A natural, non-supersymmetric solution to the hierarchy problem generically requires fermionic partners of the top quark with masses not much heavier than 500 GeV. We study the pair production and detection at the LHC of the top partners with electric charge Q=5/3 (T_{5/3}) and Q=-1/3 (B), that are predicted in models where the Higgs is a pseudo-Goldstone boson. The exotic T_{5/3} fermion, in particular, is the distinct prediction of a LR custodial parity invariance of the electroweak symmetry breaking sector. Both kinds of new fermions decay to Wt, leading to a t\bar{t}WW final state. We focus on the golden channel with two same-sign leptons, and show that a discovery could come with less than 100 pb^{-1} (less than 20 fb^{-1}) of integrated luminosity for masses M=500 GeV (M=1TeV). In the case of the T_{5/3}, we present a simple strategy for its reconstruction in the fully hadronic decay chain. Although no full mass reconstruction is possible for the B, we still find that the same-sign dilepton channel offers the best chances of discovery compared to other previous searches that used final states with one or two opposite-sign leptons, and hence suffered from the large t\bar{t} background. Our analysis also directly applies to the search of 4th generation b' quarks.
Heavy partners of the top quark are a common prediction of many models in which a new strongly-coupled sector is responsible for the breaking of the EW symmetry. In this paper we investigate their experimental signature at the LHC, focusing on the particularly clean channel of same-sign dileptons. We show that, thank to a strong interaction with the top quark which allows them to be singly produced at a sizable rate, the top partners will be discovered at the LHC if their mass is below 1.5 TeV, higher masses being possible in particularly favorable (but plausible) situations. Being the partner…
The Standard Model is a successful theory but lack of a mechanism for neutrino mass generation as well as a solution to the naturalness problem. In the models that are proposed to simultaneously solve the two problems, heavy Majorana neutrinos and top partners are usually predicted that can lead to a new decay mode of the top partner mediated by the heavy Majorana neutrinos: $T\to b\,W^{+}\to b\,\ell^{+}\ell^{+}q\bar{q'}$. In this paper, we will study the observability of such a new signature via the pair production process of top partner $pp \to T\bar{T} \to 2b+ \ell^\pm \ell^\pm+4j$ in a mod…
Supersymmetry may be discovered at hadron colliders by searching for events similar to the top quark signal of two isolated leptons. In the case of gluino production, the most distinguishing feature is that in half the events the two leading leptons have the same sign. We demonstrate the remarkable sensitivity of this gluino signature at both the Fermilab Tevatron Collider and at the Superconducting Super Collider. Techniques for approximately determining the gluino mass are discussed.
New vectorlike quarks are predicted in many new physics scenarios beyond the Standard Model~(SM) and could potentially be discovered at the LHC. Based on a simplified model including a singlet vectorlike top partner with charge $2/3$, we investigate the process $pp \to TT$ via a $t$-channel induced by the couplings between the top partner with the first-generation SM quarks. We calculate the production cross section and further study the observability of the heavy top partner in the channel $T\to Wq$ at the high-luminosity LHC (HL-LHC) using final states with same-sign dileptons (electrons or …
In the littlest Higgs Model with T-parity, we discuss the pair production of the T-odd top partner ($T_{-}$) which decays almost 100\% into the top quark and the lightest T-odd particle ($A_{H}$). Considering the current constraints, we investigate the observability of the T-odd top partner pair production through the process $pp\rightarrow T_{-}\bar{T}_{-}\rightarrow t\bar{t}A_{H}A_{H}$ in final states with two leptons at 14 TeV LHC. We analyze the signal significance and find that the lower limit on the T-odd top partner mass are about 1.04 TeV, 1.14 TeV, 1.23 TeV at $2σ$ confidence level at…
We perform a model-independent study for top-antitop and top-top resonances in the dilepton channel at the Large Hadtron Collider. In this channel, we can solve the kinematic system to obtain the momenta of all particles including the two neutrinos, and hence the resonance mass and spin. For discovering top-antitop resonances, the dilepton channel is competitive to the semileptonic channel because of the good resolution of lepton momentum measurement and small standard model backgrounds. Moreover, the charges of the two leptons can be identified, which makes the dilepton channel advantageous f…
In the composite models with colored substructure of the fermions the color singlet leptons are accompanied by a composite color octet partners, which are known as leptogluons. We consider the effect of leptogluons in the dilepton production at the LHC and show that in the reachable parameter range this effect is typically dominated by t-channel leptogluon exchange (indirect channel). We show that this channel alone can give a sizable contribution to the dimuon production at the LHC for TeV scale values of the invariant mass of the muon-antimuon pairs.
We reconsider observables for discovering a heavy Higgs boson (with m_h > 2m_W) via its di-leptonic decays h -> WW -> l nu l nu. We show that observables generalizing the transverse mass that take into account the fact that both of the intermediate W bosons are likely to be on-shell give a significant improvement over the variables used in existing searches. We also comment on the application of these observables to other decays which proceed via narrow-width intermediates.
We reconsider observables for discovering and measuring the mass of a Higgs boson via its di-leptonic decays: H --> WW* --> l nu l nu. We define an observable generalizing the transverse mass that takes into account the fact that one of the intermediate W-bosons is likely to be on-shell. We compare this new variable with existing ones and argue that it gives a significant improvement for discovery in the region m_h < 2 m_W.
We determine the discovery potential of the Large Hadron Collider (LHC) at 13 TeV center-of-mass energy for a heavy scalar resonance in the dilepton channel. In particular, we consider the singlet-like heavy mass eigenstate of a mixed two Higgs doublet and scalar singlet model in the $U(1)_{B-L}$ extension of the Standard Model. We find that, despite the small coupling of the singlet scalar with the doublets, this heavy scalar can be discovered with 5$σ$ at the LHC with integrated luminosities of $\sim 300$ to $1400$ fb$^{-1}$ in the mass range between 500 GeV and 1 TeV.
At the LHC, dileptonic events may turn up new physics interacting with quarks and leptons. The poster child for this scenario is a resonant $Z'$, much anticipated in $\ell^+ \ell^-$ invariant mass spectra. However, angular spectra of dileptons may play an equal or stronger role in discovering a non-resonant species. This paper avails of their LHC measurements to corner the couplings and masses of leptoquarks (LQs), that can mediate $q \bar{q} \rightarrow \ell^+ \ell^-$ in the $t$-channel and dramatically alter Standard Model angular spectra. Also derived are constraints from alterations to $m_…
Pair production of light top squarks at the 8-TeV LHC can be used to probe the gaugino-Higgsino sector of the Minimal Supersymmetric Standard Model. The case where the lightest neutralino is a mixture of Bino and Higgsino, satisfying the thermal dark matter relic density, is investigated. In such a scenario, the lightest top squark decays mostly into $(i)$ a top quark plus the second or third lightest neutralino, and $(ii)$ a bottom quark plus the lightest chargino, instead of a decay scenario of the lightest top squark into a top quark and the lightest neutralino. Final states with $\geq 2$ j…
Fermionic third generation top partners are generic in composite Higgs models. They are likely to decay into third generation quarks and electroweak bosons. We propose a novel cut-and-count-style analysis in which we cross correlate the model-dependent single and model-independent pair production processes for the top partners $X_{5/3}$ and $B$. In the class of composite Higgs models we study, $X_{5/3}$ is very special as it is the lightest exotic fermion. A constraint on the mass of $X_{5/3}$ directly extends to constrains on all top partner masses. By combining jet substructure methods with …
In this work we reappraise the collider constraints from leptonic final states on the vectorlike colored top partners taking into account the impact of exotic colored vector resonances. These colored states are intrinsic to a broad class of models that employ a strongly interacting sector to drive electroweak symmetry breaking. We translate the recent results in the {\sl monolepton + jets} channel as reported by CMS with an integrated luminosity of 35.8 fb$^{-1}$, and {\sl dilepton + jets} and {\sl trilepton + jets} channels as reported by ATLAS with an integrated luminosity of 36.1 fb$^{-1}$ …
The Little Flavor model is a close cousin of the Little Higgs theory which aims to generate flavor structure around TeV scale. While the original Little Flavor only included the quark sector, here we build the lepton part of the Little Flavor model and explore its phenomenology. The model produces the neutrino mixing matrix and Majorana masses of the Standard Model neutrinos through coupling to heavy lepton partners and Little Higgses. We combine the usual right-handed seesaw mechanism with global symmetry protection to suppress the Standard Model neutrino masses, and identify the TeV partners…
Anomalously light fermionic partners of the top quark often appear in explicit constructions, such as the 5d holographic models, where the Higgs is a light composite pseudo Nambu-Goldstone boson and its potential is generated radiatively by top quark loops. We show that this is due to a structural correlation among the mass of the partners and the one of the Higgs boson. Because of this correlation, the presence of light partners could be essential to obtain a realistic Higgs mass. We quantitatively confirm this generic prediction, which applies to a broad class of composite Higgs models, by s…
The LHCb bounds on the branching ratio of rare decay $D^0 \to μ^+ μ^-$ and the constraints on the branching ratio of $D^+ \to π^+ μ^+ μ^-$ in the nonresonant regions enable us to improve constraints on new physics contributions. Using the effective Lagrangian approach we determine sizes of the Wilson coefficients allowed by the existing LHCb bounds on rare charm decays. Then we discuss contributions to rare charm meson decay observables in several models of new physics: a model with an additional spin-1 weak triplet, leptoquark models, Two Higgs doublets model of type III, and a $Z'$ model. He…
Motivated by UV explanations of the $B$-physics anomalies, we study a dark sector containing a Majorana dark matter candidate and a coloured coannihilation partner, connected to the Standard Model predominantly via a $U_1$ vector leptoquark. A TeV scale $U_1$ leptoquark, which couples mostly to third generation fermions, is the only successful single-mediator description of the $B$-physics anomalies. After calculating the dark matter relic surface, we focus on the most promising experimental avenue: LHC searches for the coloured coannihilation partner. We find that the coloured partner hadroni…
Solutions to the hierarchy problem usually require top partners. In standard SUSY or composite Higgs theories, the partners carry SM color and are becoming increasingly constrained by LHC searches. However, theories like Folded SUSY (FS), Twin Higgs (TH) and Quirky Little Higgs (QLH) introduce uncolored top partners, which can be SM singlets or carry electroweak charge. Their small production cross section left doubt as to whether the LHC can effectively probe such scenarios. Typically, these partners are charged under their own mirror color gauge group. In FS and QLH, the absence of light mir…
In composite Higgs models (CHMs), electroweak precision data generically push colourless composite vector resonances to a regime where they dominantly decay into pairs of light top partners. This greatly attenuates their traces in canonical collider searches, tailored for narrow resonances promptly decaying into Standard Model final states. By reinterpreting the CMS same-sign dilepton (SS2$\ell$) analysis at the Large Hadron Collider (LHC), originally designed to search for top partners with electric charge $5/3$, we demonstrate its significant coverage over this kinematical regime. We also sh…
Cai, Cheng, and Terning suggested a model in which the left-handed top quark is identified with a gaugino of an extended gauge group, and its superpartner is a spin-1 particle. We perform a phenomenological analysis of this model, with a focus on the spin-1 top partner, which we dub the "swan". We find that precision electroweak fits, together with direct searches for $Z^\prime$ bosons at the LHC, place a lower bound of at least about 4.5 TeV on the swan mass. An even stronger bound, 10 TeV or above, applies in most of the parameter space, mainly due to the fact that the swan is typically pred…
In our recent paper [1], we explored a prospect of discovering the heavy Majorana right-handed neutrinos (RHNs) at the future LHC in the context of the minimal non-exotic U(1) extended Standard Model (SM), where a pair of RHNs are created via decay of resonantly produced massive U(1) gauge boson ($Z^{\prime}$). We pointed out that this model can yield a significant enhancement of the branching ratio of the $Z^\prime$ boson to a pair of RHNs, which is crucial for discovering the RHNs under the very severe LHC Run-2 constraint from the search for the $Z^\prime$ boson with dilepton final states. …
Relatively light Top Partners are unmistakable signatures of reasonably Natural Composite Higgs models and as such they are worth searching for at the LHC. Their phenomenology is characterized by a certain amount of model-dependence, which makes the interpretation of Top Partner experimental searches not completely straightforward especially if one is willing to take also single production into account. We describe a model-independent strategy by which the interpretation is provided on the parameter space of a Simplified Model that captures the relevant features of all the explicit constructio…
We perform a detailed study of top-quark partner production in the ttbar plus large missing energy final-state at the LHC, presenting results for both scalar and fermionic top-quark partners in the semi-leptonic and dileptonic decay modes of the top quarks. We compare the results of several simulation tools: leading-order matrix elements, next-to-leading order matrix elements, leading-order plus parton shower simulations, and merged samples that contain the signal process with an additional hard jet radiated. We find that predictions from leading-order plus parton shower simulations can signif…
In supersymmetric models, Winos, partners of the SU(2) gauge bosons, may be the lightest supersymmetric particles (LSPs). For generic parameters, charged and neutral Winos are highly degenerate. Charged Winos travel macroscopic distances, but can decay to neutral Winos and extremely soft leptons or pions before reaching the muon chambers, thereby circumventing conventional trigger requirements based on energetic decay products or muon chamber hits. However, these charginos are detectable, and can be triggered on when produced in association with jets. In addition, we propose a new trigger for …
We outline a general strategy for measuring spins, couplings and mixing angles in the case of a heavy partner decay chain terminating in an invisible particle. We consider the common example of a new scalar or fermion D decaying sequentially to other new particles C, B and A by emitting a quark jet j and two leptons ln and lf. We derive analytic formulas for the dilepton {ln,lf} and the two jet-lepton ({j,ln} and {j,lf}) invariant mass distributions for most general couplings and mixing angles of the new partners. We then consider various spin assignments for the particles A, B, C and D, and d…
We present sensitivity projections for discovering a heavy resonance decaying to electron and muon pairs and for probing the charged lepton non-universality in such decays at the HL-LHC and FCC-hh. The analysis takes into account the expected differences in the reconstruction efficiencies and the dilepton mass resolutions for dielectron and dimuon final states. We demonstrate how the analyses at HL-LHC naturally paves the way for a FCC-hh machine thereby underlining its importance.
A 3 sigma excess has been recently announced by ATLAS in events with Z-peaked dilepton pairs, jets, and large transverse missing energy. We interpret this finding in the context of composite Higgs / RS theories. We find that composite Higgs theories with custodial symmetry protection to the $Zb\bar{b}$ coupling predict a significant contribution to $ZZbb$ (and to $hhbb$) final states coming from heavy gluon decays to pairs of bottom-partner vector-like quarks. The heavy gluon to vector-like quarks signal is largely accepted by the ATLAS selection if one of the $Z$ boson in the $ZZbb$ final sta…
Naturalness arguments applied to simple supersymmetric (SUSY) theories require a set of light higgsinos with mass $\sim |μ|$ not too far from $m_h$. These models have an inverted electroweakino spectrum with $|μ| \ll M_2$ which leads to a rather clean, hadronically quiet, same-sign diboson (SSdB) signature at hadron colliders arising from neutral-plus-charged wino pair production. We improve and expand our earlier studies of this signature for discovering SUSY in natural SUSY models by (i) including backgrounds which were not previously considered and which turn out to be significant, (ii) dev…
The Mirror Matter or Exact Parity Model sees every standard particle, including the physical neutral Higgs boson, paired with a parity partner. The unbroken parity symmetry forces the mass eigenstate Higgs bosons to be maximal mixtures of the ordinary and mirror Higgs bosons. Each of these mass eigenstates will therefore decay 50% of the time into invisible mirror particles, providing a clear and interesting signature for the Large Hadron Collider (LHC) which could thus establish the existence of the mirror world. However, for this effect to be observable the mass difference between the two ei…
We study the LHC potential for discovering TeV-scale SU(2)_L 5-plet fermions introduced recently to explain small neutrino masses. We show that the Drell-Yan production and the decays of new exotic Sigma leptons are testable at the LHC. Their production is abundant due to nontrivial electroweak gauge charges. For 1 fb^-1 of integrated luminosity at the present LHC sqrt{s}=7 TeV, there can be 270 Sigma-Sigmabar pairs produced for M_Sigma = 400 GeV. Besides producing same-sign dilepton events, they could lead, due to a chosen small mixing between heavy and light leptons, to ~10 golden decays Sig…
Motivated by the observed excess of the di-photon signal in Higgs searches, $σ_{γγ}/σ_{\rm SM}\simeq 1.5$, we argue that models with enhanced $Γ(h\to γγ)$ alone are the most favorable scenarios when the latest LHC/Tevatron results are all taken into account. We study the phenomenology of a supersymmetric scenario of light stau first proposed by \textit{Carena et. al. \cite{carlos}} that predicts a 125 GeV SM-like Higgs boson with enhanced diphoton decay through light stau loops. Since it is extremely challenging to search the Drell-Yan stau pair at the LHC due to the small production rate, we …
Recent clarifications of naturalness in supersymmetry robustly require the presence of four light higgsinos with mass ~100-300 GeV while gluinos and (top)-squarks may lie in the multi-TeV range, possibly out of LHC reach. We project the high luminosity (300-3000 fb^{-1}) reach of LHC14 via gluino cascade decays and via same-sign diboson production. We compare these to the reach for neutralino pair production \tz_1\tz_2 followed by \tz_2\to\tz_1\ell^+\ell^- decay to soft dileptons which recoil against a hard jet. It appears that 3000 fb^{-1} is just about sufficient integrated luminosity to pro…
We study the LHC signatures of TeV scale vectorlike quarks $b'$, $t'$ and $χ$ with electromagnetic charges -1/3, 2/3 and 5/3 that appear in many beyond the standard model (BSM) extensions. We consider warped extra-dimensional models and analyze the phenomenology of such vectorlike quarks that are the custodial partners of third generation quarks. In addition to the usually studied pair-production channels which depend on the strong coupling, we put equal emphasis on single production channels that depend on electroweak couplings and on electroweak symmetry breaking induced mixing effects betwe…
Models of warped extra dimensions with custodial symmetry usually predict the existence of a light Kaluza-Klein fermion arising as a partner of the right-handed top quark, sometimes called light custodians which we will denote $\tilde{b}_R$. The production of these particles at the LHC can give rise to multi-W events which could be observed in same-sign dilepton channels, but its mass reconstruction is challenging. In this letter we study the possibility of finding a signal for the pair production of this new particle at the LHC focusing on a rarer, but cleaner decay mode of a light custodian …
We study the prospect for discovering the $cg\to bH^+\to b A W^+$ process at the LHC. Induced by the top-flavor changing neutral Higgs coupling $ρ_{tc}$, the process may emerge if $m_{H^+} > m_A + m_{W^+}$, where $H^+$ and $A$ are charged and $CP$-odd Higgs bosons in the general two Higgs Doublet Model (g2HDM). We show that the $cg\to bH^+\to b A W^+$ process can be discovered at LHC Run 3, while the full Run 2 data at hand can constrain the parameter space significantly by searching for the same-sign dilepton final state. The process has unique implications on the hint of $gg\to A \to t \bar …
Based on a model-independent framework including the vector-like top partner (VLQ-$T$), we investigate the prospect of discovering the singlet or doublet VLQ-$T$ via the single production process $e^{-}e^{+}\to T\bar{t}+t\bar{T}$ with the $T\to Zt$ decay channel at future high energy linear $e^{+}e^{-}$ collider with $\sqrt{s}=3$ TeV. We focus on the hadronic decay of the top quark and two types of decay channel for the $Z$ boson: $Z\to \ell^{+}\ell^{-}$ and $Z\to ν\barν$. By carrying out a full simulation for the signals and the relevant SM backgrounds, the $2σ$ exclusion limit and $5σ$ disco…
We study the $W^{'}$ phenomenology in composite Higgs/ warped extra dimensional models focusing on the effect of fermionic resonances at $\sim$1 TeV. After deriving the existing bounds from the current LHC-8 analyses, we highlight the most promising signatures for $W^{'}$ discovery at the 14 TeV LHC. We find in particular very promising the study of $W^{'}$ decay modes into vector-like top partners, specifically the decay into a doublet of custodian heavy fermions, $T_{5/3} T_{2/3}$, and the decay into a heavy fermion plus a Standard Model quark. We perform a detailed parton level analysis of …
We study the collider phenomenology of the lightest Kaluza-Klein excitation of the gluon, G*, in theories with a warped extra dimension. We do so by means of a two-site effective lagrangian which includes only the lowest-lying spin-1 and spin-1/2 resonances. We point out the importance of the decays of G* to one SM plus one heavy fermion, that were overlooked in the previous literature. It turns out that, when kinematically allowed, such heavy-light decays are powerful channels for discovering the G*. In particular, we present a parton-level Montecarlo analysis of the final state Wtb that foll…
This paper investigates the $e^-e^+\to Zh_1$ sensitivity for Higgs boson's rare decay into heavy neutrinos $h_1 \to NN$ at the proposed electron-positron collider, with the focus on multi-lepton final states that contain same-sign lepton pairs. $h_1 \to NN$ decay can derive from Higgs boson's mixing with new physics scalar(s) that is complementary to the contribution from active-sterile neutrino mixings. We consider the scenario with a singlet scalar which gives the heavy neutrino mass and has a small mixing with the SM Higgs boson. We analyze the semileptonic, fully leptonic and mixed $NN$ de…
We consider the immediate or near-term experimental opportunities offered by some scenarios that could explain the new diphoton excess at the LHC. If the excess is due to a new particle $X_s$ at 750 GeV, additional new particles are required, providing further signals. If connected with naturalness, the $X_s$ may be produced in top partner decays. Then a $t'\bar t'$ signal, with $t'\to t X_s$ and $X_s\to gg$ dominantly, might be discovered by reinterpreting 13 TeV SUSY searches in multijet events with low MET and/or a lepton. If $X_s$ is a bound state of quirks, the signal events may be accomp…
