Search For Quarks Research Articles

Correlating the CDF W-mass shift with the muon g − 2 and the b → sℓ+ℓ− transitions

Motivated by the latest CDF W-mass measurement as well as the muon g−2 anomaly and the discrepancies observed in b→sℓ+ℓ− transitions, we propose an extension of the Standard Model (SM) with the SU(2)L-singlet vector-like fermion partners that are featured by additional U(1)′ gauge symmetry. The fermion partners have the same SM quantum numbers as of the right-handed SM fermions, and can therefore mix with the latter after the electroweak and the U(1)′ symmetry breaking. As a result, desirable loop-level corrections to the (g−2)μ, the W-boson mass mW and the Wilson coefficient C9 in b→sμ+μ− transitions can be obtained. The final allowed parameter space is also consistent with the constraints from the Z→μ+μ− decay, the neutrino trident production and the LHC direct searches for the vector-like quarks and leptons.

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.

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 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 the channel with at hadron colliders* *Supported by the National Natural Science Foundation of China (NNSFC) (11705048), the National Research Project Cultivation Foundation of Henan Normal University (2020PL16, 2021PL10), the Startup Foundation for Doctors of Henan Normal University (qd18115) and also powered by the High Performance Computing Center of Henan Normal University

Based on a simplified model including a singlet vector-like top quark T with charge |Q|=, we analyze the prospects of observing T via the single T production in the channel with Z decaying to neutrinos at the hadron-hadron colliders. This simplified model only includes two free parameters, the coupling constant and the T quark mass . To investigate the observability of the single T production, we perform a detailed background analysis and detector simulation for the collision energies 14 TeV, 27 TeV, and 100 TeV. We scan the parameter space and show the exclusion and discovery capabilities on the T quark with the highest integrated luminosity designed at these colliders. Moreover, the limits from the narrow-width approximation and electroweak precision observables are considered.

Constraints on Z′ solutions to the flavor anomalies with trans-Planckian asymptotic safety

Motivated by the flavor anomalies in b → s transitions, we embed minimal models with a Z′ gauge boson, vector-like fermions, and a singlet scalar in the framework of trans-Planckian asymptotic safety. The presence of a fixed point in the renormalization group flow of the models’ parameters leads to predictions for the γ/Z′ kinetic mixing, the New Physics Yukawa couplings, and the quartic couplings of the scalar potential. We derive the constraint on the kinetic mixing from the most recent high-mass dilepton resonance searches at the LHC, showing that this bound is often inescapable in this framework, unless the U(1) charges conspire to forbid the radiative generation of kinetic mixing. In the latter case, the parameter space consistent with the flavor anomalies can still be probed in depth by direct LHC searches for heavy vector-like quarks and leptons. We derive the current exclusion bounds and projections for future high-luminosity runs.

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.

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.

Search for excited quarks of light and heavy flavor in γ + jet final states in proton–proton collisions at [formula omitted

A search is presented for excited quarks of light and heavy flavor that decay to γ+jet final states. The analysis is based on data corresponding to an integrated luminosity of 35.9fb−1 collected by the CMS experiment in proton–proton collisions at s=13TeV at the LHC. A signal would appear as a resonant contribution to the invariant mass spectrum of the γ+jet system, above the background expected from standard model processes. No resonant excess is found, and upper limits are set on the product of the excited quark cross section and its branching fraction as a function of its mass. These are the most stringent limits to date in the γ+jet final state, and exclude excited light quarks with masses below 5.5 TeV and excited b quarks with masses below 1.8 TeV, assuming standard model like coupling strengths.

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.

Optimizing event selection with the random grid search

The random grid search (RGS) is a simple, but efficient, stochastic algorithm to find optimal cuts that was developed in the context of the search for the top quark at Fermilab in the mid-1990s. The algorithm, and associated code, have been enhanced recently with the introduction of two new cut types, one of which has been successfully used in searches for supersymmetry at the Large Hadron Collider. The RGS optimization algorithm is described along with the recent developments, which are illustrated with two examples from particle physics. One explores the optimization of the selection of vector boson fusion events in the four-lepton decay mode of the Higgs boson and the other optimizes SUSY searches using boosted objects and the razor variables. Program summaryProgram title: Random Grid SearchProgram Files doi:http://dx.doi.org/10.17632/mpcrnd7xb4.1Licensing provisions: GNU General Public License 3 (GPL)Programming language: c++, pythonNature of problem: We address the problem of scanning a large number of thresholds (cuts) on discriminating variables in order to find ones that maximize some measure of the degree of discrimination between classes of objects, for example, between signal and background events at the Large Hadron Collider (LHC).Solution method: The cuts searched are determined by the distribution of the objects that are the focus of an analysis. For example, if one is searching for supersymmetric events at the LHC, the cuts are determined by the predicted distribution of the variables that discriminate between the supersymmetric signal and the standard model background. In effect, we search for cuts using importance sampling determined by the signal distribution, thereby mitigating the curse of dimensionality.Additional comments including restrictions and unusual features: For cases with exceptionally large numbers of events, the program may take several hours to run. However, the system can be trivially parallelized, with no change to the program, by splitting large files into N smaller files and running the same set of cuts over the N files. The counts associated with each cut can then be summed over the N files.