw

Weakly interacting massive particle (WIMP) as a dark matter (DM) candidate is further inspired by recent AMS-02 data, which confirm the excess of positron fraction observed earlier by PAMELA and Fermi-LAT experiments. Additionally, the excess of positron+electron flux is still significant in the measurement of Fermi-LAT. For solving the problem of massive neutrinos and observed excess of cosmic-ray by DM annihilation, we study the model with an inert Higgs doublet (IHD) in the framework of type-II seesaw mechanism by imposing a Z2 symmetry on the IHD, where the lightest particle of IHD is the DM candidate while the neutrino masses origin from the Higgs triplet in type-II seesaw model. We calculate the cosmic-ray production in our model and find that if leptonic triplet decays are dominant, the observed excess of positron/electron flux could be explained well in normal ordered neutrino mass spectrum, when the constraints of DM relic density and comic-ray antiproton spectrum are taken into account.

QCD corrections to pair production of Type III Seesaw leptons at hadron colliders

If kinematically accessible, hadron collider experiments provide an ideal laboratory for the direct production of heavy lepton partners in Seesaw models. In the context of the Type III Seesaw Mechanism, the $\mathcal{O}(\alpha_s)$ rate and shape corrections are presented for the pair production of hypothetical, heavy $SU(2)_L$ triplet leptons in $pp$ collisions at $\sqrt{s}=$ 13, 14, and 100 TeV. The next-to-leading order (NLO) $K$-factors span, approximately, $K^{NLO}=1.1 - 1.4$ for both charged current and neutral current processes over a triplet mass range $m_T = 100~\text{GeV}-2~\text{TeV}$. Total production cross sections exhibit a $^{+5\%}_{-6\%}$ scale dependence at 14 TeV and $\pm1\%$ at 100 TeV. The NLO differential $K$-factors for heavy lepton kinematics are largely flat, suggesting that na\"ive scaling by the total $K^{NLO}$ is reasonably justified. The resummed transverse momentum distribution of the dilepton system is presented at leading logarithmic (LL) accuracy. The effects of resummation are large in TeV-scale dilepton systems. Discovery potential to heavy lepton pairs at 14 and 100 TeV is briefly explored: At the High-Luminosity LHC, we estimate a $4.8-6.3\sigma$ discovery potential maximally for $m_T = 1.5-1.6~\text{TeV}$ after 3000 fb$^{-1}$. With 300 (3000) fb$^{-1}$, there is $2\sigma$ sensitivity up to $m_T = 1.3-1.4~\text{TeV}~(1.7-1.8~\text{TeV})$ in the individual channels. At 100 TeV and with 10 fb$^{-1}$, a $5\sigma$ discovery can be achieved for $m_T=1.4-1.6~\text{TeV}$. Due to the factorization properties of Drell-Yan-type systems, the fixed order and resummed calculations reduce to convolutions over tree-level quantities.

Dark matter candidate in an extended type III seesaw scenario

The type III seesaw mechanism for neutrino mass generation usually makes use of at least two $Y = 0$, $SU(2)_L$ lepton triplets. We augment such a model with a third triplet and a sterile neutrino, both of which are odd under a conserved $\Z_2$ symmetry. With all new physics confined to the $\Z_2$-odd sector, whose low energy manifestation is in some higher-dimensional operators, a fermionic dark matter candidate is found to emerge. We identify the region of the parameter space of the scenario, which is consistent with all constraints from relic density and direct searches, and allows a wide range of masses for the dark matter candidate.

Triplet with a doubly-charged lepton at the LHC

We studied the signatures of an electroweak SU(2) lepton triplet which contains a doubly charged heavy lepton. The most interesting and easily detected final states are originating from the doubly charged lepton pair and single production, that we analyzed at a center of mass energy of 14 TeV. We propose useful kinematic cuts to reduce the backgrounds and computed the integrated luminosity needed to discover or exclude the doubly charged leptons for various masses.

Flavored searches for type-III seesaw mechanism at the LHC

We present a reinterpretation of CMS searches for type-III seesaw lepton triplets at the LHC, both for the normal and the inverse seesaw. We find that, in contrast with previous expectations, these searches in the trilepton final state have a good sensitivity to triplets with predominant coupling to the tau lepton. We also show that the limits resulting from direct searches can be neatly presented for arbitrary masses and general flavour mixings. Thus, it turns out that the common (and often unrealistic) simplifying assumptions about the flavour couplings of the triplets used by experimental collaborations to present their results are unnecessary and should be dropped.

Inverse seesaw and dark matter in models with exotic lepton triplets

We show that models with exotic leptons transforming as E∼(1,3,−1) under the standard model gauge symmetry are well suited for generating neutrino mass via a radiative inverse seesaw. This approach realizes natural neutrino masses and allows multiple new states to appear at the TeV scale. The exotic leptons are therefore good candidates for new physics that can be probed at the LHC. Furthermore, remnant low-energy symmetries ensure a stable dark matter candidate, providing a link between dark matter and the origin of neutrino mass.

Phenomenology of a lepton triplet

The most general phenomenological model involving a lepton triplet with hypercharge $\ifmmode\pm\else\textpm\fi{}1$ is constructed. A distinctive feature of this model is the prediction of a doubly charged lepton and a new heavy Dirac neutrino. We study the phenomenology of these exotic leptons both in low-energy experiments and at the LHC. The model predicts flavor-changing neutral-current processes such as muon and tau rare decays, which are studied in detail in order to constrain the model parameters. All the decay channels of the exotic leptons are described for a wide range of parameters. It is found that, if the mixing parameters between the exotic and light leptons are not too small ($>{10}^{\ensuremath{-}6}$), then they can be observable to a $3--5\ensuremath{\sigma}$ statistical significance at the 7 TeV LHC with $10--50\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ luminosity for a 400 GeV mass and 14 TeV with $100--300\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ luminosity for a 800 GeV mass.

Higgs quadruplet for the type III seesaw model and implications forμ→eγandμ−econversion

In Type III seesaw model the heavy neutrinos are contained in leptonic triplet representations. The Yukawa couplings of the triplet fermion and the left-handed neutrinos with the doublet Higgs field produce the Dirac mass terms. Together with the Majorana masses for the leptonic triplets, the light neutrinos obtain non-zero seesaw masses. We point out that it is also possible to have a quadruplet Higgs field to produce the Dirac mass terms to facilitate the seesaw mechanism. The vacuum expectation value of the quadruplet Higgs is constrained to be small by electroweak precision data. Therefore the Yukawa couplings of a quadruplet can be much larger than those for a doublet. We also find that unlike the usual Type III seesaw model where at least two copies of leptonic triplets are needed, with both doublet and quadruplet Higgs representations, just one leptonic triplet is possible to have a phenomenologically acceptable model because light neutrino masses can receive sizable contributions at both tree and one loop levels. Large Yukawa couplings of the quadruplet can induce observable effects for lepton flavor violating processes $\mu \to e \gamma$ and $\mu - e$ conversion. Implications of the recent $\mu \to e\gamma$ limit from MEG and also limit on $\mu - e $ conversion on Au are also given. Some interesting collider signatures for the doubly charged Higgs boson in the quadruplet are discussed.

Phenomenological constraints on minimally coupled exotic lepton triplets

By introducing a set of new triplet leptons (with nonzero hypercharge) that can Yukawa couple to their standard model counterparts, new sources of tree-level flavor changing currents are induced via mixing. In this work, we study some of the consequences of such new contributions on processes such as the leptonic decays of gauge bosons, $\ensuremath{\ell}\ensuremath{\rightarrow}3{\ensuremath{\ell}}^{\ensuremath{'}}$ and $\ensuremath{\ell}\ensuremath{\rightarrow}{\ensuremath{\ell}}^{\ensuremath{'}}\ensuremath{\gamma}$, which violate lepton flavor, and $\ensuremath{\mu}\ensuremath{-}e$ conversion in atomic nuclei. Constraints are then placed on the parameters associated with the exotic triplets by invoking the current low-energy experimental data. Moreover, the new physics contribution to the lepton anomalous magnetic moments is calculated.

LEPTOGENESIS AND LHC PHYSICS WITH TYPE III SEE-SAW

The See-Saw mechanism provides a nice way to explain why neutrino masses are so much lighter than their charged lepton partners. It also provides a nice way to explain baryon asymmetry in our universe via the leptogenesis mechanism. In this talk we review leptogenesis and LHC physics in a See-Saw model proposed in 1989, now termed the Type III See-Saw model. In this model, SU(2)L triplet leptons are introduced with the neutral particles of the triplets playing the role of See-Saw. The triplet leptons have charged partners with standard model gauge interactions resulting in many new features. The gauge interactions of these particles make it easier for leptognesis with low masses, as low as a TeV is possible. The gauge interactions also make the production and detection of triplet leptons at LHC possible. The See-Saw mechanism and leptogenesis due to Type III See-Saw may be tested at LHC.

Collider signatures for the heavy lepton triplet in the typeI+IIIseesaw mechanism

The minimal SU(5) theory augmented by the fermionic adjoint representation restores the coupling constant unification and gives realistic neutrino masses and mixing through the hybrid Type I and Type III seesaw. The crucial prediction of the theory is an SU(2) lepton triplet with the mass below TeV. We study the signature of these heavy leptons and propose the strategy to test this mechanism at the hadron and lepton colliders. The smoking gun evidence of the theory is Delta L=2 lepton number violation through events of a pair of like-sign leptons plus four jets without significant missing energy at hadron colliders. We find that via this unique channel, the heavy lepton can be searched for up to a mass of 200 GeV at the Tevatron with 8 fb^-1, and up to 450 (700) GeV at the LHC of 14 TeV C.M.energy with 10 (100) fb^-1. The signal rate at the 10 TeV LHC is reduced to 60-35% for a mass of 200-700 GeV. We also comment on how to distinguish this theory from other models with similar heavy leptons. Finally, we compare the production rates and angular distributions of heavy leptons in e+e- collisions for various models.

Heavy lepton pair production at LHC: Model discrimination with multi-lepton signals

If new leptons exist close to the electroweak scale, they can be produced in pairs at LHC through standard or new interactions. We study the production of heavy lepton pairs in SM extensions with: (i) a Majorana or Dirac lepton triplet, as those appearing in type-III seesaw; (ii) a lepton isodoublet ( N E ) L , R ; (iii) a charged isosinglet E L , R ; (iv) a Majorana or Dirac neutrino singlet N and an additional Z ′ gauge boson. It is shown that the trilepton final state ℓ ± ℓ ± ℓ ∓ , which has a small SM background, constitutes the golden channel for heavy neutrino searches, being very sensitive to Majorana or Dirac neutrinos in triplet, doublet or singlet SU ( 2 ) L representations. For higher luminosities, signals in this final state can also distinguish lepton triplets from doublets and singlets. The Majorana or Dirac nature of the heavy neutrinos is revealed by the presence or not of like-sign dilepton ℓ ± ℓ ± signals without significant missing energy. Notably, large ℓ ± ℓ ± signals but with large missing energy are characteristic of Dirac triplets, distinguishing them from the other two models with a heavy Dirac neutrino. Further discrimination is achieved with the analysis of the clean ℓ + ℓ + ℓ − ℓ − final state.

Neutrino masses and heavy triplet leptons at the LHC: Testability of the type III seesaw mechanism

We study LHC signatures of Type III seesaw in which SU(2)_L triplet leptons are introduced to supply the heavy seesaw masses. To detect the signals of these heavy triplet leptons, one needs to understand their decays to standard model particles which depend on how light and heavy leptons mix with each other. We concentrate on the usual solutions with small light and heavy lepton mixing of order the square root of the ratio of light and heavy masses, (m_\nu/M_{\nu_R})^{1/2}. This class of solutions can lead to a visible displaced vertex detectable at the LHC which can be used to distinguish small mixing and large mixing between light and heavy leptons. We show that, in this case, the couplings of light and heavy triplet leptons to gauge and Higgs bosons, which determine the decay widths and branching ratios, can be expressed in terms of light neutrino masses and their mixing. Using these relations, we study heavy triplet lepton decay patterns and production cross section at the LHC. If these heavy triplet leptons are below a TeV or so, they can be easily produced at the LHC due to their gauge interactions from being non-trivial representations of SU(2)_L. We consider two ideal production channels, 1) E^+E^- \to \ell^+\ell^+ \ell^-\ell^- jj (\ell=e,\mu,\tau) and 2) E^\pm N \to \ell^\pm \ell^\pm jjjj in detail. For case 1), we find that with one or two of the light leptons being \tau it can also be effectively studied. With judicious cuts at the LHC, the discovery of the heavy triplet leptons as high as a TeV can be achieved with 100 fb^{-1} integrated luminosity.