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Register a new userNew physics effect on the top-Yukawa coupling at ILC
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Measurement of the top-Yukawa coupling is important to understand the fermion mass generation mechanism and dynamics of electroweak symmetry breaking. We discuss the top quark anomalous couplings which can be described by higher dimensional operators. We investigate the process $e^-e^+ to W^-W^+ ubar u to t bar t ubar u$ to study the contribution of the anomalous top-Higgs coupling to the cross section. The effect of the dimension-six top-Higgs interaction on the cross section can be a few hundred percent greater than the SM prediction. Such a large effect can be measured at the International Linear Collider.
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We propose a measurement of the top Yukawa coupling at a 100 TeV hadron collider, based on boosted Higgs and top decays. We find that the top Yukawa coupling can be measured to 1%, with excellent handles for reducing systematic and theoretical uncertainties, both from side bands and from $tbar{t}H/tbar{t}Z$ ratios.
The cross section for the reaction $e^+e^- to tbar{t} H$ depends sensitively on the top quark Yukwawa coupling $lambda_t$. We calculate the rate for $tbar{t}H$ production, followed by the decay $Hto bbar{b}$, for a Standard Model Higgs boson with 100 < m_H <130 GeV. We interface with ISAJET to generate QCD radiation, hadronization and particle decays. We also calculate the dominant $tbar{t}bbar{b}$ backgrounds from electroweak and QCD processes. We consider both semileptonic and fully hadronic decays of the $tbar{t}$ system. In our analysis, we attempt full reconstruction of the top quark and W boson masses in the generated events. The invariant mass of the remaining b-jets should show evidence of Higgs boson production. We estimate the accuracy with which $lambda_t$ can be measured at a linear e^+e^- collider. Our results, including statistical but not systematic errors, show that the top quark Yukawa coupling can be measured to 6-8 % accuracy with 1000 fb^{-1} at $E_{CM}=1 TeV$, assuming 100 % efficiency for b-jet tagging. The accuracy of the measurement drops to 17-22 % if only a 60 % efficiency for b-tagging is achieved.
Some aspects of electroweak physics at the International Linear Collider (ILC) are reviewed. The importance of precision measurements in the Higgs sector and in top-quark physics is emphasized, and the physics potential of the GigaZ option of the ILC is discussed. It is shown in particular that even in a scenario where the states of new physics are so heavy that they would be outside of the reach of the LHC and the first phase of the ILC, the GigaZ precision on the effective weak mixing angle may nevertheless allow the detection of quantum effects of new physics.
Full one-loop electroweak-corrections for an $e^-e^+rightarrow t bar{t}$ process associated with sequential $trightarrow b mu u_mu$ decay are discussed. At the one-loop level, the spin-polarization effects of the initial electron and positron beams are included in the total and differential cross sections. A narrow-width approximation is used to treat the top-quark production and decay while including full spin correlations between them. We observed that the radiative corrections due to the weak interaction have a large polarization dependence on both the total and differential cross sections. Therefore, experimental observables that depend on angular distributions such as the forward-backward asymmetry of the top production angle must be treated carefully including radiative corrections. We also observed that the energy distribution of bottom quarks is majorly affected by the radiative corrections.
Six top signatures provide a novel probe of new physics. We discuss production of six top quarks as the decay products of a pair of top partners in the setting of a composite Higgs model, and argue that the six top signal may generically provide one of the first final states to show a discrepancy. We construct an analysis based on quantities such as $H_T$ and the numbers of jets which are tagged as boosted tops, $W$s, or containing $b$-tags, and show that the LHC with 3~ab$^{-1}$ can discover top partners with masses up to around 2.5 TeV in the six top signature.
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