Flavor-changing Couplings Research Articles

Top quark forward-backward asymmetry, flavor-changing neutral-current decays, and like-sign pair production as a joint probe of new physics

The anomaly of the top quark forward-backward asymmetry $A^t_{FB}$ observed at the Tevatron can be explained by the t-channel exchange of a neutral gauge boson (Z') which has sizable flavor changing coupling for top and up quarks. This gauge boson can also induce the top quark flavor-changing neutral-current (FCNC) decays and the like-sign top pair production at the LHC. In this work we focus on two models which predict such a Z', namely the left-right model and the U(1)_X model, to investigate the correlated effects on $A^t_{FB}$, the FCNC decays $t -> u V$ ($V=g,Z,\gamma$) and the like-sign top pair production at the LHC. We also pay special attention to the most recently measured $A^t_{FB}$ in the large top pair invariant mass region. We find that under the current experimental constraints both models can alleviate the deviation of $A^t_{FB}$ and, meanwhile, enhance the like-sign top pair production to the detectable level of the LHC. We also find that the two models give different predictions for the observables and their correlations, and thus they may even be distinguished by jointly studying these top quark observables.

Effective axial-vector coupling of gluon as an explanation to the top quark asymmetry

We explore the possibility that the large $t\overline{t}$ forward-backward asymmetry measured by the CDF detector at Tevatron could be due to a universal effective axial-vector coupling of gluon. Using an effective field theory approach we show model independently how such a log-enhanced coupling occurs at 1-loop level. The interference with QCD gluon vector coupling naturally induces the observed positive forward-backward $t\overline{t}$ asymmetry that grows with $t\overline{t}$ invariant mass and is consistent with the cross section measurements. This scenario does not involve new flavor changing couplings nor operators that interfere with QCD, and, therefore, is not constrained by the LHC searches for 4-quark contact interactions. We predict top quark polarization effects that grow with energy and allow to test this scenario at the LHC. Our proposal offers a viable alternative to new physics scenarios that explain the $t\overline{t}$ forward-backward asymmetry anomaly with the interference between QCD and tree level new physics amplitudes.

Top quark asymmetry and dijet resonances

CDF recently reported an anomaly in the ${m}_{jj}$ distribution of dijet events produced in association with a $W$ boson. A single $u\ensuremath{-}t\ensuremath{-}V$ flavor-changing coupling can contribute to the ${m}_{jj}$ anomaly while being consistent with other resonance searches. Furthermore, it gives a potential explanation of the observed forward-backward asymmetry in top quark production.

Family symmetries and alignment in multi-Higgs doublet models

The exact alignment of the Yukawa structures on multi-Higgs doublet models provides cancellation of tree-level flavour changing couplings of neutral scalar fields. We show that family symmetries can provide a suitable justification for the Yukawa alignment.

Constraints on a new light spin-one particle from rareb→stransitions

The anomalously large like-sign dimuon charge asymmetry in semileptonic b-hadron decays recently measured by the D0 Collaboration may be hinting at the presence of CP-violating new physics in the mixing of B_s mesons. It has been suggested that the effect of a nonstandard spin-1 particle lighter than the b quark with flavor-changing couplings to b and s quarks can reproduce the D0 result within its one-sigma range. Here we explore the possibility that the new particle also couples to charged leptons l=e,mu and thus contributes to rare b -> s processes involving the leptons. We consider in particular constraints on its couplings from existing experimental data on the inclusive B -> X_s l^+ l^- and exclusive B -> K^{(*)} l^+ l^- decays, as well as the anomalous magnetic moments of the leptons. We find that there is parameter space of the particle that is allowed by the current data. Future measurements of these B transitions and rare decays of the B_s meson, such as B_s -> (phi,eta,eta') l^+ l^- and B_s -> l^+ l^-, at LHCb and next-generation B factories can probe its presence or couplings more stringently.

Flavor-changing top-charm associated productions at the ILC in the littlest Higgs model withTparity

The littlest Higgs model with T-parity (LHT) has new flavor-changing (FC) couplings with the Standard Model (SM) quarks, which do not suffer strong constraints from electroweak precision data. So these FC interactions may enhance the cross sections of some flavor-changing neutral-current (FCNC) processes. In this work, we study the FC top-charm associated productions via $e^{-}\gamma$ collision at the ILC. We find that the cross sections are sensitive to the mirror quark masses. With reasonable values of the parameters, the cross sections may reach the detectable level and provide useful information about the relevant parameters in the LHT model, especially setting an upper limit on the mirror quark masses.

Top quark forward-backward asymmetry from the 3-3-1 model

The forward-backward asymmetry $A_{FB}$ in top quark pair production, measured at the Tevatron, is probably related to the contribution of new particles. The Tevatron result is more than a $2\sigma$ deviation from the standard model prediction and motivates the application of alternative models introducing new states. However, as the standard model predictions for the total cross section $\sigma_{tt}$ and invariant mass distribution $M_{tt}$ for this process are in good agreement with experiments, any alternative model must reproduce these predictions. These models can be placed into two categories: One introduces the s-channel exchange of new vector bosons with chiral couplings to the light quarks and to the top quark and another relies on the t-channel exchange of particles with large flavor-violating couplings in the quark sector. In this work we employ a model which introduces both s- and t-channel nonstandard contributions for the top quark pair production in proton antiproton collisions. We use the minimal version of the $SU(3)_C \otimes SU(3)_L \otimes U (1)_X$ model (3-3-1 model) that predicts the existence of a new neutral gauge boson, called $Z^\prime$. This gauge boson has both flavor-changing couplings to up and top quarks and chiral coupling to the light quarks and to the top quark. This very peculiar model coupling can correct the $A_{FB}$ for top quark pair production for two ranges of $Z^\prime$ mass while leading to cross section and invariant mass distribution quite similar to the standard model ones. This result reinforces the role of the 3-3-1 model for any new physics effect.

THE FLAVOR-CHANGING BOTTOM AND ANTISTRANGE QUARK PRODUCTION IN THE ONE-GENERATION TECHNICOLOR MODEL AT THE ILC

The Technicolor (TC) model is a dynamical theory without fundamental scalars. Due to the pseudo-Goldstone bosons, there exist some new flavor-changing (FC) couplings in the TC model which can contribute to the production rates of some FC processes. In this paper, we study the [Formula: see text] production process in the one-generation technicolor model (OGTM) at the ILC, i.e. [Formula: see text]. We find that the cross section [Formula: see text] with the OGTM's contributions can be largely enhanced and the [Formula: see text] signal should be observable at the ILC. Our predictions can be viewed as valid estimates of other TC models. Therefore, the process [Formula: see text] may provide a unique window for studying the TC theory.

An improved observable for the forward-backward asymmetry in B → K ∗ℓ+ℓ− and B s → ϕ ℓ+ℓ−

We study the decay B → K ∗ℓ+ℓ− in the QCD factorization approach and propose a new integrated observable whose dependence on the form factors is almost negligible, consequently the non-perturbative error is significantly reduced and indeed its overall theoretical error is dominated by perturbative scale uncertainties. The new observable we propose is the ratio between the integrated forward-backward asymmetry (FBA) in the [4, 6] GeV2 and [1, 4] GeV2 dilepton invariant mass bins. This new observable is particularly interesting because, when compared to the location of the zero of the FBA spectrum, it is experimentally easier to measure and its theoretical uncertainties are almost as small; moreover it displays a very strong dependence on the phase of the Wilson coefficient C10 that is otherwise only accessible through complicated CP violating asymmetries. We illustrate the new physics sensitivity of this observable within the context of few extensions of the Standard Model (SM), namely the SM with four generations (SM4), an MSSM with non-vanishing source of flavor changing neutral currents in the down squark sector and a Z′ model with tree level flavor changing couplings.

The lepton flavor violating signal of the extra gauge boson Z′ in photon-photon collision at the ILC

The hitherto unconstrained lepton flavor mixings, induced by the new gauge boson Z′, which are the prediction of many new physics models, such as topcolor-assisted technicolor (TC2) models and flavor-universal TC2 models, may lead to the lepton flavor violating productions of τ $$ \bar \mu $$ , τ $$ \bar e $$ and µ $$ \bar e $$ in photon-photon collision at the proposed International Linear Collider (ILC). Through a comparative analysis of these processes, we find that the better channel to probe the new physics models is the production of τ $$ \bar \mu $$ or τ $$ \bar e $$ which occurs at a much higher rate than µ $$ \bar e $$ production due to the large mixing angle and the large flavor changing coupling, and may reach the detectable level of the ILC for a large part of the parameter space. Since the rates predicted by the Standard Model are far below the detectable level, these processes may serve as a sensitive probe for such new physics models.

Top quark forward-backward asymmetry from newt-channel physics

Motivated by recent measurements of the top quark forward-backward asymmetry at the Tevatron, we study how t-channel new physics can contribute to a large value. We concentrate on a theory with an abelian gauge boson possessing flavor changing couplings between up and top quarks, but satisfies flavor physics constraints. Collider constraints are strong, but can be consistent with the aid of small flavor diagonal couplings. We find that M_Z' ~ 160 GeV can yield a total lab-frame asymmetry of ~18% without being in conflict with other observables. There are implications for future collider searches, including exotic top quark decays, like-sign top quark production, and detailed measurements of the top production cross section. An alternate model with a gauged non-Abelian flavor symmetry would have similar phenomenology, but lacks the like-sign top signal.

Lepton flavor violation production of new particles from a topcolor scenario via eγ collision

The topcolor scenario generally predicts the existence of the neutral top-pion π0t and the nonuniversal gauge boson Z′, which have flavor changing couplings to ordinary fermions at tree-level. Taking into account the constraints of the lepton flavor violation (LFV) processes μ → eγ and μ → 3e on the relevant free parameters, we consider production of these new particles via the LFV processes eγ → τπ0t and eγ → τZ′ at the international linear e+e− collider (ILC) experiments. We find that the effective cross section for the LFV process eγ → τπ0t is smaller than 0.2fb. While, the effective cross section of the LFV process eγ → τZ′ can reach 12.2fb with reasonable values of the free parameters.

Yukawa alignment in the two-Higgs-doublet model

In multi-Higgs-doublet models the alignment in flavour space of the relevant Yukawa matrices guarantees the absence of tree-level flavour-changing couplings of the neutral scalar fields. We analyze the consequences of this condition within the two-Higgs-doublet model and show that it leads to a generic Yukawa structure which contains as particular cases all known specific implementations of the model based on Z_2 symmetries. All possible freedom in the Yukawa sector gets parametrized in terms of three complex couplings. In spite of having flavour conservation in the neutral scalar couplings, the phases of these three parameters represent potential new sources of CP violation.

Top Quark Decay t → cbb̄ in Topcolor-Assisted Technicolor Models

The topcolor-assisted technicolor (TC2) model predicts the existence of the top-pions and the CP-even top-Higgs with large flavor-changing couplings to the top quark, which at tree-level can mediate the top quark three-body decay t → cbb̄. We study this decay, show the dependence of the decay rate on the relevant TC2 parameters and compare the results with the predictions in the minimal supersymmetric model. We find that the decay rate in the TC2 model is much larger than that in the minimal supersymmetric model if the new particles are not too heavy. However, in consideration of the CDF and the LEP experiment limits, the top-pion mass is greater than 340 GeV in a more realistic parameter space, so the decay width of the channel t → cbb̄ intermediated by top-pions will be depressed greatly and difficult to be detected at the future collider. Thus, to observe this channel at the future collider, considering the top-higgs contribution may be the possible way when the masses of the top-pions and the top-higgs are not degenerate.

Electroweak and flavour structure of a warped extra dimension with custodial protection

We present the electroweak and flavour structure of a model with a warped extra dimension and the bulk gauge group SU(3) x SU(2)_L x SU(2)_R x P_LR x U(1)_X. The presence of SU(2)_R implies an unbroken custodial symmetry in the Higgs system allowing to eliminate large contributions to the T parameter, whereas the P_LR symmetry and the enlarged fermion representations provide a custodial symmetry for flavour diagonal and flavour changing couplings of the SM Z boson to left-handed down-type quarks. We diagonalise analytically the mass matrices of charged and neutral gauge bosons including the first KK modes. We present the mass matrices for quarks including heavy KK modes and discuss the neutral and charged currents involving light and heavy fields. We give the corresponding complete set of Feynman rules in the unitary gauge.

Radion mediated flavor changing neutral currents

In the context of a warped extra-dimension with Standard Model fields in the bulk, we obtain the general flavor structure of the Radion couplings to fermions and show that the result is independent on the particular nature of the Higgs mechanism (bulk or brane localized). These couplings will be generically misaligned with respect to the fermion mass matrix when the fermion bulk mass parameters are not all degenerate. When the Radion is light enough, the generic size of these tree-level flavor changing couplings will be strongly constrained by the experimental bounds on $\Delta F=2$ processes. At the LHC the possibility of a heavier Radion decaying into top and charm quarks is then considered as a promising signal to probe the flavor structure of both the Radion sector and the whole scenario.

Top quark flavor-changing neutral-current decays and productions at LHC in the littlest Higgs model withTparity

In the littlest Higgs model with $T$-parity (LHT) the newly introduced mirror quarks have flavor-changing couplings with the standard model (SM) quarks and may enhance the flavor-changing neutral-current (FCNC) top-quark interactions which are extremely suppressed in the SM. In this work we perform a comprehensive study for the contributions of these mirror fermions to various top-quark FCNC decays and productions at the LHC, which includes the decays $t\ensuremath{\rightarrow}cV$ ($V=g$, $\ensuremath{\gamma}$, $Z$), $t\ensuremath{\rightarrow}cgg$ and the productions proceeding through the parton processes $cg\ensuremath{\rightarrow}t$, $gg\ensuremath{\rightarrow}t\overline{c}$, $cg\ensuremath{\rightarrow}tg$, $cg\ensuremath{\rightarrow}t\ensuremath{\gamma}$ and $cg\ensuremath{\rightarrow}tZ$. We find that although these FCNC processes can be greatly enhanced by the LHT contributions, they are hardly accessible at the LHC. Therefore, the LHT model may not cause the FCNC problem in the top-quark sector if the top-quark property is proved to be SM-like at the LHC.

Flavor physics in the Randall-Sundrum model I. Theoretical setup and electroweak precision tests

A complete discussion of tree-level flavor-changing effects in the Randall-Sundrum (RS) model with brane-localized Higgs sector and bulk gauge and matter fields is presented. The bulk equations of motion for the gauge and fermion fields, supplemented by boundary conditions taking into account the couplings to the Higgs sector, are solved exactly. For gauge fields the Kaluza-Klein (KK) decomposition is performed in a covariant R_xi gauge. For fermions the mixing between different generations is included in a completely general way. The hierarchies observed in the fermion spectrum and the quark mixing matrix are explained naturally in terms of anarchic five-dimensional Yukawa matrices and wave-function overlap integrals. Detailed studies of the flavor-changing couplings of the Higgs boson and of gauge bosons and their KK excitations are performed, including in particular the couplings of the standard W and Z bosons. A careful analysis of electroweak precision observables including the S and T parameters and the Zbb couplings shows that the simplest RS model containing only Standard Model particles and their KK excitations is consistent with all experimental bounds for a KK scale as low as a few TeV, if one allows for a heavy Higgs boson and/or for an ultra-violet cutoff below the Planck scale. The study of flavor-changing effects includes analyses of the non-unitarity of the quark mixing matrix, anomalous right-handed couplings of the W bosons, tree-level flavor-changing neutral current couplings of the Z and Higgs bosons, the rare decays t-->c(u)+Z and t-->c(u)+h, and the flavor mixing among KK fermions. The results obtained in this work form the basis for general calculations of flavor-changing processes in the RS model and its extensions.

THE ANALYSIS OF B→ηK* AND B→ϕKS DECAYS IN A FLAVOR CHANGING Z′ MODEL

We study the large branching ratios of B→ηK* decays and the time-dependent CP asymmetry SϕKS anomaly of B→ϕKS decay in a flavor changing Z′ model associated with flavor changing neutral currents at the tree level. Considering the right-handed flavor changing couplings and setting ξLL = ξRL = ξLR = ξRR = ξ, we obtain 0.011 < ξ < 0.0131, this parameter region could give a simultaneous explanation to these anomalies of B→ηK* and B→ϕKS decays. The results are consistent with the constraints and the assumptions of some references.

Probing the topcolor-assisted technicolor model via the singlet-quark production at the CERN LHC

The single t-quark production in proton-proton collisions can proceed through three distinct processes: tq, tb, tW productions. With the running of the Large Hadron Collider (LHC) at CERN, it has good potential to measure each single t-quark production mode. The topcolor-assisted technicolor (TC2) model, one of the promising dynamical theories, predicts some new particles at the several hundred GeV scale: three top-pion bosons ({pi}{sub t}{sup {+-}}, {pi}{sub t}{sup 0}) and one top-Higgs boson (h{sub t}). These particles are regarded as the typical feature of the TC2 model and can contribute to some processes. In this paper, we systematically study the contribution of the TC2 model to the single t-quark production at the Hadron colliders, especially at the LHC. The TC2 model can contribute to the cross section of the single t-quark production in two different ways. First, the existence of the top pions and top Higgs can modify the Wtb coupling via their loop contributions, and such modification can cause the correction to the cross sections of all three production modes. Our study shows that this kind of correction is negative and very small in all cases. Thus it is difficult to observe such a correction even at the LHC.more » On the other hand, there exist the tree-level flavor-changing (FC) couplings in the TC2 model which can also contribute to the cross sections of the tq and tb production processes. The resonant effect can greatly enhance the cross sections of the tq and tb productions. The first evidence of the single t-quark production has been reported by the D0 collaboration and the measured cross section for the single t-quark production of {sigma}(pp{yields}tb+X,tqb+X) is compatible at the 10% level with the standard model prediction. Because the light top pion can make a great contribution to the tb production, the top-pion mass should be very large in order to make the predicted cross section in the TC2 model be consistent with the Tevatron experiments. More detailed information about the top-pion mass and the FC couplings in the TC2 model should be obtained with the running of the LHC.« less