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Register a new userA study of top-quark Yukawa coupling measurement in e+e- -> t tbar H at sqrt(s) = 500 GeV
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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: $(-0.8,+0.3)$.
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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.
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.
We report the analysis of the three-body e+e- => B B-bar pi, B B*-bar pi, and B* B*-bar pi processes, including the first observation of the Zb+-(10610) =>[B B*-bar+c.c.]+- and Zb+-(10650) => [B*B*-bar]+- transitions. We measure visible cross sections for the three-body production of sigma_vis(e+e- => [B B*-bar+c.c.]+-pi-+=(11.2+-1.0(stat.)+-1.2(syst.)) pb and sigma_vis(e+e- => [B*B*-bar]+-pi-+)=(5.61+-0.73(stat.)+-0.66(syst.)) pb and set a 90% C.L. upper limit of sigma_vis(e+e- => [BB-bar]+-pi-+)<2.1 pb. The results are based on a 121.4 1/fb data sample collected with the Belle detector at a center-of-mass energy near the Y(5S) peak.
The process $e^- e^+ to tau^- tau^+$ is of particular interest because the tau lepton polarisation can be reconstructed, allowing its chiral nature to be probed. This note reports on a study of the reconstruction of the di-tau final state at ILC-500, its selection and the reduction of backgrounds, the identification of the tau leptons decay mode, and on the extraction of the tau leptons polarisation. The performance of this analysis is studied in two models of the ILD detector, one larger (IDR-L) the other smaller (IDR-S), which differ in the outer radius of the TPC and of the subdetectors beyond, and in the magnetic field strength of the detector solenoid. We find that the high-mass tau-pair events in which at least one tau decays haronically can be selected with an efficiency of around 60%, with a remaining background from non-di-tau processes at the few-% level. Single-prong decay modes $tau^pm to pi^pm u, tau^pm to pi^pm pi^0 u, tau^pm to pi^pm pi^0 pi^0 u$ can be correctly identified in around 60-90% of cases, with sample purities in the range 50-90%, depending on decay mode. The sensitivity to tau polarisation was estimated in the four beam polarisation datasets envisaged for the $4 ab^{-1}$ of data forseen for ILC-500. Statistical precisions on the polarisation in the different datasets are predicted to be between 0.5 and 2%. While some small performance differences between the two detector models are seen, they have very similar final sensitivity to the polarisation measurement.
The cross section of the process $e^{+} e^{-} rightarrow K^{+} K^{-}$ is measured at a number of center-of-mass energies $sqrt{s}$ from 2.00 to 3.08 GeV with the BESIII detector at the Beijing Electron Positron Collider (BEPCII). The results provide the best precision achieved so far. A resonant structure around 2.2 GeV is observed in the cross section line shape. A Breit-Wigner fit yields a mass of $M=2239.2 pm 7.1 pm 11.3$~and a width of $Gamma=139.8pm12.3pm20.6$ MeV, where the first uncertainties are statistical and the second ones are systematic. In addition, the time-like electromagnetic form factor of the kaon is determined at the individual center-of-mass energy points.
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