Quark Flavor Changing Neutral Current Research Articles

CP violation in a model with Higgs triplets

We discuss CP-violation in a model with a real and a complex isospin triplet Higgs fields without introducing any symmetries except for the electroweak gauge symmetry. This corresponds to the minimal extension of the Higgs sector with the following properties: (i) providing new source of CP violation, (ii) absence of quark flavor changing neutral currents at tree level, and (iii) enabling the electroweak rho parameter to be unity at tree level in the scenario without imposing any new symmetries. Our model can be regarded as the generalized version of the Georgi-Machacek model, in which the global SU(2)L × SU(2)R symmetry is explicitly broken due to CP-violating terms in the potential. We present analytic formulas for theoretical constraints from perturbative unitarity and vacuum stability as well as contributions to the electron electric dipole moment (EDM) and the neutron EDM from all the Barr-Zee type diagrams. We then examine the parameter space allowed by the constraints mentioned above and also those from the uniqueness of the vacuum, measurements at Tevatron and LHC by using HEPfit to perform a global parameter fit. We find that the decays of the two lightest extra neutral (singly-charged) scalars, H1 and {H}_2left({H}_1^{pm}right) , into hZ (WZ) can be significant at the same time under the constraints, which can serve as direct evidence of CP violation in our model, but not from models with multi-doublet extensions.

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 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.

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.

Constraining top quark flavor violation and dipole moments through three and four top quark productions at the LHC

In this paper, we examine the sensitivity of the three-top quark production at the LHC to the top quark flavor-changing neutral currents (FCNC) as well as the sensitivity of the four-top production to the strong and weak dipole moments of the top quark. Upper limits at $95\%$ CL on the branching fractions of $\mathcal{B}(t\rightarrow qX)$, where $X = g, Z, \gamma, H$ and $q=u,c$, are set by performing an analysis on three-top events in the same-sign dilepton channel. We consider the main sources of the background processes and a realistic detector simulation is performed at the center-of-mass energy of 14 TeV. In the second part of this work, based on the recent upper limits which have been set on the four-top quark cross section by the ATLAS and CMS collaborations from 13 TeV data, we constrain the top quark strong and weak dipole moments. The bounds on the top quark dipole moments are presented using the future LHC prospects for four-top quark cross section measurement.

Semialigned two Higgs doublet model

In the left-right symmetric model based on $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ gauge symmetry, there appear heavy neutral scalar particles mediating quark flavor changing neutral currents (FCNCs) at tree level. We consider a situation where such FCNCs give the only sign of the left-right model while $W_R$ gauge boson is decoupled, and name it "semi-aligned two Higgs doublet model" because the model resembles a two Higgs doublet model with mildly-aligned Yukawa couplings to quarks. We predict a correlation among processes induced by quark FCNCs in the model, and argue that future precise calculation of meson-antimeson mixings and CP violation therein may hint at the semi-aligned two Higgs doublet model and the left-right model behind it.

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.

Impact of future lepton flavor violation measurements in the minimal supersymmetric standard model

Working within the context of the minimal supersymmetric standard model, we compare current bounds from quark flavor changing processes with current and upcoming bounds on lepton flavor violation. We assume supersymmetry breaking approximately respects CP invariance. Under the further assumption that flavor violating insertions in the quark and lepton scalar masses are comparable, we explore when lepton flavor violation provides the strongest probe of new physics. We quote results both for spectra with all superpartners near the TeV scale and where scalars are multi-TeV. Constraints from quark flavor changing neutral currents are in many cases already stronger than those expected from future lepton flavor violation bounds, but large regions of parameter space remain where the latter could provide a discovery mode for supersymmetry.

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.

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.

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.

Search for top quark FCNC couplings in Z′ models at the LHC and CLIC

The top quark is the heaviest particle to date discovered, with a mass close to the electroweak symmetry breaking scale. It is expected that the top quark would be sensitive to the new physics at the TeV scale. One of the most important aspects of the top quark physics can be the investigation of the possible anomalous couplings. Here, we study the top quark flavor changing neutral current (FCNC) couplings via the extra gauge boson Z' at the Large Hadron Collider (LHC) and the Compact Linear Collider (CLIC) energies. We calculate the total cross sections for the signal and the corresponding Standard Model (SM) background processes. For an FCNC mixing parameter x=0.2 and the sequential Z' mass of 1 TeV, we find the single top quark FCNC production cross sections 0.38(1.76) fb at the LHC with sqrt{s_{pp}}=7(14) TeV, respectively. For the resonance production of sequential Z' boson and decays to single top quark at the Compact Linear Collider (CLIC) energies, including the initial state radiation and beamstrahlung effects, we find the cross section 27.96(0.91) fb at sqrt{s_{e^{+}e^{-}}}=1(3) TeV, respectively. We make the analysis to investigate the parameter space (mixing-mass) through various Z' models. It is shown that the results benefit from the flavor tagging.

Flavor physics in the Randall-Sundrum model: II. Tree-level weak-interaction processes

A comprehensive analysis of tree-level weak interaction processes at low energy is presented for the Randall-Sundrum (RS) model with SU(2) L × U(1) Y bulk gauge symmetry and brane-localized Higgs sector. The complete form of the effective weak Hamiltonian is obtained, which results from tree-level exchange of Kaluza-Klein (KK) gluons and photons, the W ± and Z 0 bosons and their KK excitations, as well as the Higgs boson. Exact expressions are used for the bulk profiles of the various fields, and for the exchange of entire towers of KK gauge-boson states. A detailed phenomenological analysis is performed for potential new-physics effects in neutral-meson mixing and in rare decays of kaons and B mesons, including both inclusive and exclusive processes. We find that while the predictions for ∆F = 2 observables are rather model-independent, ∆F = 1 processes depend sensitively on the exact realizations of the electroweak gauge and the fermionic sector. In this context, we emphasize that the localization of the right-handed top quark in the extra dimension plays a crucial role in the case of rare Z 0-mediated decays, as it determines the relative size of left- to right-handed couplings. We also extend earlier studies of quark flavor-changing neutral currents by examining observables which up to now attracted little attention. These include $$ D - \overline D $$ mixing, B → τντ, B → X s (K*) l + l -, ϵ′ K /ϵ K , $$ \overline B \to \pi \overline K $$ , $$ {\overline B^0} \to \phi {K_S} $$ , $$ {\overline B^0} \to \eta '{K_S} $$ , and B + → π+π0.

Grand unification of quark and lepton flavor changing neutral currents.

In the context of supersymmetric grand unified theories with soft breaking terms arising at the Planck scale, it is generally possible to link flavor changing neutral current and CP violating processes occurring in the leptonic and hadronic sectors. We study the correlation between flavor changing squark and slepton mass insertions in models à la SU(5). We show that the constraints coming from lepton flavor violation exhibit a strong impact on CP-violating B decays.

Slepton Flavor Physics at Linear Colliders

If low energy supersymmetry is realized in nature it is possible that a first generation linear collider will only have access to some of the superpartners with electroweak quantum numbers. Among these, sleptons can provide sensitive probes for lepton flavor violation through potentially dramatic lepton violating signals. Theoretical proposals to understand the absence of low energy quark and lepton flavor changing neutral currents are surveyed and many are found to predict observable slepton flavor violating signals at linear colliders. The observation or absence of such sflavor violation will thus provide important indirect clues to very high energy physics. Previous analyses of slepton flavor oscillations are also extended to include the effects of finite width and mass differences.

Search for light-to-heavy quark flavor changing neutral currents inνμNandν¯μNscattering at the Fermilab Tevatron

We report on a search for flavor-changing neutral-currents (FCNC) in the production of heavy quarks in deep inelastic νμN and ν¯μN scattering by the NuTeV experiment at the Fermilab Tevatron. This measurement, made possible by the high-purity NuTeV sign-selected beams, probes for FCNC in heavy flavors at the quark level, and is uniquely sensitive to neutrino couplings of potential FCNC mediators. All searches are consistent with zero, and limits on the effective mixing strengths |Vuc|2, |Vdb|2, and |Vsb|2 are obtained.Received 31 July 2000DOI:https://doi.org/10.1103/PhysRevD.63.012001©2000 American Physical Society

Top quark flavor-changing neutral currents at CERN LEP-200: Signals and backgrounds

Current experimental limits on FCNC allow for the single top quark production process at CERN LEP-200. We show that the rates from standard model electroweak diagrams are negligible. In order to estimate the background due to misidentification of the b quark, we calculate the 4-quark process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}c\overline{s}\ifmmode \bar{u}\else \={u}\fi{}d$ within the EW sector of the SM. We show that it is possible to handle this background using different kinematical cuts on the quarks. We conclude that LEP-200 offers the possibility of improving the current bounds on flavor-violating couplings but that one will likely have to work with semileptonic decay modes of the top quark in order to avoid rather large QCD backgrounds.

Full range of predictions forBphysics from isosinglet down quark mixing

We extend the range of predictions of the isosinglet (or vector) down quark model to the fully allowed physical ranges, and also update this with the effect of new physics constraints. We constrain the present allowed ranges of sin(2*beta) and sin(2*alpha), gamma, x_s, and A_{B_s}. In models allowing mixing to a new isosinglet down quark (as in E_6) flavor changing neutral currents are induced that allow a Z^0 mediated contribution to B-Bbar mixing and which bring in new phases. In (rho, eta), (x_s, sin(gamma)), and (x_s, A_{B_s}) plots for the extra isosinglet down quark model which are herein extended to the full physical range, we find new allowed regions that will require experiments on sin(gamma) and/or x_s to verify or to rule out an extra down quark contribution.