Trilepton Events Research Articles

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.

Return of the WIMP: Missing energy signals and the Galactic Center excess

In a recent work, we emphasized that an excess in tri-lepton events plus missing energy observed by the ATLAS experiment at the LHC could be interpreted as a signal of low energy supersymmetry. In such a scenario the lightest neutralino mass is approximately $m_\chi \simeq 60$ GeV and the direct Dark Matter detection cross section is naturally below the current bound. In this work we present simple extensions of this scenario that lead to an explanation of the gamma ray excess at the center of the galaxy observed by Fermi-LAT, as well as the anti-proton excess observed by AMS-02. These extensions include the addition of a small CP violating phase in the neutralino sector or the addition of a light CP-odd Higgs scalar. Our study is of special relevance in view of a recent analysis that casts doubt on the previously accepted preference for mili-second pulsars as the origin of the galactic center excess.

Signatures of Dirac and Majorana sterile neutrinos in trilepton events at the LHC

Heavy sterile neutrinos with masses below $M_W$ can induce trilepton events at the 14 TeV LHC through purely leptonic $W$ decays of $W^\pm \to e^\pm e^\pm \mu^\mp \nu$ and $\mu^\pm \mu^\pm e^\mp \nu$ where the heavy neutrino will be in an intermediate state on its mass shell. Discovery and exclusion limits for the heavy neutrinos are found using both Cut-and-Count (CC) and a Multi-Variate Analysis (MVA) methods in this study. We also show that it is possible to discriminate between a Dirac and a Majorana heavy neutrino, even when lepton number conservation cannot be directly tested due to unobservability of the final state neutrino. This discrimination is done by exploiting a combined set of kinematic observables that differ between the Majorana vs. Dirac cases. We find that the MVA method can greatly enhance the discovering and discrimination limits in comparison with the CC method. For a 14-TeV $pp$ collider with integrated luminosity of 3000 ${\rm fb}^{-1}$, sterile neutrinos can be found with 5$\sigma$ significance if heavy-to-light neutrino mixings $|U_{Ne}|^2 \sim |U_{N\mu}|^2 \sim 10^{-6}$, while the Majorana vs. Dirac type can be distinguished if $|U_{Ne}|^2 \sim |U_{N\mu}|^2 \sim 10^{-5}$ or even $|U_{N\ell}|^2\sim 10^{-6}$ if one of the mixing elements is at least an order of magnitude smaller than the other.

Probing the MSSM explanation of the muon g-2 anomaly in dark matter experiments and at a 100 TeV pp collider

We explore the ability of current and future dark matter and collider experiments in probing anomalous magnetic moment of the muon, $(g-2)_\mu$, within the Minimal Supersymmetric Standard Model (MSSM). We find that the latest PandaX-II/LUX-2016 data gives a strong constraint on parameter space that accommodates the $(g-2)_{\mu}$ within $2\sigma$ range, which will be further excluded by the upcoming XENON-1T (2017) experiment. We also find that a 100 TeV $pp$ collider can cover most of our surviving samples that satisfy DM relic density within $3\sigma$ range through $Z$ or $h$ resonant effect by searching for trilepton events from $\tilde{\chi}^0_2\tilde{\chi}^+_1$ associated production. While the samples that are beyond future sensitivity of trilepton search at a 100 TeV $pp$ collider and the DM direct detections are either higgsino/wino-like LSPs or bino-like LSPs co-annihilating with sleptons. Such compressed regions may be covered by the monojet(-like) searches at a 100 TeV $pp$ collider.

New limits on heavier electroweakinos and their LHC signatures

We investigate the heavier electroweakino sectors in several versions of the MSSM, which has not been explored so far in the light of the LHC data, and obtain new bounds using the ATLAS Run I constraints in the $3l + {E\!\!\!\!/_T}$ channel. We also venture beyond the trilepton events and predict several novel multilepton + ${E\!\!\!\!/_T}$ signatures of these electroweakinos which may show up before the next shutdown of the LHC.

Same-sign trileptons as a signal of sneutrino lightest supersymmetric particle

Contrary to common expectation, a left-sneutrino can occasionally be the lightest supersymmetric particle. This has important implications in both collider and dark matter studies. We show that same-sign tri-lepton (SS3L) events at the Large Hadron Collider, with any lepton having opposite sign vetoed, distinguish such scenarios, up to gluino masses exceeding 2 TeV. The jets+MET signal rate is somewhat suppressed in this case, thus enhancing the scope of leptonic signals.

Search for new physics in trilepton events and limits on the associated chargino-neutralino production at CDF

We perform a search for new physics using final states consisting of three leptons and a large imbalance in transverse momentum resulting from proton-antiproton collisions at 1.96 TeV center-of-mass energy. We use data corresponding to 5.8 fb-1 of integrated luminosity recorded by the CDF II detector at the Tevatron collider. Our main objective is to investigate possible new low-momentum (down to 5 GeV/c) multi-leptonic final states not investigated by LHC experiments. Relative to previous CDF analyses, we expand the geometric and kinematic coverage of electrons and muons and utilize tau leptons that decay hadronically. Inclusion of tau leptons is particularly important for supersymmetry (SUSY) searches. The results are consistent with standard-model predictions. By optimizing our event selection to increase sensitivity to the minimal supergravity (mSUGRA) SUSY model, we set limits on the associated production of chargino and neutralino, the SUSY partners of the electroweak gauge bosons. We exclude cross sections up to 0.1 pb and chargino masses up to 168 GeV/c^2 at 95% CL, for a suited set of mSUGRA parameters. We also exclude a region of the two-dimensional space of the masses of the neutralino and the supersymmetric partner of the tau lepton, not previously excluded at the Tevatron.

Top quark effects in composite vector pair production at the LHC

In the context of strongly coupled Electroweak Symmetry Breaking, composite light scalar singlet and composite triplet of heavy vectors may arise from an unspecified strong dynamics and the interactions among themselves and with the Standard Model gauge bosons and fermions can be described by a SU(2) L ×SU(2) R /SU(2) L+R effective chiral Lagrangian. In this framework, the production of the V + V − and V 0 V 0 final states at the LHC by gluon fusion mechanism is studied in the region of parameter space consistent with the unitarity constraints in the elastic channel of longitudinal gauge boson scattering and in the inelastic scattering of two longitudinal Standard Model gauge bosons into Standard Model fermions pairs. The expected rates of same-sign di-lepton and tri-lepton events from the decay of the V 0 V 0 final state are computed and their corresponding backgrounds are estimated. It is of remarkable relevance that the V 0 V 0 final state can only be produced at the LHC via a gluon fusion mechanism since this state is absent in the Drell–Yan process. It is also found that the V + V − final-state production cross section via gluon fusion mechanism is comparable with the V + V − Drell–Yan production cross section. The comparison of the V 0 V 0 and V + V − total cross sections will be crucial for distinguishing the different models since the vector pair production is sensitive to many couplings. This will also be useful to determine if the heavy vectors are only composite vectors or are gauge vectors of a spontaneously broken gauge symmetry.

A “composite” scalar–vector system at the LHC

In the framework of a strong dynamics for Electro-Weak Symmetry Breaking (EWSB), both vector and scalar degrees of freedom have been studied in the literature within an effective Lagrangian approach. Here we consider the case in which both a scalar, h, and a vector, V – respectively an iso-singlet and an iso-triplet under a custodial SU ( 2 ) – are relevant with a mass below the cut-off Λ ≈ 4 π v . We study the amplitudes for the processes W L W L → W L W L , h h , V L V L , V L h . Requiring unitarity for the elastic channel W L W L → W L W L we can reduce the parameter space to two free parameters for given masses of the heavy vector and the scalar. We study the numerical total cross sections for the associated productions p p → V h j j by Vector Boson Fusion (VBF) and p p → V h by Drell–Yan (DY) annihilation as functions of these free parameters. The expected rates of same-sign di-lepton and tri-lepton events from the decay of the Vh final state are also given.

Environmentally selected WIMP dark matter with high-scale supersymmetry breaking

We explore the possibility that both the weak scale and the thermal relic dark matter abundance are environmentally selected in a multiverse. An underlying supersymmetric theory containing the states of the minimal supersymmetric standard model (MSSM) and singlets, with supersymmetry and $R$ symmetry broken at unified scales, has just two realistic low-energy effective theories. One theory ($\mathrm{SM}+\stackrel{\texttildelow{}}{w}$) is the standard model augmented only by the wino, having a mass near 3 TeV, and has a Higgs boson mass in the range of (127--142) GeV. The other theory ($\mathrm{SM}+\stackrel{\texttildelow{}}{h}/\stackrel{\texttildelow{}}{s}$) has Higgsinos and a singlino added to the standard model. The Higgs boson mass depends on the single new Yukawa coupling of the theory, $y$, and is near 141 GeV for small $y$ but grows to be as large as 210 GeV as this new coupling approaches strong coupling at high energies. Much of the parameter space of this theory will be probed by direct detection searches for dark matter that push 2 orders of magnitude below the present bounds; furthermore, the dark matter mass and cross section on nucleons are correlated with the Higgs boson mass. The indirect detection signal of monochromatic photons from the Galactic center is computed, and the range of parameters that may be accessible to LHC searches for trilepton events is explored. Taking a broader view, allowing the possibility of $R$ symmetry protection to the TeV scale or axion dark matter, we find four more theories: ($\mathrm{SM}+\mathrm{\text{axion}}$), two versions of split supersymmetry, and the E-MSSM, where a little supersymmetric hierarchy is predicted. The special Higgs mass value of $(141\ifmmode\pm\else\textpm\fi{}2)\text{ }\text{ }\mathrm{GeV}$ appears in symmetry limits of three of the six theories, ($\mathrm{SM}+\mathrm{\text{axion}}$), ($\mathrm{SM}+\stackrel{\texttildelow{}}{w}$), and ($\mathrm{SM}+\stackrel{\texttildelow{}}{h}/\stackrel{\texttildelow{}}{s}$), motivating a comparison of other signals of these three theories.

Multi-leptons with high transverse momentum at HERA

Events with at least two high transverse momentum leptons (electrons or muons) are studied using the H1 and ZEUS detectors at HERA with an integrated luminosity of 0.94 fb−1. The observed numbers of events are in general agreement with the Standard Model predictions. Seven di- and tri-lepton events are observed in e+p collision data with a scalar sum of the lepton transverse momenta above 100 GeV while 1.94±0.17 events are expected. Such events are not observed in e−p collisions for which 1.19±0.12 are predicted. Total visible and differential di-electron and di-muon photoproduction cross sections are extracted in a restricted phase space dominated by photon-photon collisions.

Search for new physics in theμμ+e/μ+E̸Tchannel with a low-pTlepton threshold at the Collider Detector at Fermilab

A search for new physics using three-lepton (trilepton) data collected with the CDF II detector and corresponding to an integrated luminosity of 976 pb-1 is presented. The standard model predicts a low rate of trilepton events, which makes some supersymmetric processes, such as chargino-neutralino production, measurable in this channel. The mu+mu+l signature is investigated, where l is an electron or a muon, with the additional requirement of large missing transverse energy. In this analysis, the lepton transverse momenta with respect to the beam direction (pT) are as low as 5 GeV/c, a selection that improves the sensitivity to particles which are light as well as to ones which result in leptonically decaying tau leptons. At the same time, this low-p_T selection presents additional challenges due to the non-negligible heavy-quark background at low lepton momenta. This background is measured with an innovative technique using experimental data. Several dimuon and trilepton control regions are investigated, and good agreement between experimental results and standard-model predictions is observed. In the signal region, we observe one three-muon event and expect 0.4+/-0.1 mu+mu+l events

Multi-lepton production and a search for doubly-charged Higgs at HERA

Events with two or more leptons (electrons or muons) with high transverse momentum are measured in electron-proton collisions at HERA using the data samples collected in the period 1994-2007. Yields of di-lepton and tri-lepton events are investigated and a general good agreement is found with the Standard Model (SM) prediction. Multi-lepton events at high transverse momenta, where the SM prediction is low, are of special interest as these signature might reveal new physics beyond the Standard Model. An example is the single production of a doubly-charged Higgs boson (H±±) decaying into a high mass pair of same charge leptons. This possibility has been investigated and H±± decays involving electrons, muons and taus are considered. No evidence for doubly-charged Higgs production is observed and mass-dependent upper limits are derived on the Yukawa couplings hij of the Higgs boson to leptons of flavour i and j.

Leptons and photons at the LHC: cascades through spinless adjoints

We study the hadron collider phenomenology of (1, 0) Kaluza-Klein modes along two universal extra dimensions compactified on the chiral square. Cascade decays of spinless adjoints proceed through tree-level 3-body decays involving leptons as well as one-loop 2-body decays involving photons. As a result, spectacular events with as many as six charged leptons, or one photon plus four charged leptons are expected to be observed at the LHC. Unusual events with relatively large branching fractions include three leptons of same charge plus one lepton of opposite charge, or one photon plus two leptons of same charge. We estimate the current limit from the Tevatron on the compactification scale, set by searches for trilepton events, to be around 270 GeV.

Neutralino signatures of the singlet extended MSSM

Abstract Extending the Higgs sector of the MSSM by the addition of a gauge singlet scalar field can remedy the μ problem. We explore the implications of extended models for both the spectrum of the neutralinos and the cascade decays of the neutralinos and charginos. Extra steps due to light decoupled neutralinos in the cascade decays of both neutralinos and charginos allow an excess of trilepton events compared to the MSSM and the existence of events with higher lepton multiplicity. Additionally, displaced vertices of the χ 2 0 due to small decay widths in some models may be observable.

Multi-lepton production in ep collisions at HERA

Abstract Multi-lepton events containing at least two electrons or two muons are studied in ep scattering at HERA using HERA I data samples at centre of mass energies of √s = 301 GeV and √s = 319 GeV and corresponding to integrated luminosities of 115 pb−1 (H1) and 130 pb−1 (ZEUS). Cross sections are derived from H1 data samples for the production of di-muons at √s = 319 GeV and for electron pair production. In di- and tri-lepton events, mass distributions are studied. High-mass multi-electron events are observed in a region where the Standard Model prediction is smaller. A possible interpretation of this excess in terms of doubly charged Higgs boson production is ruled out.

Discovery potential for supersymmetry in CMS* **Available on CMS information server

This work summarizes and puts in an overall perspective studies done within the compact muon solenoid (CMS) concerning the discovery potential for squarks and gluinos, sleptons, charginos and neutralinos, supersymmetric (SUSY) dark matter, lightest Higgs, sparticle mass determination methods and the detector design optimization in view of SUSY searches. It represents the status of our understanding of these subjects as of summer 1997. As a benchmark we used the minimal supergravity-inspired supersymmetric standard model (mSUGRA) with a stable lightest supersymmetric particle (LSP). Discovery of supersymmetry at the large hadron collider should be relatively straightforward. It may occur through the observation of large excesses of events in missing ET plus jets, or with one or more isolated leptons. An excess of trilepton events or isolated dileptons with missing ET, exhibiting a characteristic signature in the l+l− invariant mass distribution, could also be the first manifestation of SUSY production. Squarks and gluinos can be discovered for masses in excess of 2 TeV. Charginos and neutralinos can be discovered from an excess of events in dilepton or trilepton final states. Inclusive searches can give early indications from their copious production in squark and gluino cascade decays. Indirect evidence for sleptons can also be obtained from inclusive dilepton studies. Isolation requirements and a jet veto would allow detection of both the direct chargino/neutralino production and the directly produced sleptons. Squark and gluino production may also represent a copious source of Higgs bosons through cascade decays. The lightest SUSY Higgs h → bb̄ may be reconstructed with a signal/background ratio of order 1 thanks to hard cuts on ETmiss justified by escaping LSPs. The LSP of SUSY models with conserved R-parity represents a very good candidate for cosmological dark matter. The region of parameter space where this is true is well covered by our searches, at least for tanβ = 2.If supersymmetry exists at the electroweak scale, it could hardly escape detection in CMS and the study of supersymmetry will form a central part of our physics program.

Trilepton signal for supersymmetry at the Fermilab Tevatron reexamined

Within a wide class of models, the CERN LEP2 lower limit of 95 GeV on the chargino mass implies gluinos are heavier than $\ensuremath{\sim}300 \mathrm{GeV}.$ In this case electroweak ${W}_{1}W{\overline{\ifmmode \tilde{}\else \~{}\fi{}}}_{1}$ production and ${W}_{1}{Z}_{2}$ production are the dominant supersymmetry (SUSY) processes at the Fermilab Tevatron, and the extensively examined isolated trilepton signal from ${W}_{1}{Z}_{2}$ production assumes an even greater importance. We update our previous calculations of the SUSY reach of luminosity upgrades of the Fermilab Tevatron in this channel incorporating (i) decay matrix elements in the computation of the momenta of leptons from chargino and neutralino decays, (ii) the trilepton background from ${W}^{*}{Z}^{*}$ and ${W}^{*}{\ensuremath{\gamma}}^{*}$ production which, though neglected in previous analyses, turns out to be the dominant background, and finally, (iii) modified sets of cuts designed to reduce these new backgrounds and increase the range of model parameters for which the signal is observable. We show our improved projections for the reach for SUSY of both the Fermilab Main Injector and the proposed TeV33 upgrade. We also present opposite sign same flavor dilepton invariant mass distributions as well as the ${p}_{T}$ distributions of leptons in SUSY trilepton events, and comment upon how the inclusion of decay matrix elements impacts upon the Tevatron reach, as well as upon the extraction of neutralino masses.

Trilepton signature of minimal supergravity at the upgraded Fermilab Tevatron

The prospects for detecting trilepton events ($\ell = e$ or $\mu$) from chargino-neutralino ($\chi^\pm_1 \chi^0_2$) associated production are investigated for the upgraded Fermilab Tevatron Collider in the context of the minimal supergravity model (mSUGRA). In some regions of parameter space, $\chi^\pm_1$ and $\chi^0_2$ decay dominantly into final states with $\tau$ leptons and the contributions from $\tau-$leptonic decays enhance the trilepton signal substantially when soft cuts on lepton transverse momenta are used. Additional sources of the mSUGRA trilepton signal and dominant irreducible backgrounds are discussed. The dilepton ($\ell^+\ell^-$) invariant mass distribution near the endpoint is considered as a test of mSUGRA mass relations. Discovery contours for $p\bar{p} \to 3\ell +X$ at 2 TeV with an integrated luminosity of 2 fb$^{-1}$ to 30 fb$^{-1}$ are presented in the mSUGRA parameter space of $(m_0,m_{1/2})$ for several choices of $\tan\beta$.

Supersymmetry reach of Fermilab Tevatron upgrades: The largetanβcase

The Yukawa couplings of the tau lepton and the bottom quark become comparable to, or even exceed, electroweak gauge couplings for large values of the SUSY parameter $\tan\beta$. As a result, the lightest tau slepton $\ttau_1$ and bottom squark $\tb_1$ can be significantly lighter than corresponding sleptons and squarks of the first two generations. Gluino, chargino and neutralino decays to third generation particles are significantly enhanced when $\tan\beta$ is large. This affects projections for collider experiment reach for supersymmetric particles. In this paper, we evaluate the reach of the Fermilab Tevatron $p\bar p$ collider for supersymmetric signals in the framework of the mSUGRA model. We find that the reach via signatures with multiple isolated leptons ($e$ and $\mu$) is considerably reduced. For very large $\tan\beta$, the greatest reach is attained in the multi-jet$+\eslt$ signature. Some significant extra regions may be probed by requiring the presence of an identified $b$-jet in jets$+\eslt$ events, or by requiring one of the identified leptons in clean trilepton events to actually be a hadronic 1 or 3 charged prong tau. In an appendix, we present formulae for chargino, neutralino and gluino three body decays which are valid at large $\tan\beta$.