Vector-like Quarks Research Articles

On the Impact of the LHC Run 2 Data on General Composite Higgs Scenarios

We study the impact of Run 2 LHC data on general composite Higgs scenarios, where nonlinear effects, mixing with additional scalars, and new fermionic degrees of freedom could simultaneously contribute to the modification of Higgs properties. We obtain new experimental limits on the scale of compositeness, the mixing with singlets and doublets with the Higgs, and the mass and mixing angle of top-partners. We also show that for scenarios where new fermionic degrees of freedom are involved in electroweak symmetry breaking, there is an interesting interplay among Higgs coupling measurements, boosted Higgs properties, SMEFT global analyses, and direct searches for single and double production of vector-like quarks.

Vector boson dark matter from trinification

We show how trinification models based on the gauge group SU(3)C× SU(3)L× SU(3)R realized near the TeV scale can provide naturally a variety of dark matter (DM) candidates. These models contain a discrete T parity which may remain unbroken even after spontaneous symmetry breaking. The lightest T-odd particle, which could be a fermion, a scalar, or a gauge boson, can constitute the dark matter of the universe. This framework naturally admits a doublet-singlet fermionic DM, a singlet scalar DM, or a vector boson DM. Here we develop the vector boson DM scenario wherein the DM couples off-diagonally with the usual fermions and vector-like fermions present in the theory. We show consistency of this framework with dark matter relic abundance and direct detection limits as well as LHC constraints. We derive upper limits of 900 GeV on the vector gauge boson DM mass and 4.5 TeV on the vector-like quark masses. We also show the consistency of spontaneous gauge symmetry breaking down to Standard Model times an extra U(1) while preserving the T-parity.

Single production of vectorlike B quarks at the CLIC

The vector-like quarks are predicted in many new physics scenarios beyond the Standard Model~(SM) and could be seen potential signatures of new physics at the TeV energy scale. In this work, we study single production of exotic singlet and doublet vectorlike bottom quarks (VLQ-$B$) at future Compact Linear Collider~(CLIC) via the process $e^{+}e^{-}\to B\bar{b}$ with the decay channel $B\to bZ$ and two types of modes: $Z\to \ell^{+}\ell^{-}$ and $Z\to \nu\bar{\nu}$. We calculate the cross sections of signal and relevant SM backgrounds. After a fast simulation of the signal and background events, the exclusion limit at 95\% confidence level and $5\sigma$ discovery prospects on the parameters (the coupling strength $\kappa_{B}$ and the VLQ-$B$ mass) have been, respectively, presented at the future CLIC with centre of mass energy $\sqrt{s}=3$ TeV and integrated luminosity of 5~ab$^{-1}$.

Collider signatures of vector-like fermions from a flavor symmetric model

We propose a model with two Higgs doublets and several SU(2) scalar singlets with a global non-Abelian flavor symmetry {mathcal{Q}}_6times {mathcal{Z}}_2 . This discrete group accounts for the observed pattern of fermion masses and mixing angles after spontaneous symmetry breaking. In this scenario only the third generation of fermions get their masses as in the Standard Model (SM). The masses of the remaining fermions are generated through a seesaw-like mechanism. To that end, the matter content of the model is enlarged by introducing electrically charged vector-like fermions (VLFs), right handed Majorana neutrinos and several SM scalar singlets. Here we study the processes involving VLFs that are within the reach of the Large Hadron Collider (LHC). We perform collider studies for vector-like leptons (VLLs) and vector-like quarks (VLQs), focusing on double production channels for both cases, while for VLLs single production topologies are also included. Utilizing genetic algorithms for neural network optimization, we determine the statistical significance for a hypothetical discovery at future LHC runs. In particular, we show that we can not safely exclude VLLs for masses greater than 200 GeV. For VLQ’s in our model, we show that we can probe their masses up to 3.8 TeV, if we take only into account the high-luminosity phase of the LHC. Considering Run-III luminosities, we can also exclude VLQs for masses up to 3.4 TeV. We also show how the model with predicted VLL masses accommodates the muon anomalous magnetic moment.

Renormalizable models of flavor-specific scalars

New light singlet scalars with flavor-specific couplings represent a phenomenologically distinctive and flavor-safe alternative to the well-studied possibility of Higgs-portal scalars. However, in contrast to the Higgs portal, flavor-specific couplings require an ultraviolet completion involving new heavy states charged under the Standard Model gauge symmetries, leading to a host of additional novel phenomena. Focusing for concreteness on a scenario with up quark-specific couplings, we investigate two simple renormalizable completions, one with an additional vector-like quark and another featuring an extra scalar doublet. We consider the implications of naturalness, flavor- and CP-violation, electroweak precision observables, and direct searches for the new states at the LHC. These bounds, while being model-dependent, are shown to probe interesting regions in the parameter space of the scalar mass and its low-energy effective coupling, complementing the essential phenomenology of the low-energy effective theory at a variety of low and medium energy experiments.

Impact of dimension-eight SMEFT contributions: A case study

The use of the SMEFT Lagrangian to quantify possible Beyond the Standard Model (BSM) effects is standard in LHC and future collider studies. One of the usual assumptions is to truncate the expansion with the dimension-$6$ operators. The numerical impact of the next terms in the series, the dimension-$8$ operators, is unknown in general. We consider a specific BSM model containing a charge-${2 3}$ heavy vector-like quark and compute the operators generated at dimension-$8$. The numerical effects of these operators are studied for the $t\bar{t}h$ process, where they contribute at tree level and we find effects at the ${\cal{O}}(0.5-2\%)$ level for allowed values of the parameters.

Spontaneously stabilised dark matter from a fermiophobic U(1)\u2032 gauge symmetry

We consider the possibility that dark matter is stabilised by a discrete Z2 symmetry which arises from a subgroup of a U(1)′ gauge symmetry, spontaneously broken by integer charged scalars, and under which the chiral quarks and leptons do not carry any charges. A chiral fermion χ with half-integer charge is odd under the preserved Z2, and hence becomes a stable dark matter candidate, being produced through couplings to right-handed neutrinos with vector-like U(1)′ charges, as in the type Ib seesaw mechanism. We calculate the relic abundance in such a low energy effective seesaw model containing few parameters, then consider a high energy renormalisable model with a complete fourth family of vector-like fermions, where the chiral quark and lepton masses arise from a seesaw-like mechanism. With the inclusion of the fourth family, the lightest vector-like quark can contribute to the dark matter production, enlarging the allowed parameter space that we explore.

Flavor constraints for a vector-like quark of Nelson-Barr type

The Nelson-Barr (NB) mechanism to solve the strong CP problem assumes CP conservation, arranges vanishing overline{theta} at tree-level and requires vector-like quarks (VLQs) to transmit the CP breaking to the SM. We analyze the flavor constraints coming from the presence of one such down type VLQ of NB type by performing a global fit on the relevant flavor observables. A comparison is made to the case of one generic VLQ. We find that the allowed parameter space for the VLQ Yukawa couplings and the mixing to the SM are confined to a region much smaller than in the generic case, making the NB case falsifiable in principle.

Single production of vectorlike Y quarks at the HL-LHC

We study single production of exotic vectorlike Y quark with electric charge |QY|=4/3 and its subsequent decay at the High Luminosity LHC (HL-LHC). Most of the vector like quark (VLQ) decays have the electroweak W bosons in the intermediate state. Besides their direct productions singly or pairs, the W-bosons are involved in decay chains as a result of the decay of a top quark which contributes to the background. This is particularly the case since vectorlike Y quark, which is estimated to be produced with a high cross-section, can only decay via a W boson and a down type quark (d,s,b). We calculate the cross sections of signal (for different couplings and mass values) and relevant Standard Model (SM) backgrounds. After a fast simulation of signal and background events, estimations of the sensitivity to the parameters (mass range 1000-2500 GeV for coupling value κY=0.5, and mass range 500-2000 GeV for coupling values κY=0.3 and κY=0.15) have been presented at the HL-LHC with centre of mass energy s=14 TeV and integrated luminosity projections of 300 fb−1, 1000 fb−1 and 3000 fb−1.

TeV-scale vector leptoquark from Pati-Salam unification with vectorlike families

In this paper, we show an explicit way to realize a TeV-scale vector leptoquark from the Pati-Salam (PS) unification with extra vectorlike families. The leptoquark mass is constrained to be heavier than PeV-scale by the measurement of a flavor violating kaon decay, $K_L \to \mu e$, in conventional models. This strong constraint can be avoided by introducing vectorlike families consistently with the quark and lepton masses and CKM and PMNS matrices. The other flavor violating processes are also suppressed. In this model, the vector leptoquark can be sufficiently light to explain the recent $b\to s\mu\mu$ anomaly, while the $b\to c\tau\nu$ anomaly is difficult to be explained due to the strong constraints from the $Z^\prime$ boson and vectorlike quark searches at the LHC. When the $b\to s\mu\mu$ anomaly is explained, we show that $\mathcal{O}({0.2})$ % tuning is required in the fermion matrix, the future experiments in $\mu\to e\gamma$ and $\mu$-$e$ conversions will cover the most available parameter space, and sizable neutral meson mixings, induced by the extra Higgs doublets, are unavoidable.

Are the CKM anomalies induced by vector-like quarks? Limits from flavor changing and Standard Model precision tests

Recent high precision determinations of Vus and Vud indicate towards anomalies in the first row of the CKM matrix. Namely, determination of Vud from beta decays and of Vus from kaon decays imply a violation of first row unitarity at about 3σ level. Moreover, there is tension between determinations of Vus obtained from leptonic Kμ2 and semileptonic Kℓ3 kaon decays. These discrepancies can be explained if there exist extra vector-like quarks at the TeV scale, which have large enough mixings with the lighter quarks. In particular, extra vector-like weak singlets quarks can be thought as a solution to the CKM unitarity problem and an extra vector-like weak doublet can in principle resolve all tensions. The implications of this kind of mixings are examined against the flavour changing phenomena and SM precision tests. We consider separately the effects of an extra down-type isosinglet, up-type isosinglet and an isodoublet containing extra quarks of both up and down type, and determine available parameter spaces for each case. We find that the experimental constraints on flavor changing phenomena become more stringent with larger masses, so that the extra species should have masses no more than few TeV. Moreover, only one type of extra multiplet cannot entirely explain all the discrepancies, and some their combination is required, e.g. two species of isodoublet, or one isodoublet and one (up or down type) isosinglet. We show that these scenarios are testable with future experiments. Namely, if extra vector-like quarks are responsible for CKM anomalies, then at least one of them should be found at scale of few TeV, and anomalous weak isospin violating Z-boson couplings with light quarks should be detected if the experimental precision on Z hadronic decay rate is improved by a factor of 2 or so.

Vector-like quarks decaying into singly and doubly charged bosons at LHC

We investigate the production of vector-like quarks with charge 5/3 at the LHC and their subsequent decays into new singly or doubly charged bosons plus a heavy quark (top or bottom). In particular, we explore final states with same-sign di-leptons (electron or muon pairs), with the leptons coming from the decay of the new bosons and, in the case of production of singly charged bosons, from top quarks as well. These processes are predicted by classes of models based on extensions of the gauge group of the Standard Model, such as the 331 Model, where the electroweak symmetry is described by SU(3)L × U(1)X, X being a new abelian charge. For this purpose, a CMS search for vector-like partners with charge 5/3 decaying into Wt is recast to obtain model-independent bounds and projected reaches at future luminosity stages of the LHC. The results are then interpreted as mass bounds for the new particles predicted in the 331 Model and as a constraint on the scale of its spontaneous symmetry breaking. The complete set of model-independent results are provided as recast efficiencies, to allow for reinterpretation in different scenarios.

High-quality axions in solutions to the μ problem

Solutions to the $\ensuremath{\mu}$ problem in supersymmetry based on the Kim-Nilles mechanism naturally feature a Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) axion with a decay constant of the order of the geometric mean of the Planck and TeV scales, consistent with astrophysical limits. We investigate minimal models of this type with two gauge-singlet fields that break a Peccei-Quinn symmetry and extensions with an extra vectorlike quark and lepton supermultiplets consistent with gauge coupling unification. We show that there are many anomaly-free discrete symmetries, depending on the vectorlike matter content, that protect the Peccei-Quinn symmetry to sufficiently high order to solve the strong $CP$ problem. We study the axion couplings in this class of models. Models of this type that are automatically free of the domain wall problem require at least one pair of strongly interacting vectorlike multiplets with mass at the intermediate scale and predict axion couplings that are greatly enhanced compared to the minimal supersymmetric DFSZ models, putting them within reach of proposed axion searches.

Single production of vector-like quarks: the effects of large width, interference and NLO corrections

We provide a comprehensive discussion, together with a complete setup for simulations, relevant for the production of a single vector-like quark at hadron colliders. Our predictions include finite width effects, signal-background interference effects and next-to-leading order QCD corrections. We explicitly apply the framework to study the single production of a vector-like quark T with charge 2/3, but the same procedure can be used to analyse the single production of vector-like quarks with charge −4/3, −1/3, 2/3 and 5/3, when the vector-like quark interacts with the Standard Model quarks and electroweak bosons. Moreover, this procedure can be straightforwardly extended to include additional interactions with exotic particles. We provide quantitative results for representative benchmark scenarios characterised by the T mass and width, and we determine the role of the interference terms for a range of masses and widths of phenomenological significance. We additionally describe in detail, both analytically and numerically, a striking feature in the invariant mass distribution appearing only in the T → th channel.

Looking for a vectorlike B quark at the LHC using jet substructure

Vectorlike quarks have been shown to resolve certain long-standing discrepancies pertaining to the bottom sector. We investigate, here, the prospects of identifying the existence of a topless vectorlike doublet $(B,~Y)$, as is preferred by the electroweak precision measurements. Concentrating on single production, $viz.$ $B \bar b$ with $B \to b + Z/H$ subsequently, we find that the fully hadronic decay-channel is susceptible to discovery provided jet substructure observables are used. At the 13 TeV LHC with an integrated luminosity of 300 fb$^{-1}$, a modest value of the chromomagnetic transition moments allows for the exclusion of $M \lesssim 1.8(2.2)$ TeV in the $Z$ and $H$ channels respectively.

Tau-jet signatures of vectorlike quark decays to heavy charged and neutral Higgs bosons

We study 4b + 2τ and 4b + 1τ signatures of heavy neutral and charged Higgs bosons originating from cascade decays of pair-produced new quarks. Decays of vectorlike quarks through heavy Higgses can easily dominate in the two Higgs doublet model of type-II, and the studied signatures are common to many possible decay chains. We design search strategies for these final states and discuss the mass ranges of heavy Higgs bosons and new quarks that can be explored at the Large Hadron Collider as functions of branching ratios in a model independent way. We further combine the results with a similar study focusing on decays which lead to a 6b final state and interpret the sensitivity to charged and neutral Higgs bosons and vectorlike quarks in the type-II two Higgs doublet model. We find that the LHC reach for their masses extends to well above 2 TeV in the case of an SU(2) doublet quark and to at least 1.8 TeV for a bottom-like SU(2) singlet quark in the whole range of tan β between 1 and 50.

Dark matter, Z′, and vector-like quark at the LHC and b → sμμ anomaly * *Supported by the National Natural Science Foundation of China (11975013, 11775025), the Natural Science Foundation of Shandong province (ZR2017JL002, ZR2017MA004) and the Project of Shandong Province Higher Educational Science and Technology Program (2019KJJ007)

Combining the anomaly and dark matter observables, we study the capability of the LHC to test dark matter, , and a vector-like quark. We focus on a local model with a vector-like doublet quark Q and a complex singlet scalar whose lightest component is a candidate of dark matter. After imposing relevant constraints, we find that the anomaly and the relic abundance of dark matter favor GeV and GeV for 2 TeV and 2 TeV (the heavy partner of ). Current searches for jets and missing transverse momentum at the LHC sizably reduce the mass ranges of the vector-like quark, and is required to be larger than 1.7 TeV. Finally, we discuss the possibility of probing these new particles at the high luminosity LHC via the QCD process followed by , , and then . Taking a benchmark point of = 1.93 TeV, GeV, and 145 GeV, we perform a detailed Monte Carlo simulation and find that this benchmark point can be accessed at the 14 TeV LHC with an integrated luminosity of 3000 fb .

Combined Explanation of the Z→bb[over ¯] Forward-Backward Asymmetry, the Cabibbo Angle Anomaly, and τ→μνν and b→sℓ^{+}ℓ^{-} Data.

In this Letter, we propose a simple model that can provide a combined explanation of the Z→bb[over ¯] forward-backward asymmetry, the Cabibbo angle anomaly (CAA), τ→μνν and b→sℓ^{+}ℓ^{-} data. This model is obtained by extending the standard model (SM) by two heavy vectorlike quarks (an SU(2)_{L} doublet (singlet) with hypercharge -5/6 (-1/3), two new scalars (a neutral and a singly charged one), and a gauged L_{μ}-L_{τ} symmetry. The mixing of the new quarks with the SM ones, after electroweak symmetry breaking, does not only explain Z→bb[over ¯] data, but also generates a lepton flavor universal contribution to b→sℓ^{+}ℓ^{-} transitions. Together with the lepton flavor universality violating effect, generated by loop-induced Z^{'} penguins involving the charged scalar and the heavy quarks, it gives an excellent fit to data (6.1σ better than the SM). Furthermore, the charged scalar (neutral vector) gives a necessarily constructive tree-level (loop) effect in μ→eνν (τ→μνν), which can naturally account for the CAA (Br[τ→μνν]/Br[τ→eνν] and Br[τ→μνν]/Br[μ→eνν]).

Super-soft CP violation

Solutions of the Strong CP Problem based on the spontaneous breaking of CP must feature a non-generic structure and simultaneously explain a coincidence between a priori unrelated CP-even and CP-odd mass scales. We show that these properties can emerge from gauge invariance and a CP-conserving, but otherwise generic, physics at the Planck scale. In our scenarios no fundamental scalar is introduced beyond the Standard Model Higgs doublet, and CP is broken at naturally small scales by a confining non-abelian dynamics. This approach is remarkably predictive: robustness against uncontrollable UV corrections to the QCD topological angle requires one or more families of vector-like quarks below a few 10’s of TeV, hence potentially accessible at colliders. Because CP violation is communicated to the SM at these super-soft scales, our solution of the Strong CP Problem is not spoiled by the presence of heavy new states motivated by other puzzles in physics beyond the Standard Model. In addition, these models generically predict a dark sector that may lead to interesting cosmological signatures.

Discovering the origin of Yukawa couplings at the LHC with a singlet Higgs and vector-like quarks

Although the 125 GeV Higgs boson discovered at the LHC is often heralded as the origin of mass, it may not in fact be the origin of Yukawa couplings. In alternative models, Yukawa couplings may instead arise from a seesaw type mechanism involving the mixing of Standard Model (SM) chiral fermions with new vector-like fermions, controlled by the vacuum expectation value (VEV) of a new complex Higgs singlet field 〈Φ〉. For example, the largest third family (t, b) quark Yukawa couplings may be forbidden by a U(1)′ gauge or global symmetry, broken by 〈Φ〉, and generated effectively via mixing with a vector-like fourth family quark doublet (T, B). Such theories predict a new physical Higgs singlet ϕ, which we refer to as the Yukon, resulting from 〈Φ〉, in the same way that the Higgs boson h0 results from 〈H〉. In a simplified model we discuss the prospects for discovering the Yukon ϕ in gluon-gluon fusion production, with (t, b) and (T, B) quarks in the loops, and decaying in the channels ϕ → γγ, Zγ and ϕ → tT → tth0, ttZ. The potential for discovery of the Yukon ϕ is studied at present or future hadron colliders such as the LHC (Run 3), HL-LHC, HE-LHC and/or FCC. For example, we find that a 300–350 GeV Yukon ϕ could be accessed at LHC Run 3 in the di-photon channel in the global model, providing a smoking gun signature of the origin of Yukawa couplings. The tth0, ttZ channels are more involved and warrant a more sophisticated analysis.