Multi-lepton Signals Research Articles

Search for a leptophobic doubly charged Higgs in same-sign four-lepton and six-lepton signatures in a left-right symmetric model

We investigate the possibility of multi-lepton (four and six) signatures, including an exotic signature of same-sign four-lepton (SS4L) as signals of pair production of a doubly charged Higgs in the minimal left-right symmetric model, extended with two doublet scalars. The right-handed neutrino masses are generated in this model through a dimension-5 lepton-number violating operator allowing the triplet scalar interactions with leptons to become negligibly small. This leads to interesting six-lepton and SS4L signatures that can be observed at the high-luminosity phase of the Large Hadron Collider (HL-LHC) with almost no background for doubly charged Higgs with mass below 500 GeV.

Prospects of gluino searches in multi-lepton channels in light of the ongoing LHC RUN-III

This study investigates the prospect of discovering strongly interacting gluinos in different multi-lepton channels with lepton multiplicities greater than or equal to 2 at LHC RUN-III, considering several pMSSM scenarios. The effectiveness of the multivariate analysis (MVA) method with the boosted decision tree (BDT) algorithm is explored to obtain a better significance for different models. Promising results are obtained for the 3-lepton channels, indicating that the use of MVA methods can improve the sensitivity of the search for gluinos at LHC RUN-III. The study probes the multi-lepton signatures arising from gluinos via intermediate eweakinos and sleptons at an early stage of the LHC RUN-III. The heavier eweakinos can give rise to three or four lepton signals, in which the squark hierarchy between the L and R types plays a crucial role. The study considers two sets of benchmark points that satisfy all the collider constraints obtained from the LHC RUN-II data. Moreover, these sets of benchmark points are mostly consistent with WMAP/PLANCK data and the muon (g-2) constraint. The corresponding results from the MVA technique demonstrate that, even for an integrated luminosity of 270 , the 5 σ discovery prospect of for TeV in the wino type model is promising. The study also presents various other models that may emerge at the early stage of LHC RUN-III. Wino type models in the scenario where left squarks are light and right squarks are heavy exhibit the best prospect of discovering gluinos in multi-lepton channels in the LHC RUN-III experiment. The findings of this study provide crucial insights into the potential discovery of gluinos in multi-lepton channels.

Multilepton signatures from dark matter at the LHC

Leptonic signatures of Dark Matter (DM) are one of the cleanest ways to discover such a secluded form of matter at high energy colliders. We explore the full parameter space relevant to multi-lepton (2- and 3-lepton) signatures at the Large Hadron Collider (LHC) from representative minimal consistent models with scalar and fermion DM. In our analysis, we suggest a new parametrisation of the model parameter spaces in terms of the DM mass and mass differences between DM and its multiplet partners. This parametrisation allows us to explore properties of DM models in their whole parameter space. This approach is generic and quite model-independent since the mass differences are related to the couplings of the DM to the Standard Model (SM) sector. We establish the most up-to-date LHC limits on the inert 2-Higgs Doublet Model (i2HDM) and Minimal Fermion DM (MFDM) model parameter spaces, by using the complementary information stemming from 2- and 3-lepton signatures. We provide a map of LHC efficiencies and cross-section limits for such 2- and 3-lepton signatures allowing one to easily make model-independent reinterpretations of LHC results for analogous classes of models. We also present combined constraints from the LHC, DM relic density and direct search experiments indicating the current status of the i2HDM and MFDM model.

Probing an intrinsically flavorful ALP via tau-lepton flavor physics

Any axionlike particle (ALP) intrinsically possesses flavorful couplings to the standard model (SM) fermions arising as a consequence of the right-handed flavor rotation within the SM. In this paper we discuss this intrinsically flavored ALP, and explore the correlation of a minimal set of the couplings in a view of coherence in flavor physics observables. We focus particularly on the tau-lepton flavor violation (LFV). The ALP is assumed to be tau-philic on a current-eigenstate basis, a la Pecci-Quinn. The ALP has the intrinsic flavorful coupling structure for fermions, which allows coupling also to muon and electron only in a right-handed specific manner. Several LFV processes are generated including radiative tau decays and also anomalous magnetic moments of electron and muon. We first pay attention to two separated limits: electron scenario with the ALP coupled to tau which mixes only with right-handed electron, and muon scenario as the muonic counterpart of the electron scenario. It turns out that those scenarios are highly constrained by experimental limits, to require a mu or electron - tau flipped feature in the mass eigenbasis when coupled to the ALP. We then examine a hybrid scenario, and find a fully viable parameter space on the ALP mass-photon coupling plane, which limits the ALP mass to be (1.7 - 10) GeV and the ALP decay constant $f_a$ to be (12.8 - 67.9) GeV. We find that the same-sign multilepton signal at Belle II is a smoking-gun to probe the present ALP signal, and the polarization asymmetry in LFV radiative $\tau$ decay is a punchline, which definitely predicts preference of the right-handed polarization, in sharp contrast to the SM plus massive Dirac neutrinos having the highly left-handed preference, and also other light-new physics candidates. Possible model-building to underlie the present third-generation specific ALP is also briefly addressed.

Modeling uncertainties of tt¯W± multilepton signatures

In light of recent discrepancies between the modeling of $t\bar{t} W^\pm$ signatures and measurements reported by the Large Hadron Collider (LHC) experimental collaborations, we investigate in detail theoretical uncertainties for multi-lepton signatures. We compare results from the state-of-the-art full off-shell calculation and its Narrow Width Approximation to results obtained from the on-shell $t\bar{t} W^\pm$ calculation, with approximate spin-correlations in top-quark and $W$ decays, matched to parton showers. In the former case double-, single-, and non-resonant contributions together with interference effects are taken into account, while the latter two cases are only based on the double resonant top-quark contributions. The comparison is performed for the LHC at $\sqrt{s} = 13$ TeV for which we study separately the multi-lepton signatures as predicted from the dominant NLO contributions at the perturbative orders $\mathcal{O}(\alpha_s^3\alpha^6)$ and $\mathcal{O}(\alpha_s\alpha^8)$. Furthermore, we combine both contributions and propose a simple way to approximately incorporate the full off-shell effects in the NLO computation of on-shell $pp\to t\bar{t} W^\pm$ matched to parton showers.

On improving NLO merging for mathrm{t}overline{mathrm{t}}mathrm{W} production

We introduce an improvement to the FxFx matrix element merging procedure for pp → toverline{t}W production at NLO in QCD with one and/or two additional jets. The main modification is an improved treatment of jets that are not logarithmically enhanced in the low transverse-momentum regime. We provide predictions for the inclusive cross section and the toverline{t}W differential distributions including parton-shower effects. Taking also the NLO EW corrections into account, this results in the most-accurate predictions for this process to date. We further proceed to include the on-shell LO decays of the toverline{t}W including the tree-level spin correlations within the narrow-width approximation, focusing on the multi-lepton signatures studied at the LHC. We find a ∼30% increase over the NLO QCD prediction and large non-flat K-factors to differential distributions.

Dark Matter and Collider Searches in S_3-Symmetric 2HDM with Vector Like Lepton

We study the S_3-symmetric two Higgs doublet model by adding two generations of vector like leptons (VLL) which are odd under a discrete Z_2-symmetry. The lightest neutral component of the VLL acts as a dark matter (DM) whereas the full VLL set belongs to a dark sector with no mixings allowed with the standard model fermions. We analyse the model in light of dark matter and collider searches. We show that the DM is compatible with the current relic density data as well as satisfying all direct and indirect dark matter search constraints. We choose some representative points in the model parameter space allowed by all aforementioned dark matter constraints and present a detailed collider analysis of multi-lepton signals viz. the mono-lepton, di-lepton, tri-lepton and four-lepton along with missing transverse energy in the final state using both the cut-based analysis and multivariate analysis respectively at the high luminosity 14 TeV LHC run.

New flavor physics in di- and trilepton events from single-top production at the LHC and beyond

The associated production of a single-top with opposite-sign same-flavor (OSSF) di-leptons, $pp \to t \ell^+ \ell^-$ and $ pp \to t \ell^+ \ell^- + j$ ($j=$light jet), can lead to striking tri-lepton $pp \to \ell^\prime \ell^+ \ell^- + X$ and di-lepton $pp \to \ell^+ \ell^- + j_b + X$ ($j_b=b$-jet) events at the LHC, after the top decays. Although these rather generic multi-lepton signals are flavor-blind, they can be generated by new 4-Fermi flavor changing (FC) $u_i t \ell \ell$ scalar, vector and tensor interactions ($u_i \in u,c$), which we study in this paper; we match the FC $u_i t \ell \ell$ 4-Fermi terms to the SMEFT operators and also to different types of FC underlying heavy physics. The main backgrounds to these di- and tri-lepton signals arise from $t \bar t$, $Z$+jets and $VV$ ($V=W,Z$) production, but they can be essentially eliminated with a sufficiently high invariant mass selection on the OSSF di-leptons, $m_{\ell^+ \ell^-}^{\tt min}(OSSF) > 1$ TeV; the use of $b$-tagging as an additional selection in the di-lepton final state case also proves very useful. We find, for example, that the expected 95\% CL bounds on the scale of a tensor(vector) $u t \mu \mu$ interaction, with the current $\sim 140$ fb$^{-1}$ of LHC data, are $\Lambda < 5(3.2) $ TeV or $\Lambda < 4.1(2.7)$ TeV, if analyzed via the di-muon $\mu^+ \mu^- + j_b$ signal or the $e \mu^+ \mu^-$ tri-lepton one, respectively. The expected reach at the HL-LHC with 3000 fb$^{-1}$ of data is $\Lambda < 7.1(4.7)$ TeV and $\Lambda < 2.4(1.5)$ TeV for the corresponding $u t \mu \mu$ and $c t \mu \mu$ operators. We also study the potential sensitivity at future 27 TeV and 100 TeV high-energy LHC successors and also discuss the possible implications of this class of FC 4-Fermi effective interactions on lepton non-universality tests at the LHC.

Multi-lepton signatures of vector-like leptons with flavor

We investigate collider signatures of standard model extensions featuring vector-like leptons and a flavorful scalar sector. Such a framework arises naturally within asymptotically safe model building, which tames the UV behavior of the standard model towards the Planck scale and beyond. We focus on values of Yukawa couplings and masses which allow to explain the present data on the muon and electron anomalous magnetic moments. Using a CMS search based on 77.4 , text {fb}^{-1} at the sqrt{s}=13 TeV LHC we find that flavorful vector-like leptons are excluded for masses below around 300 GeV if they are singlets under SU(2)_L, and around 800 GeV if they are doublets. Exploiting the flavor-violating-like decays of the scalars, we design novel null test observables based on opposite sign opposite flavor invariant masses. These multi-lepton distributions allow to signal new physics and to extract mass hierarchies in reach of near-future searches at the LHC and the HL-LHC.

Subleading EW corrections and spin-correlation effects in tbar{t}W multi-lepton signatures

Recently a slight tension between data and predictions has been reported in tbar{t}W production by both the CMS and ATLAS collaborations. We revisit the theoretical predictions for this process, focussing on the following two effects. We disentangle various effects that lead to asymmetries among the leptonic decay products of the (anti-)top quarks and W bosons, for which we find that the spin correlations in the top-quark pair are the dominant source. We also discuss the impact of the large, formally subleading, electroweak corrections to tbar{t}W production at the LHC. We find that this effect changes the tbar{t}W cross section significantly in the signature phase-space regions, and should therefore be included differentially in the theory to data comparisons.

Long-lived charged particles and multilepton signatures from neutrino mass models

Lepton number violation (LNV) is usually searched for by the LHC collaborations using the same-sign dilepton plus jet signature. In this paper, we discuss multilepton signals of LNV that can arise with experimentally interesting rates in certain loop models of neutrino mass generation. Interestingly, in such models, the observed smallness of the active neutrino masses, together with the high multiplicity of the final states, leads in large parts of the viable parameter space of such models to the prediction of long-lived charged particles, which leave highly ionizing tracks in the detectors. We focus on one particular one-loop neutrino mass model in this class and discuss its LHC phenomenology in some detail.

Multilepton dark matter signals

The signatures of dark matter at the LHC commonly involve, in simplified scenarios, the production of a single particle plus large missing energy, from the undetected dark matter. However, in Z′-portal scenarios anomaly cancellation requires the presence of extra dark leptons in the dark sector. We investigate the signatures of the minimal scenarios of this kind, which involve cascade decays of the extra Z′ boson into the dark leptons, identifying a four-lepton signal as the most promising one. We estimate the sensitivity to this signal at the LHC, the high-luminosity LHC upgrade, a possible high-energy upgrade, as well as a future circular collider. For Z′ couplings compatible with current dijet constraints the multilepton signals can reach the 5σ level already at Run 2 of the LHC. At future colliders, couplings two orders of magnitude smaller than the electroweak coupling can be probed with 5σ sensitivity.

Testing the 2HDM explanation of the muon g – 2 anomaly at the LHC

The discrepancy between the measured value and the Standard Model pre- diction for the muon anomalous magnetic moment is one of the important issues in the particle physics. In this paper, we consider a two Higgs doublet model (2HDM) where the extra Higgs doublet couples to muon and tau in lepton flavor violating (LFV) way and the one-loop correction involving the scalars largely contributes to the muon anomalous magnetic moment. The couplings should be sizable to explain the discrepancy, so that the extra Higgs bosons would dominantly decay into μτ LFV modes, which makes the model testable at the LHC through multi-lepton signatures even though they are produced via the electroweak interaction. We discuss the current status and the future prospect for the extra Higgs searches at the LHC, and demonstrate the reconstruction of the mass spectrum using the multi-lepton events.

Scalar phenomenology in type-II seesaw model

In this work we study the viable parameter space of the scalar sector in the type-II seesaw model. In identifying the allowed parameter space, we employ constraints from low energy precision measurements, theoretical considerations and the 125-GeV Higgs data. These tools prove effective in constraining the model parameter space. Moreover, the triplet also offers a rich collider phenomenology from having additional scalars that have unique collider signatures. We find that direct collider searches for these scalars can further probe various parts of the viable parameter space. These parts can be parametrized by the electroweak scalar triplet vacuum expectation value, the mass splitting of the singly- and doubly-charged scalars, and the doubly-charged Higgs mass. We find that different regions of the viable parameter space give rise to different collider signatures, such as the same-sign dilepton, the same-sign W and the multilepton signatures. By investigating various LEP and LHC measurements, we derive the most updated constraints over the whole range of parameter space of the type-II seesaw model.

Prospects for vectorlike leptons at future proton-proton colliders

Vectorlike leptons are an intriguing possibility for physics beyond the Standard Model. We study the reach for discovering (at 5$\sigma$ significance) or excluding (at 95 per cent confidence) models of charged vectorlike leptons that mix predominantly with the tau, using multi-lepton signatures at various future proton-proton collider options: a high-luminosity LHC with $\sqrt{s} =$ 14 TeV, a high-energy LHC with $\sqrt{s} =$ 27 TeV, and possible new longer-tunnel colliders with $\sqrt{s} =$ 70 or 100 TeV. For weak isodoublet vectorlike leptons, we estimate that a 27 TeV high-energy LHC with 15 ab$^{-1}$ could exclude masses up to about 2300 GeV, or discover them if the mass is less than about 1700 GeV, while a 100 TeV collider with 30 ab$^{-1}$ could exclude masses up to about 5750 GeV, or make a discovery if the mass is less than about 4000 GeV. However, we find that weak isosinglet vectorlike leptons present a much more difficult challenge, with some reach for exclusion, but not for discovery at any of the collider options considered.

Search for associated production of a Higgs boson and a single top quark in proton-proton collisions at s=13 TeV

A search is presented for the production of a Higgs boson in association with a single top quark, based on data collected in 2016 by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, which corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The production cross section for this process is highly sensitive to the absolute values of the top quark Yukawa coupling, $y_t$, the Higgs boson coupling to vector bosons, $g_\mathrm{HVV}$, and, uniquely, to their relative sign. Analyses using multilepton signatures, targeting H $\to$ WW, H $\to$ $\tau\tau$, and H $\to$ ZZ decay modes, and signatures with a single lepton and a $\mathrm{b\overline{b}}$ pair, targeting the H $\to$ $\mathrm{b\overline{b}}$ decay, are combined with a reinterpretation of a measurement in the H $\to$ $\gamma\gamma$ channel to constrain $y_\mathrm{t}$. For a standard model-like value of $g_\mathrm{HVV}$, the data favor positive values of $y_\mathrm{t}$ and exclude values of $y_\mathrm{t}$ below about $-$0.9 $y_\mathrm{t}^\mathrm{SM}$.

Electroweak superpartners scrutinized at the LHC in events with multi-leptons

We analyze a multi-lepton signal plus missing transverse energy from neutrinos expected at the LHC for a bino-like neutralino as the lightest supersymmetric particle (LSP), when the left sneutrino is the next-to-LSP and hence a suitable source of binos. The discussion is carried out in the framework of the μνSSM, where the presence of R-parity violating (RPV) couplings involving right-handed neutrinos solves the μ problem and can reproduce simultaneously the neutrino data. Left sneutrinos/sleptons are pair-produced at pp collisions decaying to binos, with the latter decaying via RPV to Wℓ or Zν. This signal can be compared with LHC searches for electroweak superpartners through chargino–neutralino production. The reduced cross section of the sneutrino/slepton production in comparison with the one of the latter process, limits the sensitivity of the searches to small sneutrino/slepton masses. Although the resulting compressed spectrum typically evades the aforementioned searches, we show that analyses using recursive jigsaw reconstruction are sensitive to these scenarios. As a by-product, we find that the region of bino masses 110–120 GeV and sneutrino masses 120–140 GeV can give rise to a tri-lepton signal compatible with the local excess recently reported by ATLAS.

The past, present and future of the heavier electroweakinos in the light of LHC and other data

The aim of this paper is to showcase the novel multilepton signals, hitherto unexplored at the LHC, arising from the heavier electroweakinos, in several generic pMSSMs at the upcoming LHC experiments. We first briefly review our old constraints on the full electroweakino sector of these models, containing both lighter and heavier sparticles, using the ATLAS trilepton data from the LHC Run I. Next we derive new stronger constraints on this sector for the first time using the ATLAS Run II data. We identify some benchmark points and explore the prospect of observing multilepton events in future LHC experiments. Our focus is on the channels with n > 3 which are the hallmarks of the heavier electroweakinos. If the spectrum of the lighter electroweakinos is compressed, these signals might very well be the discovery channels of the electroweakinos at the high luminosity LHC. We also discuss the implications of the new LHC constraints for the observed dark matter relic density of the universe, the measured value of the anomalous magnetic moment of the muon and the dark matter direct detection experiments.

Multi-lepton signatures of additional scalar bosons beyond the Standard Model at the LHC

Following a prediction made in von Buddenbrock et al (2015 arXiv:1506.00612), Kumar et al (2017 J. Phys.: Conf. Ser. 802 012007), von Buddenbrock et al (2016 Eur. Phys. J. C 76 580), this paper focuses on multi-lepton signatures arising from two new hypothetical scalar bosons, H and S, at the Large Hadron Collider. These two new bosons are an extension to the Standard Model and interact with the SM Higgs boson, h. We consider two production modes for H, one being gluon fusion and the other being in association with top quarks. The decay mode is considered, where leptonic final states are studied. The CP properties of S are characterised by considering effective couplings derived from dimension six operators through SWW vertices. The nature of the S boson is considered in two separate contexts. Firstly in a simplified model, it is considered to have Higgs-like couplings. Secondly, we consider a heavy neutrino model and its interactions with the Z, W and S bosons. The predictions of the models are compared both to ATLAS and CMS results at and 13 TeV, where appropriate. The data is interpreted using a simplified model where all the signal comes from , assuming S to be Higgs-like, mH = 270 GeV and mS = 150 GeV. The combined result yields gives a best fit value for the parameter βg (the strength of the Yukawa coupling of H to top quarks), . A number of regions of the phase space are suggested to the experiments for further exploration.

Searching for left sneutrino LSP at the LHC

We analyze relevant signals expected at the LHC for a left sneutrino as the lightest supersymmetric particle (LSP). The discussion is carried out in the “[Formula: see text] from [Formula: see text]” supersymmetric standard model [Formula: see text], where the presence of [Formula: see text]-parity breaking couplings involving right-handed neutrinos solves the [Formula: see text] problem and reproduces neutrino data. The sneutrinos are pair produced via a virtual [Formula: see text], [Formula: see text] or [Formula: see text] in the [Formula: see text] channel. From the prompt decay of a pair of left sneutrinos LSPs of any family, a significant diphoton signal plus missing transverse energy (MET) from neutrinos can be present in the mass range 118–132 GeV, with 13 TeV center-of-mass energy and an integrated luminosity of 100 fb[Formula: see text]. In addition, in the case of a pair of tau left sneutrinos LSPs, given the large value of the tau Yukawa coupling diphoton plus leptons and/or multileptons can appear. We find that the number of expected events for the multilepton signal, together with properly adopted search strategies, is sufficient to give a significant evidence for a sneutrino of mass in the range 130–310 GeV, even with the integrated luminosity of 20 fb[Formula: see text]. In the case of the signal producing diphoton plus leptons, an integrated luminosity of 100 fb[Formula: see text] is needed to give a significant evidence in the mass range 95–145 GeV. Finally, we discuss briefly the presence of displaced vertices and the associated range of masses.