Searches For Vector-like Quarks Research Articles

Global analysis and LHC study of a vectorlike extension of the standard model with extra scalars

We perform a global analysis of a vectorlike extension of the Standard Model, which also features additional doublet and singlet scalars. The usual Yukawa interactions are forbidden in this setup by an extra U(1) global symmetry and the masses of the second and third family quarks and leptons are generated via the mixing with the vectorlike sector. We identify three best-fit benchmark scenarios which satisfy the constraints imposed by the stability of the scalar potential, the perturbativity of the coupling constants, the measurement of the muon anomalous magnetic moment and the nonobservation of the flavor violating tau decays. We show that dominant contributions to the muon (g−2) originate in this model from the charged Higgs/neutral lepton one-loop diagrams, thus correcting an inaccurate statement than can be found in the literature. We also perform a detailed LHC analysis of the benchmark scenarios. We investigate the experimental constraints stemming from direct searches for vectorlike quarks, vectorlike leptons, and exotic scalars. While we show that the model is not currently tested by any collider experiment, we point out that decays of a heavy Higgs boson into two tau leptons may offer a smoking gun signature for the model verification in upcoming runs at the LHC. Published by the American Physical Society 2024

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.

Looking beyond the Standard Model with Third Generation Quarks at the LHC

The Large Hadron Collider (LHC) is at the frontier of collider physics today, probing new physics at unprecedented energy scales. Many theories of physics beyond the Standard Model seek to elucidate the underlying mechanism of electroweak symmetry breaking. Given their large Yukawa couplings to the Higgs boson, third generations quarks of the Standard Model, and especially the top quark, play a key role in such theories. Therefore, new particles predicted by these theories often couple preferentially to top and bottom quarks. The favoured coupling to third generation can also be used to explain recently observed flavour physics anomalies in the LHCb, Babar or Belle experiments. This article will review recent searches for new physics performed by the ATLAS and CMS experiments at the LHC, in final states containing top and bottom quarks. In particular, searches for vector-like quarks, leptoquarks, and heavy scalar and gauge bosons will be discussed.

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.

Kinetic mixing and portal matter phenomenology

Dark photons (DP) are interesting as potential mediators between the dark matter (DM) sector and the fields of the Standard Model (SM). The interaction of the DP, described by a broken $U(1)_D$ gauge symmetry, with the SM is usually generated at the one-loop level via kinetic mixing through the existence of portal matter (PM), here assumed to be fermionic, which carries both a dark charge as well as a SM $U(1)_Y$ hypercharge. For theoretical consistency, as well as for many phenomenological reasons, this PM must be vector-like with respect to the SM and dark gauge groups and, in particular, is shown to be allowed only to transform as vector-like copies of the usual SM fields. The dark Higgs that is responsible for the breaking of $U(1)_D$ can then generate a mixing between the PM and SM fields with the same electric charge thus altering the DP interactions with (at least some of) the SM fields and also providing a path for the PM fields to decay. In this paper we briefly explore the phenomenology of some specific simple models of this PM including, for the case where the PM is leptonic in nature, their potential impact on experiments probing low energy parity-violation and the g-2 of the muon. In the case of color-triplet, bottom quark-like PM, their direct pair- and single-production at the LHC is shown to be observable in final states that include missing $E_T$ and/or very highly boosted lepton-jets together with pairs of high $p_T$ b-jets that can be used to trigger on such events. These signatures are quite distinct from those usually employed in the search for vector-like quarks at the LHC and, furthermore, we demonstrate that the conventional signal channels for vector-like quarks involving the SM Higgs and gauge fields are essentially closed in the case of PM.

Interplay between collider searches for vector-like quarks and dark matter searches in composite Higgs models

We review the interplay between collider searches for vector-like quarks and dark matter direct detection experiments in composite Higgs models. With focus on a future 100 TeV collider, we also extend previous analyses to improve the reach for heavy quarks in the low mass region.

Search for vector-like quarks in events with two oppositely charged leptons and jets in proton\u2013proton collisions at sqrt{s} = 13,text {Te}text {V}

A search for the pair production of heavy vector-like partners mathrm {T} and mathrm {B} of the top and bottom quarks has been performed by the CMS experiment at the CERN LHC using proton–proton collisions at sqrt{s} = 13,text {Te}text {V} . The data sample was collected in 2016 and corresponds to an integrated luminosity of 35.9,text {fb}^{-1}. Final states studied for mathrm {T} overline{mathrm {T}} production include those where one of the mathrm {T} quarks decays via mathrm {T} rightarrow mathrm {t}mathrm {Z} and the other via mathrm {T} rightarrow mathrm {b}mathrm {W}, mathrm {t}mathrm {Z}, or mathrm {t}mathrm {H} , where mathrm {H} is a Higgs boson. For the mathrm {B} overline{mathrm {B}} case, final states include those where one of the mathrm {B} quarks decays via mathrm {B} rightarrow mathrm {b}mathrm {Z} and the other mathrm {B} rightarrow mathrm {t}mathrm {W}, mathrm {b}mathrm {Z}, or mathrm {b}mathrm {H} . Events with two oppositely charged electrons or muons, consistent with coming from the decay of a mathrm {Z} boson, and jets are investigated. The number of observed events is consistent with standard model background estimations. Lower limits at 95% confidence level are placed on the masses of the mathrm {T} and mathrm {B} quarks for a range of branching fractions. Assuming 100% branching fractions for mathrm {T} rightarrow mathrm {t}mathrm {Z}, and mathrm {B} rightarrow mathrm {b}mathrm {Z}, mathrm {T} and mathrm {B} quark mass values below 1280 and 1130,text {Ge}text {V}, respectively, are excluded.

Modelling top partner-vector resonance phenomenology

We have analysed the observable consequences of the interactions of spin-1 resonances coupled to the invariant fermionic currents that arise in an SO(5) Composite Higgs set-up. The phenomenology entailed by such interactions is thoroughly analysed by studying heavy vector resonances production and decay modes in the viable resonance mass range. Additionally, the production of double and single-composite fermion final states has been scanned along the fermion mass scale. Such production is mediated by the SM gauge and Higgs interactions, and also by charged and neutral vector resonances. The coupling between the new fermions and vector resonances induces a sizeable effect in the production rates. We use the recent 13 TeV LHC searches for vector-like quarks to constrain our parameter space. Specifically, we explore the allowed regions by analysing the decays of a heavy vector-like quark in the Wb-channel. We conclude that generically the impact of the couplings between the spin-1 and spin-1/2 resonances will substantially reduce the permitted regions, leading us to test the sensitivity of the parametric dependence in the light of exotic matter interactions.

Search for pair and single production of vectorlike quarks in final states with at least one Z boson decaying into a pair of electrons or muons in pp collision data collected with the ATLAS detector at s=13 TeV

A search for vectorlike quarks is presented, which targets their decay into a Z boson and a third-generation Standard Model quark. In the case of a vectorlike quark T (B) with charge +2/3e (-1/3e), the decay searched for is T→Zt (B→Zb). Data for this analysis were taken during 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb-1 of pp collisions at s=13 TeV. The final state used is characterized by the presence of b-tagged jets, as well as a Z boson with high transverse momentum, which is reconstructed from a pair of opposite-sign same-flavor leptons. Pair and single production of vectorlike quarks are both taken into account and are each searched for using optimized dileptonic exclusive and trileptonic inclusive event selections. In these selections, the high scalar sum of jet transverse momenta, the presence of high-transverse-momentum large-radius jets, as well as - in the case of the single-production selections - the presence of forward jets are used. No significant excess over the background-only hypothesis is found and exclusion limits at 95% confidence level allow masses of vectorlike quarks of mT>1030 GeV (mT>1210 GeV) and mB>1010 GeV (mB>1140 GeV) in the singlet (doublet) model. In the case of 100% branching ratio for T→Zt (B→Zb), the limits are mT>1340 GeV (mB>1220 GeV). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vectorlike quark masses. © 2018 CERN.

Dark matter search in nucleon, pion, and electron channels from a proton beam dump with MiniBooNE

A search for vectorlike quarks is presented, which targets their decay into a $Z$ boson and a third-generation Standard Model quark. In the case of a vectorlike quark $T$ ($B$) with charge $+2/3e$ ($-1/3e$), the decay searched for is $T \rightarrow Zt$ ($B \rightarrow Zb$). Data for this analysis were taken during 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV. The final state used is characterized by the presence of $b$-tagged jets, as well as a $Z$ boson with high transverse momentum, which is reconstructed from a pair of opposite-sign same-flavor leptons. Pair and single production of vectorlike quarks are both taken into account and are each searched for using optimized dileptonic exclusive and trileptonic inclusive event selections. In these selections, the high scalar sum of jet transverse momenta, the presence of high-transverse-momentum large-radius jets, as well as - in the case of the single-production selections - the presence of forward jets are used. No significant excess over the background-only hypothesis is found and exclusion limits at 95% confidence level allow masses of vectorlike quarks of $m_T > 1030$ GeV ($m_T > 1210$ GeV) and $m_B > 1010$ GeV ($m_B > 1140$ GeV) in the singlet (doublet) model. In the case of 100% branching ratio for $T\rightarrow Zt$ ($B\rightarrow Zb$), the limits are $m_T > 1340$ GeV ($m_B > 1220$ GeV). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vectorlike quark masses.

Search for vector-like quarks in a fermionic dark matter model with pseudoscalar: A resonance case

We propose a minimal model of a fermionic dark matter with a pseudoscalar mediator and N generation of vector-like quarks. We calculate the relic density and obtain a new constraint on the generation of the vector-like quarks. From the viewpoint of phenomenology, we probe the proposed model via direct and indirect approaches. Finally, as an illustrative example, we evaluate a resonance case — the subject of major experiments designed for detection of new particles. Our analysis results in significant constraints on the coupling strength of vector-like quarks.

Searches for vector-like quarks at future colliders and implications for composite Higgs models with dark matter

Many composite Higgs models predict the existence of vector-like quarks with masses outside the reach of the LHC, e.g. mQ ≳ 2 TeV, in particular if these models contain a dark matter candidate. In such models the mass of the new resonances is bounded from above to satisfy the constraint from the observed relic density. We therefore develop new strategies to search for vector-like quarks at a future 100 TeV collider and evaluate what masses and interactions can be probed. We find that masses as large as ∼ 6.4 (∼9) TeV can be tested if the fermionic resonances decay into Standard Model (dark matter) particles. We also discuss the complementarity of dark matter searches, showing that most of the parameter space can be closed. On balance, this study motivates further the consideration of a higher-energy hadron collider for a next generation of facilities.

Direct bounds on heavy toplike quarks with standard and exotic decays

Heavy vector-like quarks with electric charge $Q=2/3$ (also called \textit{heavy tops}) appear naturally in many extensions of the Standard Model. Although these typically predict the existence of further particles below the TeV scale, direct searches for heavy tops have been performed assuming that they decay only into SM particles. The aim of this paper is to overcome this situation. We consider the most constraining experimental LHC searches for vector-like quarks, including analyses of the 36 fb$^{-1}$ of data collected in the latest run at 13 TeV of center of mass energy, as well as searches sensitive to heavy tops decaying into a new scalar, $S$. Combining all these, we derive bounds for arbitrary values of the heavy top branching ratios. A simple code that automatizes this process is also provided. At the physics level, we demonstrate that bounds on heavy tops are not inevitably weaker in the presence of new light scalars. We find that heavy tops with masses below $\sim 900$ GeV are excluded by direct searches, independently of whether they decay into $Zt, Ht, Wb$ or $St$ (with $S$ giving either missing energy of bottom quarks) or into any combination of them.

Searches for Vector-like Quarks, [formula omitted] and tb resonances with the ATLAS Detector

Various extensions of the Standard Model predict the existence of new types of quarks or bosons. We report on searches for vector-like quarks decaying to a Z boson and a top quark or bottom quark and new resonances decaying to a top-antitop pair and a top-antibottom pair. Several of these searches include the use of boosted top quark reconstruction techniques. These searches use the data sample recorded in 2012 at s=8 TeV centre-of-mass energy by the ATLAS experiment at CERN's Large Hadron Collider. In these searches no evidence for new physics is observed.

Search for the production of single vector-like and excited quarks in the Wt final state in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

A search for vector-like quarks and excited quarks in events containing a top quark and a $W$ boson in the final state is reported here. The search is based on $20.3 {fb}^{-1}$ of proton--proton collision data taken at the LHC at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector. Events with one or two leptons, and one, two or three jets are selected with the additional requirement that at least one jet contains a $b$-quark. Single-lepton events are also required to contain at least one large-radius jet from the hadronic decay of a high-$p_\mathrm{T}$ $W$ boson or a top quark. No significant excess over the expected background is observed and upper limits on the cross-section times branching ratio for different vector-like quark and excited-quark model masses are derived. For the excited-quark production and decay to $Wt$ with unit couplings, quarks with masses below 1500 GeV are excluded and coupling-dependent limits are set.

Direct and indirect signals of natural composite Higgs models

We present a comprehensive numerical analysis of a four-dimensional model with the Higgs as a composite pseudo-Nambu-Goldstone boson that features a calculable Higgs potential and protective custodial and flavour symmetries to reduce electroweak fine-tuning. We employ a novel numerical technique that allows us for the first time to study constraints from radiative electroweak symmetry breaking, Higgs physics, electroweak precision tests, flavour physics, and direct LHC bounds on fermion and vector boson resonances in a single framework. We consider four different flavour symmetries in the composite sector, one of which we show to not be viable anymore in view of strong precision constraints. In the other cases, all constraints can be passed with a sub-percent electroweak fine-tuning. The models can explain the excesses recently observed in $WW$, $WZ$, $Wh$ and $\ell^+\ell^-$ resonance searches by ATLAS and CMS and the anomalies in angular observables and branching ratios of rare semi-leptonic $B$ decays observed by LHCb. Solving the $B$ physics anomalies predicts the presence of a dijet or $t\bar t$ resonance around 1 TeV just below the sensitivity of LHC run 1. We discuss the prospects to probe the models at run 2 of the LHC. As a side product, we identify several gaps in the searches for vector-like quarks at hadron colliders, that could be closed by reanalyzing existing LHC data.

Interpretation of vector-like quark searches: heavy gluons in composite Higgs models

Pair production of new vector-like quarks in pp collisions is considered model independent as it is usually dominated by QCD production. We discuss the interpretation of vector-like quark searches in the case that QCD is not the only relevant production mechanism for the new quarks. In particular we consider the effect of a new massive color octet vector boson with sizeable decay branching ratio into the new quarks. We pay special attention to the sensitivity of the Large Hadron Collider experiments, both in run-1 and early run-2, to differences in the kinematical distributions from the different production mechanisms. We have found that even though there can be significant differences in some kinematical distributions at the parton level, the differences are washed out at the reconstruction level. Thus, the published experimental results can be reinterpreted in models with heavy gluons by simply rescaling the production cross section.

Search for Vector-like Quarks using a Combination of Decay Channels in pp Collisions at $\sqrt{s}$ = 8 TeV Collected with the ATLAS Detector

Search for Vector-like Quarks using a Combination of Decay Channels in pp Collisions at $\sqrt{s}$ = 8 TeV Collected with the ATLAS Detector

Peccei-Quinn symmetry from a gauged discreteRsymmetry

The axion solution to the strong CP problem calls for an explanation as to why the Lagrangian should be invariant under the global Peccei-Quinn symmetry, U(1)_PQ, to such a high degree of accuracy. In this paper, we point out that the U(1)_PQ can indeed survive as an accidental symmetry in the low-energy effective theory, if the standard model gauge group is supplemented by a gauged and discrete R symmetry, Z_N^R, forbidding all dangerous operators that explicitly break the Peccei-Quinn symmetry. In contrast to similar approaches, the requirement that the Z_N^R symmetry be anomaly-free forces us, in general, to extend the supersymmetric standard model by new matter multiplets. Surprisingly, we find a large landscape of viable scenarios that all individually fulfill the current experimental constraints on the QCD vacuum angle as well as on the axion decay constant. In particular, choosing the number of additional multiplets appropriately, the order N of the Z_N^R symmetry can take any integer value larger than 2. This has interesting consequences with respect to possible solutions of the mu problem, collider searches for vector-like quarks and axion dark matter.

Light Higgs and vector-like quarks without prejudice

Light vector-like quarks with non-renormalizable couplings to the Higgs are a common feature of models trying to address the electroweak (EW) hierarchy problem by treating the Higgs as a pseudo-goldstone boson of a global (approximate) symmetry. We systematically investigate the implications of the leading dimension five operators on Higgs phenomenology in presence of dynamical up- and down-type weak singlet as well as weak doublet vector-like quarks. After taking into account constraints from precision EW and flavour observables we show that contrary to the renormalizable models, significant modifications of Higgs properties are still possible and could shed light on the role of vector-like quarks in solutions to the EW hierarchy problem. We also briefly discuss implications of higher dimensional operators for direct vector-like quark searches at the LHC.