Flavor Changing Neutral Current Couplings Research Articles

Bs → μ+μ− in a two-Higgs-doublet model with flavour-changing up-type Yukawa couplings

We present a Two-Higgs-Doublet Model in which the structure of the quark Yukawa matrices is governed by three spurions breaking the flavour symmetries of the quark Yukawa sector. The model naturally suppresses flavour-changing neutral current (FCNC) amplitudes in the down-type sector, but permits sizable FCNC couplings in the up sector. We calculate the branching ratio of Bs → μ+μ− to leading and next-to-leading order of QCD for the case with FCNC couplings of the heavy neutral Higgs bosons to up-type quarks and verify that all counterterms follow the pattern dictated by the spurion expansion of the Yukawa matrices. We find correlations between Bs → μ+μ−, b → sγ, and the Higgs masses. The \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${B}_{s}-{\\overline{B} }_{s}$$\\end{document} mixing amplitude is naturally suppressed in the model but can probe a portion of the parameter space with very heavy Higgs bosons.

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.

Contributions to ZZV^* (V=gamma ,Z,Z') couplings from CP violating flavor changing couplings

The one-loop contributions to the trilinear neutral gauge boson couplings ZZV^* (V=gamma ,Z,Z'), parametrized in terms of one CP-conserving f_5^{V} and one CP-violating f_4^{V} form factors, are calculated in models with CP-violating flavor changing neutral current couplings mediated by the Z gauge boson and an extra neutral gauge boson Z'. Analytical results are presented in terms of Passarino-Veltman scalar functions. Constraints on the vector and axial couplings of the Z gauge boson left| g_{{VZ}}^{tu}right| < 0.0096 and left| g_{{VZ}}^{tc}right| <0.011 are obtained from the current experimental data on the trightarrow Z q decays. It is found that in the case of the ZZgamma ^* vertex the only non-vanishing form factor is f_5^{gamma }, which can be of the order of 10^{-3}, whereas for the ZZZ^* vertex both form factors f_5^{Z} and f_4^{Z} are non-vanishing and can be of the order of 10^{-6} and 10^{-5}, respectively. Our estimates for f_5^{gamma } and f_5^{Z} are smaller than those predicted by the standard model, where f_4^{Z} is absent up to the one loop level. We also estimate the ZZ{Z'}^{*} form factors arising from both diagonal and non-diagonal Z' couplings within a few extension models. It is found that in the diagonal case f_{5}^{Z'} is the only non-vanishing form factor and its real and imaginary parts can be of the order of 10^{-1}–10^{-2} and 10^{-2}–10^{-3}, respectively, with the dominant contributions arising from the light quarks and leptons. In the non-diagonal case f_{5}^{Z^prime } can be of the order of 10^{-4}, whereas f_4^{Z'} can reach values as large as 10^{-7}–10^{-8}, with the largest contributions arising from the Z'tq couplings.

Signatures of a flavor changing Z′ boson in Bq → γZ′

Rare B meson decays offer an opportunity to probe a light hidden Z′ boson. In this work we explore a new channel Bq→γZ′ (q=d,s) followed by a cascade decay of Z′ into an invisible (neutrino or dark matter) or charged lepton pair ℓ+ℓ− (ℓ=e,μ). The study is based on a simplified effective model where the down quark sector has tiny flavor-changing neutral current couplings with Z′. For the first time, we calculate BR(Bq→γZ′) at the leading power of 1/mb and 1/Eγ. Confronting with the strong constraints from semi-invisible decays of B meson, we find that the branching ratio for Bd→invisible+γ can be larger than its Standard Model background prediction, leaving a large room for new physics, in particular for light dark matter. Additionally, the branching ratio for Bd→e+e−γ can also be sizable when the corresponding flavor violating Z′ coupling to quarks is of the axial-vector type. On the other hand, the predicted branching ratios of Bd→μ+μ−γ and Bs→ℓ+ℓ−γ are severely constrained by the experimental measurements.

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.

Investigation of the Higgs boson anomalous FCNC interactions in the simple 3-3-1 model

We study phenomenological constraints on the simple 3-3-1 model with flavor-violating Yukawa couplings. Both Higgs triplets couple to leptons and quarks, which generates flavor-violating signals in both lepton and quark sectors. We have shown that this model allows for a large Higgs lepton flavor-violating rate decay h rightarrow mu tau and also may reach perfect agreement with other experimental constraints such as tau rightarrow mu gamma and (g-2)_mu . The contributions of flavor-changing neutral current couplings, Higgs–quark–quark couplings, mixing to the mesons are investigated. Br(h rightarrow q q^prime ) can be enhanced acknowledging the measurements of meson mixing. The branching ratio for t rightarrow q h can reach up to 10^{-3}, but it could be as low as 10^{-8}.

Higgs boson production through FCNC interactions

Since top quark has large Yukawa coupling, investigation the Higgs-top sector is highly interesting as it looks for any deviations from the standard model predictions. In this paper, we study the Higgs boson production in the gluon-gluon fusion channel and in the presence of top quark Flavor Changing Neutral Current (FCNC) interactions at the LHC. We utilize the standard model effective field theory framework to probe the new physics effects. The amplitude for Higgs boson production via FCNC interactions and the theoretical expression for its signal strength are calculated. Then, by comparing this theoretical expression with the experimental value reported by LHC collaborations, we find the allowed region for those FCNC couplings that play a role in Higgs boson production

Top quark anomalous FCNC production via tqg couplings at an FCC-hh

We investigate the top quark anomalous Flavor Changing Neutral Current (FCNC) $tqg$ interactions to probe limits on the couplings $\zeta_c$ and $\zeta_u$ through the $qg \to l\nu b$ signal suprocess at FCC-hh collider with center of mass energy of 100 TeV. To separate signal from relevant Standard Model background processes, selection criteria based on Boosted Decision Trees (BDT) is used with a set of useful kinematic variables. The sensitivities on the anomalous top FCNC couplings $\zeta_u$ and $\zeta_c$ are found to be $1.239\times10^{-4}$ and $1.149\times10^{-4}$ for FCC-hh with $L_{int}$=10 ab$^{-1}$ at 95 \% C.L. including realistic detector effects of the FCC-hh baseline detector, respectively. The branchings $BR (t \to ug)$ and $BR (t \to c g)$ converted from obtained limits for FCNC couplings are at the order of $10^{-7}$ which is at least one order of magnitude better than the projected limits of HL-LHC with $L_{int}$= 3 ab$^{-1}$.

Flavor violating Higgs couplings in minimal flavor violation

Motivated by the recent LHC data on the lepton-flavor violating (LFV) decays h → ℓ1ℓ2 and Bs,d → ℓ1ℓ2, we study the Higgs-mediated flavor-changing neutral current (FCNC) interactions in the effective field theory (EFT) approach without and with the minimal flavor violation (MFV) hypothesis, and concentrate on the later. After considering the B and K physics data, the various LFV processes, and the LHC Higgs data, severe constraints on the Higgs FCNC couplings are derived, which are dominated by the LHC Higgs data, the {B}_s-{overline{B}}_s mixing, and the μ → eγ decay. In the general case and the MFV framework, allowed ranges of various observables are obtained, such as mathrm{mathcal{B}} (Bs → ℓ1ℓ2), mathrm{mathcal{B}} (h → ℓ1ℓ2), mathrm{mathcal{B}} (h → q1q2), and the branching ratio of μ → e conversion in Al. Future prospects of searching for the Higgs FCNC interactions at the low-energy experiments and the LHC are discussed.

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.

Search for Flavour Changing Neutral Current Couplings of Higgs-up Sector Quarks at Future Circular Collider (FCC-eh)

Search for Flavour Changing Neutral Current Couplings of Higgs-up Sector Quarks at Future Circular Collider (FCC-eh)

Probing the Anomalous FCNC tqγ Couplings at Large Hadron Electron Collider

We investigate the anomalous flavour changing neutral current (FCNC) interactions of top quark through the process e-p→e-W±q+X. We calculate the signal and background cross sections in electron-proton collisions at Large Hadron electron Collider (LHeC) with a 7 TeV proton beam from the LHC and a new 60 GeV electron beam from energy recovery linac (ERL). We study the relevant background processes including one electron and three jets in the final state. The distributions of the invariant mass of two jets and an additional jet tagged as b-jet are used to account for signal and background events after the analysis cuts. We find upper bounds on anomalous FCNC couplings λ of the order of 10-2 at LHeC for a luminosity projection of 500 fb-1 together with the fast simulation of detector effects. As a matter of interest, we analyze the sensitivity to the couplings (λu, λc) and find an enhanced sensitivity to λc at the LHeC when compared to the results from the HERA.

Constraints and implications on Higgs FCNC couplings from precision measurement of Bs→μ+μ− decay

We study constraints and implications of the recent LHCb measurement of ${\cal B}(B_s \to \mu^+\mu^-)$ for tree-level Higgs-mediated flavor-changing neutral current (FCNC) interactions. Combined with experimental data on $B_s$ mass difference $\Delta m_s$, the $h \to \mu \tau$, and the $h \to \tau^+\tau^-$ decay branching ratios from the LHC, we find that the Higgs FCNC couplings are severely constrained. The allowed regions for $B_s \to \mu \tau$, $\tau\tau$ and $h \to sb$ decays are obtained. Current data allow large CP violation in the $h \to \tau^+ \tau^-$ decay. Consequences of the Cheng-Sher ansatz for the Higgs Yukawa couplings are discussed in some detail.

Using Spin Observables and Polarizations to Probe Top-Higgs FCNC Couplings at Colliders

We present how the polarized linear colliders can be used, complementary to the LHC, to fully determine the top-Higgs flavor changing neutral current (FCNC) couplings by using produced asymmetries and top spin polarizations and correlations.

Search for anomalous Wtb couplings and flavour-changing neutral currents in t-channel single top quark production in pp collisions at sqrt{s}=7 and 8 TeV

Single top quark events produced in the t channel are used to set limits on anomalous Wtb couplings and to search for top quark flavour-changing neutral current (FCNC) interactions. The data taken with the CMS detector at the LHC in proton-proton collisions at sqrt{s}=7 and 8 TeV correspond to integrated luminosities of 5.0 and 19.7 fb−1, respectively. The analysis is performed using events with one muon and two or three jets. A Bayesian neural network technique is used to discriminate between the signal and backgrounds, which are observed to be consistent with the standard model prediction. The 95% confidence level (CL) exclusion limits on anomalous right-handed vector, and left- and right-handed tensor Wtb couplings are measured to be |fVR| < 0.16, |fTL| < 0.057, and − 0.049 < fTR < 0.048, respectively. For the FCNC couplings κtug and κtcg, the 95% CL upper limits on coupling strengths are |κtug|/Λ < 4.1 × 10− 3 TeV−1 and |κtcg|/Λ < 1.8 × 10− 2 TeV−1, where Λ is the scale for new physics, and correspond to upper limits on the branching fractions of 2.0 × 10−5 and 4.1 × 10−4 for the decays t → ug and t → cg, respectively.

Searches for the FCNC couplings from top-Higgs associated production signal with h→γγ at the LHC

In this paper, we study the observability of the top-Higgs flavor changing neutral current (FCNC) $tqh$ coupling through the process $pp\to qg\to t(\to \ell^{+}b\nu)h(\to \gamma\gamma)$ at the Large Hadron Collider~(LHC), where $\ell=e, \mu$. Our numerical results show that, in some parameter regions, the LHC may observe the above signals at the $5\sigma$ level. Otherwise, the branching ratios $Br(t\to uh)$ and $Br(t\to ch)$ can be respectively probed to $0.036\%$ and $0.13\%$ at $3\sigma$ level at 14 TeV LHC with the high integrated luminosity of 3000 fb$^{-1}$. On the other hand, studying the charge ratio for the lepton in top quark decay can be not only used to discriminate between signal and backgrounds, but also used to discriminate between $tuh$ and $tch$ couplings, for which anomalous single top production comes from the up initiated channel and charm initiated channel.

Searches for top-Higgs FCNC couplings via theWhjsignal withh→γγat the LHC

In this paper, we investigated the process $pp\ensuremath{\rightarrow}{W}^{\ensuremath{-}}hj$ induced by the top-Higgs flavor-changing neutral current couplings at the LHC. We found that the cross section of $pp\ensuremath{\rightarrow}{W}^{\ensuremath{-}}hj$ mainly comes from the resonant process $pp\ensuremath{\rightarrow}{W}^{\ensuremath{-}}t\ensuremath{\rightarrow}{W}^{\ensuremath{-}}hq$ due to the anomalous $tqH$ couplings (where $q$ denotes up and charm quarks). We further studied the observability of the top-Higgs flavor-changing neutral current couplings through the process $pp\ensuremath{\rightarrow}{W}^{\ensuremath{-}}(\ensuremath{\rightarrow}{\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}})h(\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})j$ and found that the branching ratios $\mathrm{Br}(t\ensuremath{\rightarrow}qh)$ can be probed to 0.16% at $3\ensuremath{\sigma}$ level at 14 TeV LHC with an integrated luminosity of $3000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$.

Effects of the FCNC Couplings in Production of New Heavy Quarks withinZ′Models at the LHC

We study the flavor changing neutral current couplings of new heavy quarks through theZ′models at the LHC. We calculate the cross sections for the signal and the corresponding standard model background processes. Considering the present limits on the mass of new heavy quarks and theZ′boson, we performed an analysis to investigate the parameter space (mixing and mass) through differentZ′models. For FCNC mixing parameterx=0.1and theZ′massMZ′=2000 GeV and new heavy quark massmt′=700 GeV at the LHC withs=13 TeV, we find the cross section for single production of new heavy quarks associated with top quarks as5.8 fb,3.3 fb,1.5 fb, and1.2 fb within theZη′,Zψ′,ZLP′, andZχ′models, respectively. It is shown that the sensitivity would benefit from the flavor tagging.

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.

PREDICTIONS FOR $B \to \tau \bar{\mu} + \mu \bar{\tau}$

The observation of [Formula: see text] at present experiments would be a clear sign of new physics. In this paper, we calculate this process in an extended Higgs sector framework where the decay is mediated by the exchange of spin zero particle with flavor changing neutral current couplings. If we identify the scalar with the newly discovered state at LHC with a mass ~125 GeV then we find that, after imposing all experimental constraints, the [Formula: see text] can be as high as ~10-6 and [Formula: see text] can be as high as ~10-7. We also calculate this process in the minimal supersymmetric standard model and find the [Formula: see text] is typically of the order ~10-8.