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Register a new userTop Yukawa coupling measurement with indefinite CP Higgs in $e^+e^-to tbar{t}Phi$
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Balasubramanian Ananthanarayan
Publication date
2014
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English
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We consider the issue of the top quark Yukawa coupling measurement in a model in dependent and general case with the inclusion of CP-violation in the coupling. Arguably the best process to study this coupling is the associa ted production of Higgs boson along with a $tbar t$ pair in a machine like the International Linear Collider (ILC). While detailed analyses of the sensitivity of the measurement assuming a Standard Model (SM) - like coupling are available in the context of ILC, conclude that th e coupling could be pinned down at about 10% level with modest luminosity, our investigations show that the scenario could be different in case of a more general coupling. The modified Lorentz structure resulting in a changed functional dependence of the cross section on the couplin g, along with the difference in the cross section itself leads to considerable deviation in the sensitivity. Our studies with an ILC of center of mass energies of 500 GeV, 800 GeV and 1000 GeV show that moderate CP-mixing in the Higgs sector could change the sensitivity to about 20 %, while it could be worsened to 75% in cases which could accommodate more dramatic changes in the coupling. While detailed considerations of the decay distributions point to a need for a relook at the analysis strategy followed for the case of SM such as for a model independent analysis of the top quark Yukawa coupling measurement. This study strongly suggests that, a joint analysis of the CP properties and the Yukawa coupling measurement would be the way forward at the ILC and that caution must be excercised in the measurem ent of the Yukawa couplings and the conclusions drawn from it.
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The recently discovered scalar resonance at the LHC is now almost confirmed to be a Higgs Boson, whose CP properties are yet to be established. At the ILC with and without polarized beams, it may be possible to probe these properties at high precision. In this work, we study the possibility of probing departures from the pure CP-even case, by using the decay distributions in the process $e^+ e^- to t bar{t} Phi$, with $Phi$ mainly decaying into a $bbar b$ pair. We have compared the case of a minimal extension of the SM case (Model I) with an additional pseudoscalar degree of freedom, with a more realistic case namely the CP-violating Two-Higgs Doublet Model (Model II) that permits a more general description of the couplings. We have considered the ILC with $sqrt{s}=800$,GeV and integrated luminosity of $300, {rm fb}^{-1}$. Our main findings are that even in the case of small departures from the CP-even case, the decay distributions are sensitive to the presence of a CP-odd component in Model II, while it is difficult to probe these departures in Model I unless the pseudoscalar component is very large. Noting that the proposed degrees of beam polarization increases the statistics, the process demonstrates the effective role of beam polarization in studies beyond the Standard Model. Further, our study shows that an indefinite CP Higgs would be a sensitive laboratory to physics beyond the SM.
We report on the feasibility of measuring the top Yukawa coupling in the process: $e^+e^-to tbar{t}H$. This measurement is crucial to test the mass generation mechanism for matter particles. Since the cross section for this process attains its maximum around $sqrt{s}=700 $GeV, most of the past studies were done assuming this energy region. It has been pointed out, however, that the QCD threshold correction enhances the cross section significantly and might enable its measurement at $sqrt{s}=500 $GeV, which will be accessible already in the first phase of the ILC project. We have implemented this threshold enhancement into our $tbar{t}H$ event generator and carried out Monte Carlo simulations. Our results show that $tbar{t}H$ events can be observed with a significance of $4.1,sigma$ with no beam polarization and $5.4 sigma$ with the $e^-$ and $e^+$ beam polarization combination: $(-0.8,+0.3)$.
In this paper we investigate methods to study the $tbar{t}$ Higgs coupling. The spin and CP properties of a Higgs boson are analysed in a model-independent way in its associated production with a $tbar{t}$ pair in high-energy $e^+e^-$ collisions. We study the prospects of establishing the CP quantum numbers of the Higgs boson in the CP-conserving case as well as those of determining the CP-mixing if CP is violated. We explore in this analysis the combined use of the total cross section and its energy dependence, the polarisation asymmetry of the top quark and the up-down asymmetry of the antitop with respect to the top-electron plane. We find that combining all three observables remarkably reduces the error on the determination of the CP properties of the Higgs Yukawa coupling. Furthermore, the top polarisation asymmetry and the ratio of cross sections at different collider energies are shown to be sensitive to the spin of the particle produced in association with the top quark pair.
We find a large CP violation effect within the Two-Higgs-Doublet-Model for the reaction $e^+e^-to tbar{t}H^0$ at future linear colliders. The CP-asymmetry arises already at the tree level as a result of interference between diagrams with $H^0$ emission from $t$ (and $bar{t}$) and its emission from a $Z^0$ and can be about 10--20%. In the best case one needs a few hundred $tbar{t}H^0$ events to observe CP violation at the 3$sigma$ level.
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.
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