Vector-like Quarks Research Articles

Heavy vector-like quarks decaying to exotic scalars: a case study with triplets

We investigate the pair production of a vector-like quark triplet with hypercharge 5/3 decaying into top quark and a complex scalar triplet with hypercharge 1 at the LHC. This novel scenario, featuring particles with exotic charges — two quarks with charge 8/3 and 5/3 and a scalar with charge 2 — serves as a unique window to models based on the framework of partial compositeness, where these particles naturally emerge as bound states around the TeV scale. Leveraging on the LHC data we establish exclusion limits on the masses of the vector-like quark and the scalar triplet. Subsequently, we design an analysis strategy aimed at improving sensitivity in the region which is still allowed. Our analysis focuses on two specific regions in the parameter space: the first entails a large mass gap between the vector-like quarks and the scalars, so that the vector-like quarks can decay into the scalars; the second involves a small mass gap, such that this decay is forbidden. To simplify the parameter space, both vector-like quarks and scalars are assumed to be degenerate or almost degenerate within the triplets, such that chain decays between fermions and scalars are suppressed. As a result, we found that final states characterized by a same-sign lepton pair, multiple jets, and high net transverse momentum (i.e. effective mass) will play a pivotal role to unveil this model and, more in general, models characterised by multiple vector-like quarks around the same mass scale during the high luminosity LHC phase.

Pair production of the singlet vector-like B quark at the CLIC

Vector-like quarks (VLQs) are a common feature of many scenarios of new physics beyond the Standard Model (SM), which generally decay into a SM third-generation quark with a SM gauge boson or a Higgs boson. The presence of a new exotic decay mode of VLQs will reduce the branching ratios of these standard decay modes and thus relax the current mass exclusion limits from LHC experiments. Based on a model-independent framework, we investigate the prospect of discovering the pair production of the weak-singlet VLQ-B at the future 3-TeV Compact Linear Collider (CLIC), by focusing on the final states including one Z boson and four b-jets via two types of modes: Z→ℓ+ℓ-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z\\rightarrow \\ell ^{+}\\ell ^{-}$$\\end{document} and Z→νν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z\\rightarrow \ u {\\bar{\ u }}$$\\end{document}. By performing a rapid detector simulation of the signal and background events, and considering the initial state radiation and beamstrahlung effects, the exclusion limit at the 95% confidence level and the 5σ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$5\\sigma $$\\end{document} discovery prospects are respectively obtained on the branching ratio of B→bZ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B\\rightarrow bZ$$\\end{document} and the VLQ-B masses at the future 3-TeV CLIC with an integrated luminosity of 5 ab-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document}.

A model with vectorlike fermions and U(1)X symmetry: CKM unitarity, b → s transitions, and prospect at Belle II

The updated analysis of the LHCb Collaboration on the lepton flavor violation suggests that the new physics should couple to muons and electrons with comparable magnitudes, resulting in the anomalies in both rare decay channels, b→sμ+μ− and b→se+e−. Meanwhile, the recent result of the Muon g−2 experiment with higher precision has increased the existing tension with the standard model prediction. In this paper, we consider an extension of the standard model with a new sector consisting of vectorlike fermions and two scalar charged under an extra U(1)X gauge symmetry. The exotic Yukawa interactions in this model lead to the quark mixing responsible for the additional contributions to the flavor changing neutral currents in B-meson decays, and solve the muon g−2 discrepancy. We derive the analytic expression of the new physics contributions to the Wilson coefficient C7 in the effective Hamiltonian, and point out that the CKM unitarity violation can be explained within this context. By calculating the branching ratio of the inclusive radiative B decay, the impact of current experimental data of the b→sγ transition on the model and the future prospect at the Belle II experiment are investigated. Taking into account the current data on the muon anomalous magnetic moment, the CKM unitarity violation, the constraints on the flavor observables relevant to the b→s transitions, the LHC searches for vectorlike quarks, and the perturbation limits of the couplings, the viable parameter regions of the model are identified.

On the importance of three-body decays of vector-like quarks

It is a common feature of vector-like extensions of the electroweak sector to have near degenerate states, such as electroweak doublets. In simplified models, it is usually assumed that these have decay widths saturated by two-body channels. As a consequence, experimental searches can be done focusing on only one of the states of the doublet. Taking as an example case the light exotic electroweak doublet present in the Minimal Composite Higgs Model, we show that including three-body decays in the pair production process makes this separation unfeasible, since both states of the doublet will be present and contribute significantly to the signal. In addition, by recasting present searches in multileptonic channels, with a simplified cut-and-count analysis, a relevant increase in discovery reach or exclusion potential is obtained; this indeed motivates a more detailed analysis. This study shows how an inclusive search strategy, taking into account both the near degeneracy and the presence of three-body decays, will have greater discovery power and be more natural from a model building perspective.

A simulated search for the singlet vector-like bottom quark decaying into tW at high energy pp colliders

In a framework of simplified model, we perform a simulated search for the singlet vector-like bottom quark B decaying into tW with t hadronic decay and W leptonic decay at the 14 TeV high-luminosity LHC (HL-LHC), 27 TeV high energy LHC (HE-LHC) and 100 TeV Future Circular Hadron Collider (FCC-hh). In this work, we focus on the case that the B only couples with the third generation quarks of the Standard Model. We make a detailed detector simulations of the signal and backgrounds and obtain the 2sigma exclusion limit and 5sigma discovery prospects. Our results show that the upcoming HL-LHC can exclude (discover) the m_B to 2100 (1900) GeV for a typical coupling strength g^{*}=0.2. For the HE-LHC (FCC-hh), this exclusion capability on the B quark mass can be expanded to more than 3000 (5000) GeV.

Search for the signal of vector-like bottom quark at LHeC in the final state with 3 b-jets

Vector-like quarks arise in many new theories beyond the Standard Model, which is driven by several theoretical issues. In a framework of the Standard Model simply extended by an SU(2) singlet vector-like bottom quark, we present a study of single production of such vector-like bottom quark at the LHeC. The work is preformed with assuming the LHeC with center of mass energy s=1.98 TeV and an integrated luminosity L=1000fb−1. We focus on the process e−p→νeB→νebH(orZ)→νebbb¯. The signal is searched in events with three b-jets within final state. Exclusion (discover) capability of the LHeC for the signal of the vector-like bottom quark are obtained at 2 σ (5 σ) confidence level. Our result demonstrates a significant improvement compared to previous relevant researches. We also discuss the capability of LHeC for the singlet B search with combined analysis from other experiments.

Search for single production of vector-like T quarks decaying into Ht or Zt in pp collisions at sqrt{s} = 13 TeV with the ATLAS detector

This paper describes a search for the single production of an up-type vector-like quark (T) decaying as T → Ht or T → Zt. The search utilises a dataset of pp collisions at sqrt{s} = 13 TeV collected with the ATLAS detector during the 2015–2018 data-taking period of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb−1. Data are analysed in final states containing a single lepton with multiple jets and b-jets. The presence of boosted heavy resonances in the event is exploited to discriminate the signal from the Standard Model background. No significant excess above the Standard Model expectation is observed, and 95% CL upper limits are set on the production cross section of T quarks in different decay channels. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength (κ) of the vector-like quark. For singlet T quarks, κ values above 0.53 are excluded for all masses below 2.3 TeV. At a mass of 1.6 TeV, κ values as low as 0.35 are excluded. For T quarks in the doublet scenario, where the production cross section is much lower, κ values above 0.72 are excluded for all masses below 1.7 TeV, and this exclusion is extended to κ above 0.55 for low masses around 1.0 TeV.

Cabibbo angle anomalies and oblique corrections: The remarkable role of the vectorlike quark doublet

Cabibbo angle anomalies and oblique corrections: The remarkable role of the vectorlike quark doublet

Search for pair-produced vector-like top and bottom partners in events with large missing transverse momentum in pp collisions with the ATLAS detector

A search for pair-produced vector-like quarks using events with exactly one lepton (e or mu ), at least four jets including at least one b-tagged jet, and large missing transverse momentum is presented. Data from proton–proton collisions at a centre-of-mass energy of sqrt{s}=13 text {TeV}, recorded by the ATLAS detector at the LHC from 2015 to 2018 and corresponding to an integrated luminosity of 139 fb^{-1}, are analysed. Vector-like partners T and B of the top and bottom quarks are considered, as is a vector-like X with charge +5/3, assuming their decay into a W, Z, or Higgs boson and a third-generation quark. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section of T and B quark pairs as a function of their mass are derived for various decay branching ratio scenarios. The strongest lower limits on the masses are 1.59 text {TeV} assuming mass-degenerate vector-like quarks and branching ratios corresponding to the weak-isospin doublet model, and 1.47 text {TeV} (1.46 text {TeV}) for exclusive T rightarrow Zt (B/X rightarrow Wt) decays. In addition, lower limits on the T and B quark masses are derived for all possible branching ratios.

Search for the singlet vector-like top quark in T → tZ channel with [formula omitted] at the 14 TeV LHC

The vector-like top quark T is an important new fermion beyond the Standard Model. In the framework of a simplified model, we study the possibility of detecting the T through the single production of T decaying into tZ with Z decaying to neutrinos at the 14 TeV Large Hadron Collider (LHC). This simplified model includes three free parameters, that is the coupling constant g⁎, the T quark mass mT and the mixing parameter RL. In order to investigate the observability of the single T production, we make a detailed detector simulation and scan the parameter space. Corresponding to an integrated luminosity of 3000 fb−1, the correlated regions g⁎∈[0.14,0.5]([0.2,0.5]) with mT∈[1200GeV,1900GeV]([1200GeV,1660GeV]) can be excluded (discovered) for RL=0 and the correlated regions g⁎∈[0.1,0.5] with mT∈[1600GeV,2520GeV]([1320GeV,2220GeV]) can be excluded (discovered) for RL=0.5.

Search for resonant and nonresonant production of pairs of dijet resonances in proton-proton collisions at sqrt{s} = 13 TeV

A search for pairs of dijet resonances with the same mass is conducted in final states with at least four jets. Results are presented separately for the case where the four jet production proceeds via an intermediate resonant state and for nonresonant production. The search uses a data sample corresponding to an integrated luminosity of 138 fb−1 collected by the CMS detector in proton-proton collisions at sqrt{s} = 13 TeV. Model-independent limits, at 95% confidence level, are reported on the production cross section of four-jet and dijet resonances. These first LHC limits on resonant pair production of dijet resonances via high mass intermediate states are applied to a signal model of diquarks that decay into pairs of vector-like quarks, excluding diquark masses below 7.6 TeV for a particular model scenario. There are two events in the tails of the distributions, each with a four-jet mass of 8 TeV and an average dijet mass of 2 TeV, resulting in local and global significances of 3.9 and 1.6 standard deviations, respectively, if interpreted as a signal. The nonresonant search excludes pair production of top squarks with masses between 0.50 TeV to 0.77 TeV, with the exception of a small interval between 0.52 and 0.58 TeV, for supersymmetric R-parity-violating decays to quark pairs, significantly extending previous limits. Here, the most significant excess above the predicted background occurs at an average dijet mass of 0.95 TeV, for which the local and global significances are 3.6 and 2.5 standard deviations, respectively.

W boson mass and grand unification via the type-II seesaw-like mechanism

We propose an SU(5) GUT model extended with two additional pairs of 10 representation vector-like fermions. The CDF collaboration W boson mass anomaly is explained by using the VEV of a real SU(2)L triplet scalar coming from the 24 representation Higgs. The vector-like fermions are decomposed partly into vector-like quark doublets. Those vector-like quark doublets acquire mass from two sources; through the Yukawa interaction with the real SU(2)L triplet via a type-II seesaw-like mechanism. And, they acquire mass from the 24 representation Higgs. We assume that the mass for the vector-like quark doublets is expressed in terms of the real triplets mass. By combining the constraints on the vector-like quark masses with those on the heavy Higgs boson masses, we can obtain the narrow allowed mass ranges for the vector-like quark doublet and the real triplet. Therefore, our model can be tested in searches for these particles in the near future. In addition, the two additional pairs of vector-like fermions allow the SM gauge couplings to unify successfully at MGUT≈5.1×1015 GeV. Our model is also testable by the future Hyper-Kamiokande experiment via the proton decay lifetime τp(p→π0e+)<1.0×1035 years.

Search for pair production of vector-like quarks in leptonic final states in proton-proton collisions at sqrt{s} = 13 TeV

A search is presented for vector-like T and B quark-antiquark pairs produced in proton-proton collisions at a center-of-mass energy of 13 TeV. Data were collected by the CMS experiment at the CERN LHC in 2016–2018, with an integrated luminosity of 138 fb−1. Events are separated into single-lepton, same-sign charge dilepton, and multi-lepton channels. In the analysis of the single-lepton channel a multilayer neural network and jet identification techniques are employed to select signal events, while the same-sign dilepton and multilepton channels rely on the high-energy signature of the signal to distinguish it from standard model backgrounds. The data are consistent with standard model background predictions, and the production of vector-like quark pairs is excluded at 95% confidence level for T quark masses up to 1.54 TeV and B quark masses up to 1.56 TeV, depending on the branching fractions assumed, with maximal sensitivity to decay modes that include multiple top quarks. The limits obtained in this search are the strongest limits to date for textrm{T}overline{textrm{T}} production, excluding masses below 1.48 TeV for all decays to third generation quarks, and are the strongest limits to date for textrm{B}overline{textrm{B}} production with B quark decays to tW.

Search for single production of vector-like top partner T → H+b and [formula omitted] at the LHC Run-III

While experimental data has not ruled out the possibility of additional Higgs bosons or gauge sectors, several alternative models have been proposed to go beyond the standard model and tackle the question of hierarchy. These models predict the existence of heavy vector-like partner quarks that exhibit vector-axial (V-A) coupling, typically on the TeV scale. In this work, we use simplified interactions to establish the fundamental components of the model and explore potential scenarios beyond the standard model. We focus on the unusual decays of the partner heavy quark, called T, into H±b, which may compete with W±b decays and create a new discovery channel at the Large Hadron Collider (LHC). Using Monte Carlo (MC) simulations, we analyze the signal-to-noise ratio of pp→qg→T+bb¯j→H+bb¯j→W+bb¯j→1ℓ+4b+1j+E̸T, and evaluate the sensitivity of the LHC to the masses of T and H± in the two-Higgs-doublet model (2HDM) plus the vector-like quark (VLQ) model with a branching ratio of 100% for T→H±b. We take into account current and predicted luminosities, as well as theoretical and experimental bounds. This paper presents a new strategy for identifying VLQs at the LHC, which goes beyond the usual search strategies that rely on decays of T into Standard Model bosons.

Exploring maverick top partner decays at the LHC

In this work, we have considered an extension of the standard model (SM) with a $SU(2{)}_{L}$ singlet vectorlike quark (VLQ) with electric charge $Q=+2/3$. The model also contains an additional local $U(1{)}_{d}$ symmetry group and the corresponding gauge boson is the dark photon. The VLQ is charged while all the SM particles are neutral under the new $U(1{)}_{d}$ gauge group. Even though in this model the VLQ possesses many properties qualitatively similar to that of the traditional top partner (${T}_{p}$), there are some compelling differences as well. In particular, its branching ratio to the traditional modes (${T}_{p}\ensuremath{\rightarrow}bW,tZ,th$) are suppressed which in turn helps to evade many of the existing bounds, mainly coming from the LHC experiments. In an earlier work, such a VLQ is referred to as ``maverick top partner''. It has been shown that the top partner in this model predominantly decays to a top quark and a dark photon/dark Higgs pair (${T}_{p}\ensuremath{\rightarrow}t{\ensuremath{\gamma}}_{d},t{h}_{d}$) over a large region of the parameter space. The dark photon can be made invisible and consequently, it gives rise to the missing transverse energy (${\overline{)E}}_{T}$) signature at the LHC detector. We have mainly focused on the LHC signatures and future prospects of such top partners. In particular, we have studied the $t\overline{t}+{\overline{)E}}_{T}$ and $t+{\overline{)E}}_{T}$ signatures in the context of the LHC via pair and single productions of the top partner, respectively at 13 and 14 TeV LHC center of mass energies assuming that the dark photon either decays into an invisible mode or it is invisible at the length scale of the detector. We have shown that one can exclude $\mathrm{sin}{\ensuremath{\theta}}_{L}\ensuremath{\sim}0.025$ (0.05) for ${m}_{{T}_{p}}\ensuremath{\le}2.0\text{ }(2.6)\text{ }\text{ }\mathrm{TeV}$ at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$ with an integrated luminosity of $3\text{ }\text{ }{\mathrm{ab}}^{\ensuremath{-}1}$ using the single top partner production channel.

Search for pair-production of vector-like quarks in pp collision events at [formula omitted] TeV with at least one leptonically decaying Z boson and a third-generation quark with the ATLAS detector

A search for the pair-production of vector-like quarks optimized for decays into a Z boson and a third-generation Standard Model quark is presented, using the full Run 2 dataset corresponding to 139fb−1 of pp collisions at s=13TeV, collected in 2015–2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a Z boson with high transverse momentum, reconstructed from a pair of same-flavour leptons with opposite-sign charges, as well as by the presence of b-tagged jets and high-transverse-momentum large-radius jets reconstructed from calibrated smaller-radius jets. Events with exactly two or at least three leptons are used, which are further categorized by the presence of boosted W, Z, and Higgs bosons and top quarks. The categorization is performed using a neural-network-based boosted object tagger to enhance the sensitivity to signal relative to the background. No significant excess above the background expectation is observed and exclusion limits at 95% confidence level are set on the masses of the vector-like partners T and B of the top and bottom quarks, respectively. The limits depend on the branching ratio configurations and, in the case of 100% branching ratio for T→Zt and 100% branching ratio for B→Zb, this search sets the most stringent limits to date, allowing mT>1.60TeV and mB>1.42TeV, respectively.

Machine learning-enhanced search for a vectorlike singlet B quark decaying to a singlet scalar or pseudoscalar

The presence of a new decay mode relaxes the current mass exclusion limits on vectorlike quarks considerably. We consider the case of a weak-singlet vectorlike $B$ quark that can decay to a singlet scalar or pseudoscalar $\Phi$. In an earlier paper [A. Bhardwaj et al., Roadmap to explore vectorlike quarks decaying to a new scalar or pseudoscalar, Phys. Rev. D, 106 (2022) 095014; arXiv:2203.13753], we mapped the possibilities to explore such setups at the LHC. We showed that it is possible for a $B$ quark to decay into $\Phi$ and the $\Phi$ to dominantly decay to a pair of gluons or $b$ quark(s) without fine-tuning the parameters. In this paper, we present a collider search strategy to look for the pair production of singlet $B$ quarks. If both $B$ quarks decay into $b\Phi$ pairs, the final state is fully hadronic: $B{B}\to(b\Phi)({b}\Phi)\to (bgg)({b}gg)/(bb{b})({b}b{b})$, which is very challenging to probe. Therefore, we consider a simpler mixed decay specific to the singlet $B$ case, $BB\to(b\Phi)(tW)$ with a lepton in the final state, to achieve the best sensitivity at the high-luminosity LHC. We use a deep neural network with a weighted cross-entropy loss to separate the signal from the huge SM background. Our analysis shows that large areas of the $M_{B}-M_{\Phi}$ parameter space are discoverable through this signature. We show how the discovery and exclusion regions scale with the branching ratio in the new decay mode. We also estimate the reach in the inclusive monolepton channel with the same network model.

Accommodating the CDF W -boson mass measurement in the beautiful mirror model

The W-boson mass measurement recently reported by the CDF II experiment exhibits a significant deviation from both the Standard Model prediction and previous measurements. There is also a long-standing deviation between the Standard Model prediction of the forward-backward asymmetry of the bottom quark ($A^{0,b}_{\rm FB}$) and its measurement at the LEP experiment. The Beautiful Mirror model, proposed to resolve the $A^{0,b}_{\rm FB}$ discrepancy, introduces vector-like quarks that modify the W-boson mass at one-loop level. In this study, we find an interesting region in the model parameter space that could potentially explain both discrepancies, which puts the new quarks in the multi-TeV region. This region is mostly consistent with current LHC bounds from direct searches and Higgs coupling measurements, but will be thoroughly probed at the High Luminosity LHC. As such, the Beautiful Mirror model as an explanation of the $m_W$ and $A^{0,b}_{\rm FB}$ discrepancies could be confirmed or falsified in the near future.

Scalar leptoquark and vector-like quark extended models as the explanation of the muon g–2 anomaly: bottom partner chiral enhancement case* *Supported by an appointment to the Young Scientist Training Program at the APCTP through the Science and Technology Promotion Fund and Lottery Fund of the Korean Government. This work was also supported by the Korean Local Governments-Gyeongsangbuk-do Province and Pohang

Leptoquark (LQ) models are well motivated solutions to the anomaly. In the minimal LQ models, only specific representations can lead to chiral enhancements. For the scalar LQs, and can lead to the top quark chiral enhancement. For the vector LQs, and can lead to the bottom quark chiral enhancement. When we consider the LQ and vector-like quark (VLQ) simultaneously, there can be more scenarios. In our previous study, we considered the scalar LQ and VLQ extended models with up-type quark chiral enhancement. Here, we study the scalar LQ and VLQ extended models with down-type quark chiral enhancement. We find two new models with B quark chiral enhancements, which originate from the bottom and bottom partner mixing. Then, we propose new LQ and VLQ search channels under the constraints of .

Top partners and scalar dark matter: A nonminimal reappraisal

A large set of models beyond the Standard Model of particle physics suggest that the top quark plays a special role in fundamental interactions. At the same time some of these models predict that a particle responsible for dark matter is present in the physical spectrum of the theory, and in particular cases dark matter may be specially linked to the top sector. Such a new physics configuration occurs, for instance, in composite frameworks that additionally feature vector-like quarks as mediators of the interactions of the dark matter with the Standard Model. We investigate the interplay of top-philic dark matter and the presence of several vector-like top partners. Such a non-minimal setup with several vector-like fermions is typical of composite scenarios, and we make use of an effective model to study to which extent such non-minimality modifies the collider and cosmological phenomenology of the minimal model, the latter only including one vector-like mediator of the dark matter interactions with the Standard Model.