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Register a new userTop quark effects on the virtual photon structure function at ILC
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Yoshio Kitadono
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We investigated top quark effects on virtual photon structure functions by pQCD. We include the top quark mass effects on the virtual photon structure function with the quark parton model and with the operator product expansion up to the next-to-leading order in QCD. We also consider the threshold effect on the running coupling constant in the calculation to the effective photon structure function with a matching condition. The numerical calculations are investigated in the kinematical region expected at the future international linear collider.
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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.
Top quark decays are of particular interest as a mean to test the standard model (SM) predictions, both for the dominant ($tto b+W$) and rare decays ($tto q+W, cV, cVV,cphi^0,bWZ$). As the latter are highly suppressed, they become an excellent window to probe the predictions of theories beyond the SM. In particular, in this paper, we evaluate the corrections from new physics to the CKM-suppressed SM top quark decay $tto q+W$ ($q=d,s$), both within the effective lagrangian approach and the MSSM and we discuss the perspectives to probe those predictions at the ILC.
Precise predictions for an $e^+e^-rightarrow tbar{t}$ cross-section are presented in the energy region from 400 GeV to 800 GeV. Cross-sections are estimated including the beam-polarization effects with full $mathcal{O}(alpha)$, and also with the effects of the initial-state photon emission. A radiator technique is used for the initial-state photon emission up to two-loop orders. In this investigation, a weak correction is defined as the full electroweak corrections without the initial-state photonic corrections. As a result, it is determined that the total cross-section of a top quark pair-production receives the weak corrections of $+4%$ over the trivial initial state corrections at a center of mass energy of 500 GeV. Among the initial state contributions, a contribution from two-loop diagrams gives less than $0.11%$ correction over the one-loop ones at the center of mass energies of from $400$ GeV to $800$ GeV. In addition, the effect of a running coupling constant is also discussed.
We study pair production of scalar top quarks in polarized photon-photon collisions with the subsequent decay of the top squarks into b-quarks and charginos. We simulate this process by using PYTHIA6.4 for an electron beam energy 2E_beam =1000 GeV. A set of criteria for physical variables is proposed which leads to a good separation of stop signal events from top quark pair production being the main background. These criteria allow us to reconstruct the mass of the top squark provided that the neutralino mass is known.
In this work we study the flavor changing neutral current(FCNC) decays of the top quark, $tto cgamma$ and $tto c g$. The Standard Model, predictions for the branching ratios of these decays are about $sim 5times 10^{-14}$, and $sim 1times 10^{-12}$, respectively. The recent study presented by the ATLAS Collaboration gives a sensitivity on these branching ratios about $sim 10^{-5}$ at $%95$ C.L. The parameter space of $lambda$, $Lambda$, and $d$ where the branching ratios of $tto cgamma$ and $tto c g$ decays exceed these predictions is obtained.
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