We investigate single top production in the presence of anomalous Wtb couplings. We explicitly show that, if these couplings arise from gauge invariant effective operators, the only relevant couplings for single top production and decay are the usual gamma^mu and sigma^mu nu q_nu terms, where q is the W boson momentum. This happens even in the single top production processes where the Wtb interaction involves off-shell top and/or bottom quarks. With this parameterisation for the Wtb vertex, we obtain expressions for the dependence on anomalous couplings of the single top cross sections, for (i) the t-channel process, performing a matching between tj and t bbar j production, where j is a light jet; (ii) s-channel t bbar production; (iii) associated tW- production, including the correction from tW- bbar. We use these expressions to estimate, with a fast detector simulation, the simultaneous limits which the measurement of single top cross sections at LHC will set on V_tb and possible anomalous couplings. Finally, a combination with top decay asymmetries and angular distributions is performed, showing how the limits can be improved when the latter are included in a global fit to Wtb couplings.
We study the possibility for the process $pp \rightarrow pγp \rightarrow pW^-t(W^+\bar t)X$ with anomalous Wtb couplings in a model independent effective Lagrangian approach at the LHC. We find 95% confidence level bounds on the anomalous coupling parameters for various values of the integrated luminosity. The improved constraints on the anomalous Wtb couplings have been obtained compared to current limits.
The potential of ep collision to prospect for anomalous Wtb vertex is discussed from the single top quark production process $ep\to t\barν+X$ for TESLA+HERAp and CLIC+LHC energies. Sensitivities to anomalous couplings $F_{2L}$ and $F_{2R}$, in the case of CLIC+LHC, are shown to be comparable with LHC.
We investigate the potential of $eγ$ collisions to probe anomalous $Wtb$ couplings via the polarized single top quark production process $e^{+} γ\to t \bar{b} \bar{ν_{e}}$. We find 95% confidence level limits on the anomalous coupling parameters $F_{2L}$ and $F_{2R}$ with an integrated luminosity of $500 fb^{-1}$ and $\sqrt{s}=0.5, 1$ and 1.5 TeV energies. The effects of top quark spin polarization on the anomalous $Wtb$ couplings are discussed. It is shown that polarization leads to a considerable improvement in the sensitivity limits.
We calculate the amplitudes of $b\to s$ transition in extension of the Standard Model with $Wtb$ anomalous couplings. We found that i) there exist the Ward identity violating terms in effective vertix of $b\to sγ$. The terms, which come from the tensor parts of $Wtb$ anomalies, and can be canceled exactly by introducing corresponding $Wtbγ$ interactions, ii) $Br(B_{s} \to μ^{+}μ^{-})$ provides unique information on $δv_L$ which is set to zero in top decay experiments, and stringent bounds on $v_R,\ g_L$ by $Br(B\to X_sγ)$ are obtained.
Results of complete tree level calculations of the single top production reaction $γe \to ν\bar{t} b$ at the Next Linear Collider, including the contribution of anomalous operators to the Wtb coupling are presented. The sensitivity for probing the structure of the Wtb coupling in a model independent way is analyzed and found to be significantly higher than for comparable measurements at the Tevatron.
We investigate several quantities, defined in the decays of top quark pairs, which can be used to explore non-standard Wtb interactions. Two new angular asymmetries are introduced in the leptonic decay of top (anti)quarks. Both are very sensitive to anomalous Wtb couplings, and their measurement allows for a precise determination of the W helicity fractions. We also examine other angular and energy asymmetries, the W helicity fractions and their ratios, as well as spin correlation asymmetries, analysing their dependence on anomalous Wtb couplings and identifing the quantities which are most se…
We explore the effects of the anomalous $tbW$ couplings on the $\bd$ mixing and recently measured $\bs$ mixing. The combined analysis of mixings via box diagrams with penguin decays provides strong constraints on the anomalous top quark couplings. We find the bound from the $\bd$ mixing data is stronger than that from the $\bs$ mixing.
We perform an exclusive study of the anomalous $Wtb$ interaction in the context of LHC. The limits on $Wtb$ anomalous couplings have been estimated via the measurements of the top-quark decay width and cross-section as well as production asymmetries in case the CP is violated. Our investigations reveal that the upper bounds on $(|C_L|, |C_R|)$ would be of about (1.82, 0.03) $\times 10^{-4}$ at $2.5 σ$ C.L., for the already collected data at the LHC with $\sqrt{S} = 13$ TeV for an integrated luminosity of 139 fb$^{-1}$. The corresponding limits for future hadron colliders, namely, High Luminosi…
Differential cross sections of secondary particles in a process of top quark pair production and decay into six fermions at a linear collider with an unpolarized and a longitudinally polarized electron beam are computed to the lowest order in the standard model and in the presence of an anomalous Wtb coupling. It is illustrated that the latter has a little impact on the differential cross sections. In particular, it is shown that the angular distribution of a secondary lepton receives practically no contribution from the anomalous Wtb coupling, even if the top quark is produced off shell and t…
Angular distributions of a mu+ and a b-quark resulting from the decay of a top quark produced at the e+e- linear collider with an unpolarized and a 100% longitudinally polarized electron beam are presented. The results of the standard model are compared with the results obtained in the presence of the anomalous Wtb coupling.
Some distributions of the mu- in the top quark pair production reaction pp -> b u anti-d anti-b mu- anti-nu_mu at the LHC are calculated to leading order in the presence of the anomalous Wtb coupling with operators of dimension up to five. The distributions in the transverse momentum, rapidity and cosine of the mu- angle with respect to the beam in the laboratory frame and with respect to the reversed momentum of the b-quark in the rest frame of W-boson are changed rather moderately by the anomalous Wtb coupling. The distributions computed with the full set of leading order Feynman diagrams pr…
An influence of the anomalous Wtb coupling on forward-backward asymmetry of top quark pair production at the Tevatron is investigated taking into account decays of the top quarks to 6 fermion final states containing one charged lepton. To this end the most general effective Lagrangian of the Wtb interaction containing terms of dimension up to five is implemented into 'carlomat', a general purpose Monte Carlo program, which allows to compute automatically all necessary cross sections in the presence of anomalous vector and tensor form factors. A sample of results which illustrate little effect …
We study the effect of anomalous Wtb couplings on the B -> Xs gamma branching ratio. The considered couplings are introduced as parts of gauge-invariant dimension-six operators that are built out of the Standard Model fields only. One-loop contributions from the charged-current vertices are assumed to be of the same order as the tree-level flavour-changing neutral current ones. Bounds on the corresponding Wilson coefficients are derived.
A method to simulate anomalous fermion-boson interactions in Wtb vertex is presented with a minimal set of simulated samples of single top quark events at the LHC energies. In proposed method additional auxiliary vector fields corresponding to the Standard Model gauge fields are implemented for each type of the anomalous vertex structure. The method allows to simulate a manifestation of anomalous gauge couplings in two approaches used in experimental analyses either keeping only the linear order contributions in the anomalous couplings or keeping all contributions in numerators and denominator…
Separation of left-handed and anomalous right-handed vector operators contributions into the Wtb vertex for single and double resonant top quark production processes using a neural network
We study the sensitivity of the ATLAS experiment to Wtb anomalous couplings in top pair production with semileptonic decay, pp -> t tbar -> W+ b W- bbar, with one of the W bosons decaying leptonically and the other hadronically. Several observables are examined, including the W helicity fractions and new quantities recently introduced, such as the ratios of helicity fractions and some angular asymmetries defined in the W rest frame. The dependence on anomalous couplings of all these observables has been previously obtained. In this work we show that some of the new observables also have smalle…
The $Wtb$ vertex structure and the search for new anomalous couplings is studied using top quark measurements obtained at the LHC, for a centre-of-mass energy of 8 TeV. By combining the latest and most precise results on the single top quark production cross section and the measurements of the $W$-boson helicity fractions ($F_0$ and $F_L$), it is possible to set new limits, at 95% CL (confidence level), on the real and imaginary components of the new couplings. The combination of the LHC observables clearly improves the limits obtained when using the individual results alone. The updated measu…
The potential of LHC for investigation of the $W$-$t$-$b$ vertex through the $tW$ channel of single top quark production is studied. Unlike the other two single top quark production processes ($t-$channel and $s-$channel), the $tW$ channel provides the possibility to study the $Wtb$ vertex without receiving contamination from FCNC. This study has been done at parton level but is involved the separation of signal from backgrounds when both $W$-bosons decay to leptons. In this study $\mathcal{CP}$ is assumed to be conserved. The 68% C.L. bounds on the non-Standard Model couplings are estimated.
We study the angular correlation and the amount of top quark polarization in the production of a higgs boson in association with a single top quark in the $t-$channel at the LHC. We also study the effect of anomalous $Wtb$ couplings on the angular correlation and on the production cross section of the process. The cross section and angular correlation is almost insensitive to the variation of the Higgs boson mass within 3 GeV. The robustness of the angular correlation against the center-of-mass energy of the proton-proton collision, the variation of parton distribution functions, and the chang…
The latest and most precise top quark measurements at the LHC and Tevatron are used to establish new limits on the $Wtb$ vertex. Recent results on the measurements of the $W$-boson helicity fractions and single top quark production cross section are combined in order to establish new limits at 95% CL (confidence level). The allowed regions for these limits are presented, for the first time, in three-dimensional graphics, for both real and imaginary components of the different anomalous couplings, providing a new perspective on the impact of the combination of different physics observables. The…
We analyze constraints on the anomalous $Wtb$ couplings from $B$-physics experiments, performing a correlated analysis and allowing all anomalous couplings to differ simultaneously from their Standard-Model (SM) values. The $B$-physics observables allow one to probe three linear combinations out of the four anomalous couplings, which parameterize the $Wtb$ vertex under the assumption that the SM symmetries remain the symmetries of the effective theory. The constraints in this work are obtained by taking into account the following $B$-physics observables: the $\bar B^0-B^0$ oscillations, the le…
Kinematical distributions of decay products of the top quark carry information on the polarisation of the top as well as on any possible new physics in the decay of the top quark. We construct observables in the form of asymmetries in the kinematical distributions to probe their effects. Charged-lepton angular distributions in the decay are insensitive to anomalous couplings to leading order. Hence these can be a robust probe of top polarisation. However, these are difficult to measure in the case of highly boosted top quarks as compared to energy distributions of decay products. These are the…
Probing anomalous $Wtb$ couplings through $bW$ spin correlations in top-quark pair production at the HL-LHC
School of Physics, Beihang University（北京航空航天大学物理学院）；School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University（北京大学物理学院与核物理及技术国家重点实验室）；Center for High Energy Physics, Peking University（北京大学高能物理中心）
We use the recent measurements of top quark decay asymmetries in ATLAS and the t-channel single top cross section in CMS to set the first combined LHC limits on the Wtb vertex. This combination allows to obtain much better limits than the separate measurements. The resulting constraints are comparable, although still weaker, than the ones obtained using Tevatron data with much more statistics.
The Wtb vertex can be probed on future colliders in the processes of single top production (LHC, pp mode, NLC, $γe$ mode) and of top pair production (NLC, e^+ e^- mode). We analyse observables sensitive to anomalous Wtb couplings in the top pair production process of e^+ e^- collisions. In particular, forward-backward and spin-spin asymmetries of the top decay products and the asymmetry of the lepton energy spectum are considered. Possible bounds on anomalous couplings obtained are competitive to those expected from the upgraded Tevatron and LHC. The validity of the infinitely small width appr…
We study the sensitivity of certain observables to the anomalous right tensorial coupling in single top production at the LHC at $\sqrt{s}=13 \text{ TeV}$. The observables consist of asymmetries constructed from the energy and angles of the decay products of the top quark produced in single top production through $t$-channel. The computation is done at Leading Order (LO) and Next-to-Leading Order (NLO) in the strong coupling in the $5$ flavor scheme. We have estimated projected limits on the anomalous coupling, both at the parton level without cuts and at the particle level with cuts. We find …
We present the results of a complete tree level calculation of the processes pp(\bar p) -> Wb\bar b and Wb\bar b + jet that includes the single top signal and all irreducible backgrounds simultaneously. In order to probe the structure of the Wtb coupling with the highest possible accuracy and to look for possible deviations from standard model predictions, we identify sensitive observables and propose an optimal set of cuts which minimizes the background compared to the signal. At the LHC, the single top and the single anti-top rates are different and the corresponding asymmetry yields additio…
Top pair production at LHC is the ideal place to search for nonstandard Wtb couplings in t -> W b -> l nu b decays. The lb forward-backward asymmetry in the W rest frame is very sensitive to sigma_{mu nu} couplings, and can spot one-loop QCD corrections to the decay vertex with more than 5 sigma statistical significance. We discuss the potential of this asymmetry to signal nonstandard gamma_mu and sigma_{mu nu} couplings and compare with top-antitop spin correlation asymmetries, which have a lower sensitivity. We also briefly summarise the results for Tevatron.
In this work, we study the new physics effects arising due the presence of anomalous $Wtb$ vertex through the semileptonic decay modes of the top-quark at the Large Hadron Collider. An estimate on the sensitivities of the aforementioned interaction at 5$σ$ CL in the context of top-quark decay-width measurements and cross-section measurements would also be discussed for the pre-existing 13 TeV LHC data and its projections for the proposed LHC runs at 14 TeV, 27 TeV and 100 TeV. We also incorporate the $\mathcal CP$-violating effects to such interactions by constructing the $\mathcal CP$-violati…
We consider the production of a single top quark in association with a $W$ boson at LHeC based $γp$ collider. We compute the cross section for the process $γp\rightarrow WtX$ with the anomalous $Wtb$ and $Wtbγ$ couplings. We find that the sensitivities to anomalous couplings of top quark are shown to be comparable, even better than the ones obtained from direct searches at hadron colliders.
We study the accuracy with which the lowest order $CP$ conserving anomalous $Wtb$ couplings in the single top quark production at the proposed large hadron electron collider (LHeC) can be probed. The one dimensional distribution of various kinematic observables at the parton level MC and their asymmetries arising due to the presence of anomalous couplings both in the hadronic and leptonic $W$ decay is examined. \par We find that at 95 \% C.L. the anomalous coupling associated with the left handed vector current can be measured at an accuracy of the order of $\sim 10^{-2}-10^{-3}$, while those …
We have reexamined the tW+X single top-quark production process which is important at the LHC contrary to the Tevatron. The special attention was paid to the treatment of the 2->2[Wt] process and the part of it's next-to-leading correction: 2->3[Wtb] process. We show that 2->3[Wtb] process has to be correctly taken into account with a proper subtraction of the top pair contribution and that it has qualitatively different kinematical distributions from the 2->2[Wt] process. We present the total cross section of the tW+X production to be about 62 pb at QCD scale be taken as a top quark mass, sug…
Spin correlations in the t-channel single top quark production and its subsequent decay are investigated for the case of contributions involving anomalous Wtb couplings. We obtain analytical expressions for the differential widths for the three-particle decay of a polarized t quark in its rest frame and also expressions for the differential cross sections of the full process of production and decay of the t quark ($2\to 4$) as a function of the energy of a charged lepton and two angles of orientation of the quantization axis of the t-quark spin. The expression is presented in the most general …
We consider the production of a single top quark in association with a Higgs boson at the LHC. In particular, we compute the cross sections for the processes $ p p \to t h j$, $t h b$, $t h W$, $t h j j$, $t h j b$, $t h W j$, $t h W b$ in the presence of the anomalous $Wtb, WWh$ and $tth$ couplings. We find that the anomalous $Wtb$ and $tth$ couplings can enhance the cross sections significantly. We also analyze a few signatures and show that, if these couplings are indeed anomalous, then with enough data, one should be able to observe the production of the Higgs boson in association with sin…
We study in the effective Lagrangian approach the possibility of probing anomalous top quark charged current couplings in the single top production at a high energy eγcollider. We analyzed all possible dimension-six CP-conserving operators which can give rise to an anomalous Wtb coupling which represent new physics effect generated at a higher energy scale. For those operators which also give rise to anomalous Zbb_bar or right-handed Wtb couplings, we find that they are strongly constrained by the existing R_b and b->sγdata. As a result, a collider with a luminosity of the order of 100 fb^{-1}…
We investigate new physics effects on the Wtb effective couplings in a model-independent manner. The new physics effects are summarized as four independent couplings $f_1^L$, $f_1^R$, $f_2^L$ and $f_2^R$. Using single-top-quark productions and W-helicity fraction measurements at the LHC and Tevatron, we perform a global fit to impose constraints on top quark effective couplings. We introduce a set of parameters $x_0$, $x_m$, $x_p$ and $x_5$ to study the correlations among Wtb effective couplings. We show that (i) improving the measurements of $σ_t$ and $σ_{tW}$ is important in constraining the…
We study the sensitivities of non-standard top and Higgs couplings in the $pp\to thj$ process at the 14 TeV high-luminosity run of Large Hadron Collider (HL-LHC). We calculate top polarization and construct various lab-frame observables to study their sensitivities on anomalous couplings at HL-LHC. Both anomalous Wtb and hVV couplings contribute in the production process. However, only the former contributes in top decay and only the latter takes part in Higgs decay. We study this interesting interplay of couplings in the production and decay observables. In production, these couplings signifi…
Top quarks can be produced from the electroweak Wtb coupling, leading to a single top or antitop in the final state. We examine single top quark production at the Fermilab Tevatron and calculate the cross section as a function of top quark mass, for each of three separate production processes. We give details of the interferences between Feynman diagrams in W-gluon fusion and then study the effects of an anomalous (V+A) term in the Wtb coupling. Using predictions for the next Tevatron run, we estimate the experimental sensitivity to the CKM matrix element V_tb as a function of the strength of …
We study the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and $Wtb$ effective couplings in single-top production in association with a $W^-$ boson, followed by semileptonic decay of the top. We calculate the top polarization and the effects of these anomalous couplings on it at two centre-of-mass (cm) energies, 8 TeV and 14 TeV. As a measure of top polarization, we look at decay-lepton angular distributions in the laboratory frame, without requiring reconstruction of the rest frame of the top, and study the effect of…
We study the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and $Wtb$ effective couplings in single-top production in association with a $W^-$ boson, followed by semileptonic decay of the top. The $Wt$ single-top production mode helps to isolate the anomalous $ttg$ and $Wtb$ couplings, in contrast to top-pair production and other single-top production modes, where other new-physics effects can also contribute. We calculate the top polarization and the effects of these anomalous couplings on it at two centre-of-mass (cm…
We describe the computation of the $\mathcal{O}(α^3)$ corrections for single top (and single anti-top) production in the $t$-channel at hadron colliders. We show also The genuine one loop SUSY contribution to these processes. Such contributions are generally quite modest in the mSUGRA scenario. The experimental observables would therefore only practically depend, in this framework, on the CKM $Wtb$ coupling.
Single top quark cross section evaluations for the complete sets of tree-level diagrams in the $e^+ e^-$, $e^- e^-$, $γe$ and $γγ$ modes of the next linear collider with unpolarized and polarized beams are performed within the Standard Model and beyond. From comparison of all possibilities we conclude that the process $γ_+ e^-_L \to e^- t \bar b$ is extremely favoured due to large cross section, no $t \bar t$ background, high degrees of beam polarization, and exceptional sensitivities to $V_{tb}$ and anomalous $Wtb$ couplings. Similar reasons favour the process $e^- e^- \to e^- ν_e \bar t b$ f…
The combination of the latest and most precise measurements of several top quark properties is presented in this paper in order to establish allowed regions on anomalous contributions to the Lorentz structure of the $Wtb$ vertex. These measurements include single top production cross sections, $W$ boson helicity fractions and forward-backward asymmetries, both at Tevatron and at the Large Hadron Collider, up to a center-of-mass energy of 13 TeV. The results obtained at 95% Confidence Level for the top quark anomalous couplings are compared with the limits extracted from a combination that incl…
The paper presents a method of modeling events with anomalous fermion-boson couplings by means of an auxiliary vector fields in addition to the SM gauge field. The method allows to simulate anomalous gauge couplings in different approaches, keeping only the linear order or higher order contributions of the anomalous couplings. The effectiveness of proposed method is demonstrated by modeling the single top quark production processes with anomalous Wtb couplings.
The latest results on top quark physics, namely single top quark production cross sections, $W$-boson helicity and asymmetry measurements are used to probe the Lorentz structure of the $Wtb$ vertex. The increase of sensitivity to new anomalous physics contributions to the top quark sector of the Standard Model is quantified by combining the relevant results from Tevatron and the Large Hadron Collider. The results show that combining an increasing set of available precision measurements in the search for new physics phenomena beyond the Standard Model leads to significant sensitivity improvemen…
Single top quark is mainly produced through the $t$-channel W boson exchange $q + b \rightarrow q' + t$ at LHC. This process probes Wtb vertex directly and can be used to measure the CKM matrix element $V_{tb}$ or to constrain the bottom quark PDF. The non-factorisable contributions are the last missing piece of the NNLO corrections. In these proceedings, we discuss in a first part the ostensible importance of such corrections and the obtention of the different relevant amplitudes. In the second part, recently published results at the energy of the LHC are compared to new results for proton-pr…
Within the model-independent framework of $SU(3)\times SU(2)\times U(1)$ gauge-invariant dimension-six operators, we study flavor-changing $Wtq$ couplings ($q=d,s$) and related four-quark contact interactions involving the top. We obtain bounds on those couplings from Tevatron and LHC data for single-top production and branching fractions in top decays, as well as other experimental results on flavor-changing neutral-current processes including $B\to X_{q}γ$ and $Z\to b\bar{q}$ decays ($q=d,s$). We also update the bounds on flavor-diagonal $Wtb$ couplings using the most recent measurement of $…
We investigate the feasibility of probing anomalous top-quark couplings of $Wtb$, $Z t \bar{t}$, and $γt \bar{t}$ in terms of an effective Lagrangian with dimension-six operators at future $e^+e^-$ linear colliders with a c. m. energy $\sqrt s \sim 500-800$ GeV. We first examine the constraints on these anomalous couplings from the $Z\to b \bar{b}$ data at LEP I and from unitarity considerations. We then consider in detail the effects of anomalous couplings on $t \bar{t}$ spin correlations in the top-pair production and decay with three spin bases: the helicity, beamline and off-diagonal bases…
We present a calculation of the single top quark cross section for proton-antiproton interactions with sqrt(s) = 1.8 TeV at the Fermilab Tevatron collider. We examine the effects of top mass, parton distribution functions, QCD scale, and collision energy, on each of the component production mechanisms, and study the kinematic distributions for standard model electroweak production. At the upgraded Tevatron with sqrt(s) = 2.0 TeV and high luminosity, it will be possible to test the nature of the Wtb coupling using single top production. We estimate the sensitivity to measure the single top cros…
We have computed the complete one-loop electroweak effects in the MSSM for single top (and single antitop) production in the $t$-channel at hadron colliders, generalizing a previous analysis performed for the dominant $dt$ final state and fully including QED effects. The results are quite similar for all processes. The overall Standard Model one-loop effect is small, of the few percent size. This is due to a compensation of weak and QED contributions that are of opposite sign. The genuine SUSY contribution is generally quite modest in the mSUGRA scenario. The experimental observables would the…
We calculate the density matrix for the decay of a polarised top quark into a polarised W boson and a massive b quark, for the most general Wtb vertex arising from dimension-six gauge-invariant effective operators. We show that, in addition to the well-known W helicity fractions, for polarised top decays it is worth to define and study the transverse and normal W polarisation fractions, that is, the W polarisation along two directions orthogonal to its momentum. In particular, a rather simple forward-backward asymmetry in the normal direction is found to be very sensitive to complex phases in …
We simplify the general form of the fermion-fermion-gauge boson interactions generated by dimension-six gauge-invariant effective operators by using the equations of motion to remove redundant operators. It is found that the most general vertex for off-shell fermions fi, fj and an off-shell boson V=W,Z,gamma,g only involves gamma^mu and sigma^{mu nu} q_nu terms, with q=p_i-p_j. Examples are given for the Wtb, Ztt, gamma tt and gtt interactions, whose general expression is greatly simplified with respect to previous results in the literature. The same arguments apply to top flavour-changing neu…
We examine the potential of pp-> p gamma p -> p W q X reaction to probe anomalous WWgamma couplings at the LHC. We find 95% confidence level bounds on the anomalous coupling parameters with various values of the integrated luminosity. We show that the reaction pp -> p gamma p -> p W q X at the LHC highly improve the current limits.
We present some physics topics that can be studied at the LHC using proton tagging. We distinguish the QCD (Pomeron structure, BFKL analysis...) from the exploratory physics topics (HIggs boson, anomalous couplings between photons and $W/Z$ bosons
Some features of a program 'eett6f' for top quark pair production and decay into six fermions at linear colliders are discussed. Lowest order standard model predictions for cross sections of some six fermion channels and for the top quark decay width are confronted with the predictions obtained within a model with an anomalous Wtb coupling. The question of wether non doubly resonant background can easily be reduced by imposing kinematical cuts is addressed.
We calculate the spinor helicity amplitudes of anomalous $H\to ZZ \to 4\ell$ decay. After embedding these analytic formulas into the $\texttt{MCFM}$ package, we study the interference effects between the anomalous $gg\to H\to ZZ \to 4\ell$ process and the SM processes, which are indispensable in the Higgs off-shell region. Subsequently, the constraints on the anomalous couplings are estimated using LHC experimental data.
We report on a possible measurement at the LHC using the first data and a luminosity of 10 pb$^{-1}$ of $W$ and $Z$ pair production via two-photon exchange. This measurement allows to increase the present sensitivity on $WWγγ$ and $ZZγγ$ quartic anomalous couplings from the LEP experiments by almost three orders of magnitude.
We investigate the constraints on the anomalous quartic $ZZγγ$ couplings through the process $p p\to pγp\to p γq Z X$ at the LHC. Taking into consideration various forward detector acceptances and integrated LHC luminosities, we find 95% confidence level bounds on the anomalous coupling parameters. We show that the bounds on these couplings are at the order of $10^{-6}GeV^{-2}$ which are about four orders of magnitude more restricted with respect to current experimental bounds.
We investigate the anomalous interactions of heavy up-type quark t' in a $γγ$ collision at the LHC. We have obtained 95% confidence level(CL)limit of $t'q γ$ anomalous coupling by taking into account three forward detector acceptances: $0.0015<ξ<0.15$,$0.0015<ξ<0.5$, and $0.1<ξ<0.5$.
