Top Partners Research Articles

Common exotic decays of top partners

Many standard model extensions that address the hierarchy problem contain Dirac-fermion partners of the top quark, which are typically expected around the TeV scale. Searches for these vector-like quarks mostly focus on their decay into electroweak gauge bosons and Higgs plus a standard model quark. In this article, backed by models of composite Higgs, we propose a set of simplified scenarios, with effective Lagrangians and benchmarks, that include more exotic decay channels, which modify the search strategies and affect the bounds. Analysing several classes of underlying models, we show that exotic decays are the norm and commonly appear with significant rates. All these models contain light new scalars that couple to top partners with charge 5/3, 2/3, and −1/3.

Effective potential in ultraviolet completions for composite Higgs models

We consider a class of composite Higgs models based on asymptotically free $SO(d)$ gauge theories with $d$ odd, with fermions in two irreducible representations, and in which the Higgs field arises as a pseudo Nambu-Goldstone boson and the top quark is partially composite. The Nambu-Goldstone coset containing the Higgs field, or Higgs coset, is either $SU(4)/Sp(4)$ or $SU(5)/SO(5)$, whereas the top partners live in two-index representations of the relevant flavor group ($SU(4)$ or $SU(5)$). In both cases, there is a large number of terms in the most general four-fermion lagrangian describing the interaction of third-generation quarks with the top partners. We derive the top-induced effective potential for the Higgs coset together with the singlet pseudo Nambu-Goldstone boson associated with the non-anomalous axial symmetry, to leading order in the couplings between the third-generation quarks and the composite sector. We obtain expressions for the low-energy constants in terms of top-partner two-point functions. We revisit the effective potential of another composite Higgs model that we have studied previously, which is based on an $SU(4)$ gauge theory and provides a different realization of the $SU(5)/SO(5)$ coset. The top partners of this model live in the fundamental representation of $SU(5)$, and, as a result, the effective potential of this model is qualitatively different from the $SO(d)$ gauge theories. We also discuss the role of the isospin-triplet fields contained in the $SU(5)/SO(5)$ coset, and show that, without further constraints on the four-fermion couplings, an expectation value for the the Higgs field will trigger the subsequent condensation of an isospin-triplet field.

Loop induced single top partner production and decay at the LHC

Most searches for top partners, T , are concerned with top partner pair production. However, as these bounds become increasingly stringent, the LHC energy will saturate and single top partner production will become more important. In this paper we study the novel signature of the top partner produced in association with the SM top, ppto Toverline{t}+toverline{T} , in a model where the Standard Model (SM) is extended by a vector-like SU(2)L singlet fermion top partner and a real, SM gauge singlet scalar, S. In this model, ppto Toverline{t}+toverline{T} production is possible through loops mediated by the scalar singlet. We find that, with reasonable coupling strengths, the production rate of this channel can dominate top partner pair production at top partner masses of mT ≳ 1.5 TeV. In addition, this model allows for the exotic decay modes T → tg, T → tγ, and T → tS. In much of the parameter space the loop induced decay T → tg dominates and the top partner is quite long lived. New search strategies are necessary to cover these decay modes. We project the the sensitivity of the high luminosity LHC to ppto Toverline{t}+toverline{T} via a realistic collider study. We find with 3 ab−1, the LHC is sensitive to this process for masses mT ≲ 2 TeV. In addition, we provide appendices detailing the renormalization of this model.

The fate of the littlest Higgs model with T -parity under 13 TeV LHC data

We exploit all LHC available Run 2 data at center-of-mass energies of 8 and 13 TeV for searches for physics beyond the Standard Model. We scrutinize the allowed parameter space of Little Higgs models with the concrete symmetry of T -parity by providing comprehensive analyses of all relevant production channels of heavy vectors, top partners, heavy quarks and heavy leptons and all phenomenologically relevant decay channels. Constraints on the model, particularly the symmetry breaking scale f, will be derived from the signatures of jets and missing energy or leptons and missing energy. Besides the symmetric case, we also study the case of T-parity violation. Furthermore, we give an extrapolation to the LHC high-luminosity phase at 14 TeV as well.

The Hyperbolic Higgs

We introduce the Hyperbolic Higgs, a novel solution to the little hierarchy problem that features Standard Model neutral scalar top partners. At one-loop order, the protection from ultraviolet sensitivity is due to an accidental non-compact symmetry of the Higgs potential that emerges in the infrared. Once the general features of the effective description are detailed, a completion that relies on a five dimensional supersymmetric framework is provided. Novel phenomenology is compared and contrasted with the Twin Higgs scenario.

Top partner-resonance interplay in a composite Higgs framework

Guided us by the scenario of weak scale naturalness and the possible existence of exotic resonances, we have explored in a [Formula: see text] Composite Higgs setup the interplay among three matter sectors: elementary, top partners and vector resonances. We parametrize it through explicit interactions of spin-1 [Formula: see text]-resonances, coupled to the [Formula: see text]-invariant fermionic currents and tensors presented in this work. Such invariants are built upon the Standard Model fermion sector as well as top partners sourced by the unbroken [Formula: see text]. The mass scales entailed by the top partner and vector resonance sectors will control the low energy effects emerging from our interplaying model. Its phenomenological impact and parameter spaces have been considered via flavor-dijet processes and electric dipole moments bounds. Finally, the strength of the Nambu–Goldstone symmetry breaking and the extra couplings implied by the top partner mass scales are measured in accordance with expected estimations.

Single top partner production at lepton colliders in the left–right twin Higgs model

In the framework of the left–right twin Higgs (LRTH) model, we investigate the single top partner production at lepton colliders. We calculate the production cross-sections of the processes [Formula: see text], [Formula: see text] [Formula: see text] and [Formula: see text] [Formula: see text] at [Formula: see text] TeV, and display some typical differential distributions of the final state particles.

Search for single production of vector-like top partner decaying to Wb at egamma collision

In a simplified model including an SU(2) singlet T quark with charge 2/3, we investigate the single vector-like T production at the high energy egamma collision. We study the observability of the vector-like T focusing on the Trightarrow Wb decay channel with Wrightarrow lbar{nu } at sqrt{s}=2.0 TeV. In this analysis, only two free parameters are involved, namely the T quark coupling strength for single production g^{*} and the mass m_{T}. We scan the parameter space and find that the correlation region of g^{*}in [0.24, 0.5] and m_{T}in [800, 1360] GeV can be excluded with integrated luminosity L=100,hbox {fb}^{-1} and the correlation region of g^{*}in [0.13, 0.5] and m_{T}in [800, 1620] GeV can be excluded with integrated luminosity L=1000 ,hbox {fb}^{-1} at 2sigma level.

Neutral naturalness from the brother-Higgs model

We present a version of the twin Higgs mechanism with minimal symmetry structure and particle content. The model is built upon a composite Higgs theory with global $SO(6)/SO(5)$ symmetry breaking. The leading contribution to the Higgs potential, from the top sector, is solely cancelled via the introduction of a Standard Model neutral top partner. We show that the inherent $Z_2$ breaking of this construction is under control and of the right size to achieve electroweak symmetry breaking, with a fine-tuning at the level of $5-10\%$, and compatibly with the observed Higgs mass. We briefly discuss the particular phenomenological features of this scenario.

Search for thj production with h → γγ at the LHC in the littlest Higgs model with T-parity

In the littlest Higgs model with T-parity, we study associated production of a Higgs and a single top quark at the 14 TeV LHC. We focus on the Higgs to two photons decay and the semileptonic top decay channel. By performing a fast detector simulation, we find that the thj search in the selected channel can excluded the top partner mass mT+ up to 805 (857) GeV for case A (case B) at 2σ confidence level at 14 TeV LHC with the integrated luminosity L=3ab−1.

Stop co-annihilation in the minimal supersymmetric standard model revisited

We re-examine the stop co-annihilation scenario of the Minimal Supersymmetric Standard Model, wherein a bino-like lightest supersymmetric particle has a thermal relic density set by co-annihilations with a scalar partner of the top quark in the early universe. We concentrate on the case where only the top partner sector is relevant for the cosmology, and other particles are heavy. We discuss the cosmology with focus on low energy parameters and an emphasis on the implications of the measured Higgs boson mass and its properties. We find that the irreducible direct detection signal correlated with this cosmology is generically well below projected experimental sensitivity, and in most cases lies below the neutrino background. A larger, detectable, direct detection rate is possible, but is unrelated to the co-annihilation cosmology. LHC searches for compressed spectra are crucial for probing this scenario.

Monotop signature from a fermionic top partner

We investigate mono-top signatures arising from phenomenological models of fermionic top-partners, which are degenerate in mass and decay into a bosonic dark matter candidate, either spin-0 or spin-1. Such a model provides a mono-top signature as a smoking-gun, while conventional searches with $t\bar t$ + missing transverse momentum are limited. Two such scenarios: i) a phenomenological 3rd generation extra dimensional model with excited top and electroweak sectors, and ii) a model where only a top-partner and a dark matter particle are added to the SM, are studied in the degenerate mass regime. We find that in the case of extra dimension a number of different processes give rise to effectively the same mono-top final state, and a great gain can be obtained in the sensitivity for this channel. We show that the mono-top search can explore top-partner masses up to 630 GeV and 300 GeV for the 3rd generation extra dimensional model and the minimal fermionic top-partner model, respectively, at the high luminosity LHC.

Precision tests and fine tuning in twin Higgs models

We analyze the parametric structure of Twin Higgs (TH) theories and assess the gain in fine tuning which they enable compared to extensions of the Standard Model with colored top partners. Estimates show that, at least in the simplest realizations of the TH idea, the separation between the mass of new colored particles and the electroweak scale is controlled by the coupling strength of the underlying UV theory, and that a parametric gain is achieved only for strongly-coupled dynamics. Motivated by this consideration we focus on one of these simple realizations, namely composite TH theories, and study how well such constructions can reproduce electroweak precision data. The most important effect of the Twin states is found to be the infrared contribution to the Higgs quartic coupling, while direct corrections to electroweak observables are sub-leading and negligible. We perform a careful fit to the electroweak data including the leading-logarithmic corrections to the Higgs quartic up to three loops. Our analysis shows that agreement with electroweak precision tests can be achieved with only a moderate amount of tuning, in the range 5-10\%, in theories where colored states have mass of order 3-5 TeV and are thus out of reach of the LHC. For these levels of tuning, larger masses are excluded by a perturbativity bound, which makes these theories possibly discoverable, hence falsifiable, only at a future 100 TeV collider.

Charged composite scalar dark matter

We consider a composite model where both the Higgs and a complex scalar χ, which is the dark matter (DM) candidate, arise as light pseudo Nambu-Goldstone bosons (pNGBs) from a strongly coupled sector with TeV scale confinement. The global symmetry structure is SO(7)/SO(6), and the DM is charged under an exact U(1)DM ⊂ SO(6) that ensures its stability. Depending on whether the χ shift symmetry is respected or broken by the coupling of the top quark to the strong sector, the DM can be much lighter than the Higgs or have a weak-scale mass. Here we focus primarily on the latter possibility. We introduce the lowest-lying composite resonances and impose calculability of the scalar potential via generalized Weinberg sum rules. Compared to previous analyses of pNGB DM, the computation of the relic density is improved by fully accounting for the effects of the fermionic top partners. This plays a crucial role in relaxing the tension with the current DM direct detection constraints. The spectrum of resonances contains exotic top partners charged under the U(1)DM, whose LHC phenomenology is analyzed. We identify a region of parameters with f = 1.4 TeV and 200 GeV ≲ mχ ≲ 400 GeV that satisfies all existing bounds. This DM candidate will be tested by XENON1T in the near future.

Search for single production of the vector-like top partner at the 14 TeV LHC

The new heavy vector-like top partner (T) is one of typical features of many new physics models beyond the standard model. In this paper we study the discovery potential of the LHC for the vector-like T-quark both in the leptonic Trightarrow bW and Trightarrow t_mathrm{lep}Z_mathrm{lep} (trilepton) channels at sqrt{s}= 14 TeV in the single production mode. Our analysis is based on a simplified model including a SU(2)_L singlet with charge 2/3 with only two free parameters, namely the TWb coupling parameter g^{*} and the top-partner mass m_T. The 2sigma exclusion limits, 3sigma evidence and the 5sigma discovery reach in the parameter plane of g^{*}-m_T, are, respectively, obtained for some typical integrated luminosity at the 14 TeV LHC. Finally we analyze the projected sensitivity in terms of the production cross section times branching fraction for two decay channel.

Higgs-precision constraints on colored naturalness

The presence of weak-scale colored top partners is among the simplest solutions to the Higgs hierarchy problem and allows for a natural electroweak scale. We examine the constraints on generic colored top partners coming solely from their effect on the production and decay rates of the observed Higgs with a mass of 125 GeV. We use the latest Higgs precision data from the Tevatron and the LHC as of EPS 2017 to derive the current limits on spin-0, spin-1/2, and spin-1 colored top partners. We also investigate the expected sensitivity from the Run 3 and Run 4 of the LHC, as well from possible future electron-positron and proton-proton colliders, including the ILC, CEPC, FCC-ee, and FCC-hh. We discuss constraints on top partners in the Minimal Supersymmetric Standard Model and Little Higgs theories. We also consider various model-building aspects — multiple top partners, modified couplings between the Higgs and Standard-Model particles, and non-Standard-Model Higgs sectors — and evaluate how these weaken the current limits and expected sensitivities. By modifying other Standard-Model Higgs couplings, we find that the best way to hide low-mass top partners from current data is through modifications of the top-Yukawa coupling, although future measurements of top-quark-pair production in association with a Higgs will extensively probe this possibility. We also demonstrate that models with multiple top partners can generically avoid current and future Higgs precision measurements. Nevertheless, some of the model parameter space can be probed with precision measurements at future electron-positron colliders of, for example, the e+e− → Zhcrosssection.

Testing naturalness at 100 TeV

Solutions to the electroweak hierarchy problem typically introduce a new symmetry to stabilize the quadratic ultraviolet sensitivity in the self-energy of the Higgs boson. The new symmetry is either broken softly or collectively, as for example in supersymmetric and little Higgs theories. At low energies such theories contain naturalness partners of the Standard Model fields which are responsible for canceling the quadratic divergence in the squared Higgs mass. Post the discovery of any partner-like particles, we propose to test the aforementioned cancellation by measuring relevant Higgs couplings. Using the fermionic top partners in little Higgs theories as an illustration, we construct a simplified model for naturalness and initiate a study on testing naturalness. After electroweak symmetry breaking, naturalness in the top sector requires aT = − λt2 at leading order, where λt and aT are the Higgs couplings to a pair of top quarks and top partners, respectively. Using a multivariate method of Boosted Decision Tree to tag boosted particles in the Standard Model, we show that, with a luminosity of 30 ab−1 at a 100 TeV pp-collider, naturalness could be tested with a precision of 10% for a top partner mass up to 2.5 TeV.

A new evidence from the effects of Russia’s WTO accession on its foreign trade

The purpose of this paper is to analyze the effects of Russia’s WTO accession on Russia’s foreign trade policy with top 40 trading partners. The paper conducts a panel data estimation based on a gravity model for bilateral trade patterns in agricultural and industrial commodities between Russia and 40 trading partners during two separate periods: 2000–2011 and 2012–2015. The paper arrives at two key conclusions. First, economic size, trade remoteness and bilateral exchange rate have a stronger effect on the agricultural trade flows between Russia and its trading partners before the WTO accession of Russia, while economic size, trade remoteness and trade openness show a stronger impact on agricultural trade flows after Russia’s accession to the WTO. Second, economic size, trade remoteness and bilateral exchange rate have a stronger effect on the industrial trade flows between Russia and its top trading partners before the WTO accession, while economic size, trade remoteness and trade openness show a stronger impact on industrial trade flows after the accession. To the best of the author's knowledge, no study has looked at the Russia’s foreign trade with its top trading partners before and after WTO accession of Russia.

Search for a heavy resonance decaying to a top quark and a vector-like top quark at sqrt{s}=13 TeV

A search is presented for massive spin-1 Z′ resonances decaying to a top quark and a heavy vector-like top quark partner T. The search is based on a 2.6 fb−1 sample of proton-proton collisions at 13 TeV collected with the CMS detector at the LHC. The analysis is optimized for final states in which the T quark decays to a W boson and a bottom quark. The focus is on all-jet final states in which both the W boson and the top quark decay into quarks that evolve into jets. The decay products of the top quark and of the W boson are assumed to be highly Lorentz-boosted and cannot be reconstructed as separate jets, but are instead reconstructed as merged, wide jets. Techniques for the identification of jet substructure and jet flavour are used to distinguish signal from background events. Several models for Z′ bosons decaying to T quarks are excluded at 95% confidence level, with upper limits on the cross section ranging from 0.13 to 10 pb, depending on the chosen hypotheses. This is the first search for a neutral spin-1 heavy resonance decaying to a top quark and a vector-like T quark in the all-hadronic final state.

Understanding Canada's International Trade under the Trans-Pacific Partnership: Lessons from Gravity Redux

This article applies the traditional gravity model of trade to explore the effects of the Trans-Pacific Partnership (TPP) in the context of Canada and, subsequently, draws on the theoretical gravity framework incorporating trade costs derived by Anderson and van Wincoop (2003). It provides a decomposition of the growth of Canada's trade flow with each of its top trading partners. The growth of Canada's bilateral trade with the prospective TPP member countries can be largely attributed to output growth, whereas a vast majority of trade growth with the non-member countries is associated with a reduction in trade costs. Moreover, the elimination of trade barriers after putting into effect free trade agreements considerably diminishes trade costs and raises bilateral trade.