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Register a new userProbing Electroweak Top Quark Couplings at Hadron and Lepton Colliders
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U. Baur
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We discuss possibilities to measure the tt-gamma and ttZ couplings at hadron and lepton colliders. We also briefly describe how these measurements can be used to constrain the parameter space of models of new physics, in particular Little Higgs models.
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The International Linear Collider (ILC) will be able to precisely measure the electroweak couplings of the top in e+e- -> tt~. We compare the limits which can be achieved at the ILC with those which can be obtained in tt~gamma$ and tt~Z production at the Large Hadron Collider (LHC).
We consider QCD tbar{t}gamma and tbar{t}Z production at hadron colliders as a tool to measure the ttgamma and ttZ couplings. At the Tevatron it may be possible to perform a first, albeit not very precise, test of the ttgamma vector and axial vector couplings in tbar{t}gamma production, provided that more than 5 fb^{-1} of integrated luminosity are accumulated. The tbar{t}Z cross section at the Tevatron is too small to be observable. At the CERN Large Hadron Collider (LHC) it will be possible to probe the ttgamma couplings at the few percent level, which approaches the precision which one hopes to achieve with a next-generation e^+e^- linear collider. The LHCs capability of associated QCD tbar{t}V (V=gamma, Z) production has the added advantage that the ttgamma and ttZ couplings are not entangled. For an integrated luminosity of 300 fb^{-1}, the ttZ vector (axial vector) coupling can be determined with an uncertainty of 45-85% (15-20%), whereas the dimension-five dipole form factors can be measured with a precision of 50-55%. The achievable limits improve typically by a factor of 2-3 for the luminosity-upgraded (3 ab^{-1}) LHC.
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 tqgamma 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 lambda_q for tqgamma vertices and kappa_q 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.
The $h(125)$ boson, discovered only in 2012, is lower than the top quark in mass, hence $t to ch$ search commenced immediately thereafter, with current limits at the per mille level and improving. As the $t to ch$ rate vanishes with the $h$-$H$ mixing angle $cosgamma to 0$, we briefly review the collider probes of the top changing $tcH/tcA$ coupling $rho_{tc}$ of the exotic $CP$-even/odd Higgs bosons $H/A$. Together with an extra top conserving $ttH/ttA$ coupling $rho_{tt}$, one has an enhanced $cbH^+$ coupling alongside the familiar $tbH^+$ coupling, where $H^+$ is the charged Higgs boson. The main processes we advocate are $cg to tH/A to ttbar c,; ttbar t$ (same-sign top and triple-top), and $cg to bH^+ to btbar b$. We also discuss some related processes such as $cg to thh$, $tZH$ that depend on $cosgamma$ being nonzero, comment briefly on $gg to H/A to tbar t, tbar c$ resonant production, and touch upon the $rho_{tu}$ coupling.
In ongoing and upcoming hadron collider experiments, top quark physics will play an important role in testing the Standard Model and its possible extensions. In this work we present analytic results for the differential cross sections of top quark pair production in hadronic collisions at next-to-leading order in the QCD coupling, keeping the full dependence on the spins of the top quarks. These results are combined with the corresponding next-to-leading order results for the decay of polarized top quarks into dilepton, lepton plus jets, and all jets final states. As an application we predict double differential angular distributions which are due to the QCD-induced top quark spin correlations in the intermediate state. In addition to the analytic results, we give numerical results in terms of fit functions that can easily be used in an experimental analysis.
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