Top Quark Flavor Changing Research Articles

Studying Same-Sign Top Pair Production through Top-Higgs FCNC Interactions at the HL-LHC

We investigate the potential of the HL-LHC for discovering new physics effects via the same-sign top pair signatures. We focus on the semileptonic (electron and muon) decay of the top quarks and study the reach for a simplified model approach where top quark flavor changing could occur through a neutral scalar exchange. A relatively smaller background contribution and clean signature are the advantages of the leptonic decay mode of the same-sign W bosons in the same-sign production processes of top quark pairs. Assuming the FCNC between top quark, up-type quark, and scalar boson from the new physics interactions, the branchings could be excluded of the order O 1 0 − 4 . We use angular observables of the same-sign lepton pairs and the top quark kinematics in the process which provide the possibility of separation of new physics signal from the SM backgrounds using machine-learning techniques. We find that the same-sign top quark pair production is quite capable of testing the top-Higgs FCNCs at the HL-LHC.

Large t→cZ as a sign of vectorlike quarks in light of the W mass

The rare flavor-changing top quark decay $t\ensuremath{\rightarrow}cZ$ is a clear sign of new physics and experimentally very interesting due to the huge number of top quarks produced at the LHC. However, there are few (viable) models which can generate a sizable branching ratio for $t\ensuremath{\rightarrow}cZ$---in fact vectorlike quarks seem to be the only realistic option. In this paper, we investigate all three representations (under the Standard Model gauge group) of vectorlike quarks ($U$, ${Q}_{1}$ and ${Q}_{7}$) that can generate a sizable branching ratio for $t\ensuremath{\rightarrow}cZ$ without violating bounds from $B$ physics. Importantly, these are exactly the three vectorlike quarks which can lead to a sizable positive shift in the prediction for $W$ mass, via the couplings to the top quark also needed for a sizable $\mathrm{Br}(t\ensuremath{\rightarrow}cZ)$. Calculating and using the one-loop matching of vectorlike quarks on the Standard Model effective field theory, we find that $\mathrm{Br}(t\ensuremath{\rightarrow}cZ)$ can be of the order of ${10}^{\ensuremath{-}6}$, ${10}^{\ensuremath{-}5}$ and ${10}^{\ensuremath{-}4}$ for $U$, ${Q}_{1}$ and ${Q}_{7}$, respectively, and that in all three cases the large $W$ mass measurement can be accommodated.

Probing the Anomalous tqgamma Couplings in Photon-Proton Collisions

The top quark flavor changing neutral current processes are extremely suppressed within the Standard Model. Nevertheless, they could be enhanced in a new physics model beyond the Standard Model. Investigating the top quark’s flavor changing neutral current interactions at colliders would be an important test in terms of new physics. In this work, we examine the potentials of the processes \({e^{-}} {p} {\rightarrow } {e^{-}} {\gamma } {p} {\rightarrow } {e^{-}} W^{+} b {\gamma }{\rightarrow } {e^{-}} {j} {\bar {j}} b {\gamma }\) and \({e^{-}} {p} {\rightarrow } {e^{-}} {\gamma } {p} {\rightarrow } {e^{-}} W^{+} b {\gamma }{\rightarrow } {e^{-}} \textit {l} {\nu }_{\textit {l}} b {\gamma }\) at the FCC-eh with \(\sqrt {s}=7.08\) and 10.0 TeV to study anomalous tqγ couplings via effective Lagrangians. We obtain 95% confidence level sensitivities on the anomalous coupling parameters at the two FCC-eh energies and various integrated luminosities. We find that our limits are at least two orders of magnitude better than the current LHC experimental results.

Probing FCNC couplings in single top-quark production associated with a neutral gauge boson in future lepton colliders

In this paper, we consider a single top-quark production in association with a neutral gauge boson (Z boson or photon) in the future electron–positron colliders. We utilize these channels to probe the top-quark flavor changing neutral current (FCNC) interactions in tqZ and tqγ vertices. We perform two separate analyses for top-Z-jet and top-γ-jet channels. The standard model effective field theory approach is employed to search for these anomalous couplings. We consider parton showering, hadronization and fast detector simulation in our study and use a cut-and-count technique to separate the signal from the standard model background processes. The upper limits on the FCNC couplings at 95% confidence level for different integrated luminosities are obtained. It is shown that the future lepton collider would be able to probe the FCNC branching fractions to Br(t → qγ) < 10−4 and Br(t → qZ) < 10−4 with 3 ab−1 of integrated luminosity of data at a center of mass energy of 350 GeV.

Flavor changing top decays to charm and a Higgs boson with ττ at the LHC

We investigate the prospects of discovering the top quark decay into a charm quark and a Higgs boson ($t \to c h^0$) in top quark pair production at the CERN Large Hadron Collider (LHC). A general two Higgs doublet model is adopted to study flavor changing neutral Higgs (FCNH) interactions. We perform a parton level analysis as well as Monte Carlo simulations using \textsc{Pythia}~8 and \textsc{Delphes} to study the flavor changing top quark decay $t \to c h^0$, followed by the Higgs decaying into $\tau^+ \tau^-$, with the other top quark decaying to a bottom quark ($b$) and two light jets ($t\to bW\to bjj$). To reduce the physics background to the Higgs signal, only the leptonic decays of tau leptons are used, $\tau^+\tau^- \to e^\pm\mu^\mp +\slashed{E}_T$, where $\slashed{E}_T$ represents the missing transverse energy from the neutrinos. In order to reconstruct the Higgs boson and top quark masses as well as to effectively remove the physics background, the collinear approximation for the highly boosted tau decays is employed. Our analysis suggests that a high energy LHC at $\sqrt{s} = 27$ TeV will be able to discover this FCNH signal with an integrated luminosity $\mathcal{L} = 3$ ab$^{-1}$ for a branching fraction ${\cal B}(t \to ch^0) \agt 1.4 \times 10^{-4}$ that corresponds to a FCNH coupling $|\lambda_{tch}| \agt 0.023$. This FCNH coupling is significantly below the current ATLAS combined upper limit of $|\lambda_{tch}| = 0.064$.

Probing top quark FCNC couplings in the triple-top signal at the high energy LHC and future circular collider

Our main aim in this paper is to present detailed studies to probe the top quark flavor changing neutral current (FCNC) interactions at tqg, tqγ, tqH and tqZ(σμν,γμ) vertices in the triple-top signal pp→tt¯t(t¯tt¯) at the high energy proposal of Large Hadron Collider (HE-LHC) and future circular hadron-hadron collider (FCC-hh). To this end, we investigate the production of three top quarks which arises from the FCNC couplings taken into account the fast simulation at s=27 TeV of HE-LHC and 100 TeV of FCC-hh considering the integrated luminosities of 10, 15 and 20 ab−1. All the relevant backgrounds are considered in a cut based analysis to obtain the limits on the anomalous couplings and the corresponding branching ratios. The obtained exclusion limits on the coupling strengths and the branching ratios are summarized and compared in detail with the results in the literature, namely the most recent direct LHC experimental limits and HL-LHC projections as well. We show that, for the higher energy phase of LHC, a dedicated search for the top quark FCNC couplings can achieve much better sensitivities to the triple-top signal than other top quark production scenarios. We found that the limits for the branching ratios of tqg and tqH transitions could reach an impressive sensitivity and the obtained 95% CL limits are at least three orders of magnitude better than the current LHC experimental results as well as the existing projections of HL-LHC.

Probing the anomaloustqγ couplings through single top production at the future lepton–hadron colliders

The measurements of the top quark flavor changing neutral current interactions are one of the most important goals of the top quark physics program in the present and the future collider experiments. These measurements provide direct information on nonstandard interactions of the top quark. Within the framework of new physics beyond the Standard Model, these interactions can be defined by an effective Lagrangian. In this study, we have investigated the potential of the future [Formula: see text] colliders on the top quark flavor changing neutral current interactions through the subprocesses [Formula: see text], where [Formula: see text]. These subprocesses have been produced through the main reaction [Formula: see text] at the LHC-[Formula: see text], the FCC-[Formula: see text] and the SPPC-[Formula: see text]. For the main reaction, the total cross-sections have been calculated as a function of the anomalous [Formula: see text] couplings. In addition, sensitivities on [Formula: see text] at 95% Confidence Level have been calculated. We obtain that the best constraints on [Formula: see text] are at the order of [Formula: see text] which is four orders of magnitude better than the LHC’s experimental results.

(g − 2)μ versus flavor changing neutral current induced by the light (B − L)μτ boson

We propose the local (B − L)μτ model, which minimally retains the local B − L extension for the sake of neutrino phenomenologies, and at the same time presents an invisible gauge boson Z′ with mass ∼ \U0001d4aa (10) MeV to account for the discrepancy of the muon anomalous magnetic moment. However such a scenario is challenged by flavor physics. To accommodate the correct pattern of Cabibbo-Kobayashi-Maskawa matrix, we have to introduce either a SU(2)L doublet flavon or vector-like quarks plus a singlet flavon. In either case Z′ induces flavor changing neutral current (FCNC) in the quark sector at tree-level. We find that the former scheme cannot naturally suppress the FCNC from the down-type quark sector and thus requires a large fine-tuning to avoid the stringent K → πν overline{v} bound. Whereas the latter scheme, in which FCNC merely arises in the up-type quark sector, is still free of strong constraint. In particular, it opens a new window to test our scenario by searching for flavor-changing top quark decay mode t → u/c+(invisible), and the typical branching ratio ∼ \U0001d4aa (10−4).

Probing top quark FCNC tqγ and tqZ couplings at future electron-proton colliders

The top quark flavor changing neutral current (FCNC) processes are extremely suppressed within the Standard Model (SM) of particle physics. However, they could be enhanced in a new physics model Beyond the Standard Model (BSM). The top quark FCNC interactions would be a good test of new physics at present and future colliders. Within the framework of the BSM models, these interactions can be described by an effective Lagrangian. In this work, we study tqγ and tqZ effective FCNC interaction vertices through the process e−p→e−Wq+X at future electron proton colliders, projected as Large Hadron electron Collider (LHeC) and Future Circular Collider-hadron electron (FCC-he). The cross sections for the signal have been calculated for different values of parameters λq for tqγ vertices and κq for tqZ vertices. Taking into account the relevant background we estimate the attainable range of signal parameters as a function of the integrated luminosity and present contour plots of couplings for different significance levels including detector simulation. We find the sensitivities to the branching ratios (BR(t→qγ)=7.5×10−6, BR(t→qZ)=3.5×10−5) and (BR(t→qγ)=8.5×10−7, BR(t→qZ)=6.0×10−6) for an integrated luminosity of 2ab−1 at LHeC and FCC-he, respectively.

$t \rightarrow cg$ and $t \rightarrow cZ$ in universal extra-dimensional models

In this work we perform a complete one-loop calculation of the flavor-changing top quark decays $t\to cg$ and $t\to cZ$ in the universal extra dimensional models. We find that the branching ratios of these decays in the minimal scenario remain unaltered from the Standard Model expectations for currently allowed values of the inverse compactification radius. In the non-minimal setup, the branching ratios can be enhanced from the Standard Model due to the presence of boundary localized terms which modify the mass spectrum and couplings in the theory in a non-trivial way. We also check the compatibility of the parameter choices that result in enhancements in these rare decays with other experimental observations.

Top quark flavor changing neutral currents and dipole moments through three and four top-quark productions at the LHC

In this paper, the top quark flavor changing neutral current (FCNC) interaction is studied through the process of the three-top quark production at the LHC for center-of-mass energy of 14 TeV. We also investigate the anomalous top quark chromoelectric and chromomagnetic dipole moments through the four top-quarks production signal at the center-of-mass energy of 13 TeV. We demonstrate that these processes are powerful tools to constrain the top quark FCNC couplings as well as the top dipole moments.

Same-sign top pair plus W production in flavor changing vector and scalar models

We investigate the prospect of the LHC for discovering new physics effects via new strategies in the same-sign top pair and same-sign top pair associated with a $W$ boson signatures. Significant enhancement in production of same-sign top quarks (plus a $W$ boson) is a joint property of several models beyond the standard model. We concentrate on the leptonic (electron and muon) decay of the top quarks and study the exclusion reach of the LHC data for a simplified model approach where top quark flavor changing could occur through a $Z'$ or a neutral scalar $\phi$ exchange. Less background contributions and clean signature are the advantages of the leptonic decay mode of the top quarks in the same-sign production processes. A combination is performed on both same-sign top pair and same-sign top pair plus a $W$ boson production modes which enables us to reach a large fraction of the model parameter space. Assuming the couplings of new physics of the order of $10^{-2}$, the mass of a flavor changing $Z'$ or a neutral scalar above 1 TeV could be excluded. We propose a momentum dependent charge asymmetry and angular observables in the same-sign top process which provide the possibility of separation of new physics signal from the SM backgrounds as well as discrimination of the flavor changing $tuX$ from $tcX$, where $X = Z',\phi$.

Photon initiated single top quark production via flavor-changing neutral currents at the LHC

Single top quark production is a powerful process to search for new physics signs. In this work we propose and investigate a search for top quark flavor changing neutral currents (FCNC) via a photon using direct single top quark production events in proton-proton collisions at the LHC at CERN. We show that the direct single top quark final state can provide constraints on the strengths of tq$\gamma$ (top-quark-$\gamma$) and tqg (top-quark-gluon) FCNC couplings simultaneously. Results of a search for direct single top quark production at the LHC at a center-of-mass energy of 8 TeV performed by the ATLAS collaboration are used to set first experimental limits on the anomalous FCNC top decay branching fractions ${\cal B}$(t$\rightarrow$ u$\gamma$) $<$ 0.05% and ${\cal B}$(t$\rightarrow$ c$\gamma$) $<$ 0.14% via direct single top quark production. Finally, the sensitivity of the proposed channel for probing the tq$\gamma$ couplings at 13 TeV is presented.

Constraints on top quark flavor changing neutral currents using diphoton events at the LHC

In this paper we show that the diphoton mass spectrum in proton–proton collisions at the LHC is sensitive to the top quark flavor changing neutral current in the vertices of tuγ and tcγ. The diphoton mass spectrum measured by the CMS experiment at the LHC at a center-of-mass energy of 8 TeV and an integrated luminosity of 19.5 fb−1 is used as an example to set limits on these FCNC couplings. It is also shown that the angular distribution of the diphotons is sensitive to anomalous tuγ and tcγ couplings and it is a powerful tool to probe any value of the branching fraction of top quark rare decay to an up-type quark plus a photon down to the order of 10−4.

Search for Top Quark Flavor-Changing Neutral Currents at CMS

We present searches for the top quark flavor-changing neutral current (FCNC) interactions by the CMS experiment. The FCNC searches have been performed in the vertices of tqZ, tqg, and tqγ in decay and production processes. The results are based on the data collected from proton proton collisions at the LHC at centre-of-mass energies of 7 and 8 TeV.

Direct and indirect searches for top-Higgs FCNC couplings

Large top quark flavor changing through neutral currents is expected by many extensions of the standard model. Direct and indirect searches for flavor changing neutral currents (FCNC) in the top quark decays to an up type quark (up,charm) and a Higgs boson are presented. We probe the observability of the top-Higgs FCNC couplings through the process e$^-$e$^+ \rightarrow t (\rightarrow \ell \nu_{\ell} b) \,\, \bar{t} (\rightarrow q H)$, where $\ell$ = e, $\mu$ and $q$ reflects up and charm quarks. It is shown that the branching ratio $Br(t\rightarrow qH)$ can be probed down to $1.12\times 10^{-3}$ at $95\%$ C.L. at the center-of-mass energy of 500 GeV with the integrated luminosity of 3000 fb$^{-1}$. We also update the constraint on the top-Higgs FCNC coupling using the electroweak precision observables related to $Z\rightarrow c\bar{c}$ decay.

Exploring top quark FCNC within 2HDM type III in association with flavor physics

The top quark flavor changing neutral current (FCNC) process is an excellent probe to search for new physics in top sector since the Standard Model expectation is extremely suppressed. We explore Higgs-mediated top quark FCNC, focusing on $H$-$t$-$c$ Yukawa coupling $\lambda_{ct}$ within the general two Higgs doublet model. After electroweak symmetry breaking the top quark FCNC couplings are included in the charged Higgs Yukawa sector so that they contribute to various processes in flavor physics. To probe $\lambda_{ct}$, we study anomalous single top production and the same sign top pair production at the LHC in association with flavor physics from the tree-level processes $B\to D^{(*)}\tau\nu$, $B\to \tau \nu$ as well as from the loop-level processes $B_d \to X_s \gamma$, $B_{d,s}-{\overline B}_{d,s}$ mixing. We perform combined analysis of all the constraints regarding the fine-tuning argument to fit the data and discuss future prospect. The recently updated measurements on $B\to D^{(*)}\tau\nu$ still prefer large $\lambda_{ct}$, but we show that the current bound on the same sign top pair production at the LHC gives the most significant upper bound on $\lambda_{ct}$ to be less than $10\sim30$ depending on neutral heavy Higgs masses. We also find that for the given upper bound on $\lambda_{ct}$, $B\to D^{(*)}\tau\nu$ put significant lower bound on $H$-$\tau$-$\tau$ Yukawa coupling, and the bound is proportional to the charged Higgs mass.

Dimension-six anomalous tqγ couplings in γγ collision at the LHC

We have investigated the flavor changing top quark physics on the dimension-six anomalous tqγ (q=u,c) couplings through the process pp→pγγp→ptq¯p at the LHC by considering different forward detector acceptances. In this paper, we have also examined the effects of top quark decay. The sensitivity bounds on the anomalous couplings and t→qγ branching ratio have been obtained at the 95% confidence level for the effective Lagrangian approach. Besides, we have investigated the effect of the anomalous couplings on single top quark spin asymmetry.

SUSY induced top quark FCNC decay $$t \rightarrow { ch}$$ t → c h after Run I of LHC

In light of the Higgs discovery and the nonobservation of sparticles at the LHC, we revisit the SUSY induced top quark flavor changing decay into the Higgs boson. We perform a scan over the relevant SUSY parameter space by considering the constraints from the Higgs mass measurement, the LHC search for SUSY, the vacuum stability, the precision electro-weak observables as well as $B \to X_s \gamma$. We have the following observations: (1) In the MSSM, the branching ratio of $ t \to c h$ can only reach $3.0\times10^{-6}$, which is about one order smaller than previous results obtained before the advent of the LHC. Among the considered constraints, the Higgs mass and the LHC search for sparticles are found to play an important role in limiting the prediction. (2) In the singlet extension of the MSSM, since the squark sector is less constrained by the Higgs mass, the branching ratio of $t \to c h$ can reach the order of $10^{-5}$ in the allowed parameter space. (3) The chiral-conserving mixings $\delta_{LL}$ and $\delta_{RR}$ may have remanent effects on $t \to c h$ in heavy SUSY limit. In the MSSM with squarks and gluino above 3 TeV and meanwhile the CP-odd Higgs boson mass around 1 TeV, the branching ratio of $t\to c h$ can still reach the order of $10^{-8}$ under the constraints.

Top quark rare three-body decays in the littlest Higgs model withTparity

In the littlest Higgs model with T-parity (LHT), the mirror quarks have flavor structures and will contribute to the top quark flavor changing neutral current. In this work, we perform an extensive investigation of the top quark rare three-body decays $t\to cVV (V=\gamma,Z,g)$ and $t\to cf\bar{f} (f=b,\tau,\mu,e)$ at one-loop level. Our results show that the branching ratios of $t\to cgg$ and $t\to cb\bar{b}$ could reach $\mathcal {O}(10^{-3})$ in the favorite parameter space of the littlest Higgs model with T-parity, which implies that these decays may be detectable at the LHC or ILC, while for the other decays, their rates are too small to be observable at the present or future colliders.