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Register a new userModel-independent analysis of Higgs spin and CP properties in the process $e^+ e^- to t bar t Phi$
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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.
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Appreciable CP Asymmetries (~ 10%) can arise in the reaction e+ e- -> t bar-t Z already at TREE-LEVEL in models with two Higgs doublets. For a neutral Higgs particle, h, with a mass in the range 50 GeV < m_h < 400 GeV, it may be possible to detect a 2-3 sigma CP-odd effect in e+ e- -> t bar-t Z in ~ 1-2 years of running of a future high energy e+e- collider with c.m. energies of ~ 1-2 TeV and an integrated luminosity of 200-500 inverse fb.
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.
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.
In the context of the littlest Higgs$(LH)$ model, we study the process $ e^{+}e^{-}to tbar{t}$. We find that the new gauge bosons $Z_{H}$ and $B_{H}$ can produce significant correction effects on this process, which can be further enhanced by the suitably polarized beams. In most of the parameter space preferred by the electroweak precision data, the absolute value of the relative correction parameter $R_{B_{H}}$ is larger than 5%. As long as $1TeVleq M_{Z_{H}}leq 1.5TeV$ and $0.3leq cleq 0.5,$ the absolute value of the relative correction parameter $R_{Z_{H}}$ is larger than 5%. With reasonable values of the parameters of the $LH$ model, the possible signals of the new gauge bosons $B_{H}$ and $Z_{H}$ can be detected via the process $ e^{+}e^{-} to tbar{t}$ in the future $LC$ experiments with the c.m. energy $sqrt{S}=800GeV$. $B_{H}$ exchange and $Z_{H}$ exchange can generate significantly corrections to the forward-backward asymmetry $A_{FB}(tbar{t})$ only in small part of the parameter space.
We study the triple Higgs self-coupling at future $e^{+}e^{-}$ colliders energies, with the reactions $e^{+}e^{-}to b bar b HH$ and $e^{+}e^{-}to t bar t HH$. We evaluate the total cross section of $bbar bHH$, $tbar tHH$ and calculate the total number of events considering the complete set of Feynman diagrams at tree-level. The sensitivity of the triple Higgs coupling is considered in the Higgs mass range 110-190 $GeV$, for the energy which is expected to be available at a possible Next Linear $e^{+}e^{-}$ Collider with a center-of-mass energy $800, 1000, 1500$ $GeV$ and luminosity 1000 $fb^{-1}$.
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