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Register a new userCKM-suppressed top quark decays t -> q + W in the SM and beyond
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Top quark decays are of particular interest as a mean to test the standard model (SM) predictions, both for dominant (t -> b + W) and rare decays (t -> q + W, cV, cVV, c phi^{0}, bWZ). As the latter are highly suppressed, they become an excellent window to probe the predictions of thories beyond the SM. In particular, we evaluate the corrections from new physics to the CKM-suppressed SM top quark decay t -> q + W (q = d, s), both within the an effective model with right-handed currents and the MSSM. We also discuss the perspectives to probe those predictions at the ILC.
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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.
We study the angular distribution of the charged lepton in the top-quark decay into a bottom quark and a W boson which subsequently decays into ell u_{ell}, when a hard gluon is radiated off. The absorptive part of the t to bWg decay amplitudes, which gives rise to T-odd asymmetries in the distribution, is calculated at the one-loop level in perturbative QCD. The asymmetries at a few percent level are predicted, which may be observable at future colliders.
Stringent tests on top quark production and decay mechanisms are provided by the measurement of the top quark and W boson polarization. This paper presents a detailed study of these two measurements with the ATLAS detector, in the semileptonic (ttbar -> W W b bbar -> l nu j1 j2 b bbar) and dileptonic (ttbar -> W W b bbar -> l nu l nu b bbar) ttbar channels. It is based on leading-order Monte Carlo generators and on a fast simulation of the detector. A particular attention is paid to the systematic uncertainties, which dominate the statistical errors after one LHC year at low luminosity (10 fb^{-1}), and to the background estimate. Combining results from both channel studies, the longitudinal component of the W polarization (F_0) can be measured with a 2% accuracy and the right-handed component (F_R) with a 1% precision with 10 fb^{-1}. Even though the top quarks in ttbar pairs are not polarized, a large asymmetry is expected within the Standard Model in the like-spin versus unlike-spin pair production. A 4% precision on this asymmetry measurement is possible with 10 fb^{-1}, after combining results from both channel studies. These promising results are converted in a sensitivity to new physics, such as tWb anomalous couplings, top decay to charged Higgs boson, or new s-channels (heavy resonance, gravitons) in ttbar production.
We identify the $T$-odd structure functions that appear in the description of polarized top quark decays in the sequential decay $t(uparrow) to X_b+W^+(to ell^+ + u_ell)$ (two structure functions) and the quasi-three-body decay $t(uparrow) to X_b+ ell^+ + u_ell$ (one structure function). A convenient measure of the magnitude of the $T$-odd structure functions is the contribution of the imaginary part Im $g_R$ of the right-chiral tensor coupling $g_R$ to the $T$-odd structure functions which we work out. Contrary to the case of QCD the NLO electroweak corrections to polarized top quark decays admit of absorptive one-loop vertex contributions. We analytically calculate the imaginary parts of the relevant four electroweak one-loop triangle vertex diagrams and determine their contributions to the $T$-odd helicity structure functions that appear in the description of polarized top quark decays.
We report the combination of recent measurements of the helicity of the W boson from top quark decay by the CDF and D0 collaborations, based on data samples corresponding to integrated luminosities of 2.7 - 5.4 fb^-1 of ppbar collisions collected during Run II of the Fermilab Tevatron Collider. Combining measurements that simultaneously determine the fractions of W bosons with longitudinal (f0) and right-handed (f+) helicities, we find f0 = 0.722 pm 0.081 [pm 0.062 (stat.) pm 0.052 (syst.)] and f+ = -0.033 pm 0.046 [pm 0.034 (stat.) pm 0.031 (syst.)]. Combining measurements where one of the helicity fractions is fixed to the value expected in the standard model, we find f0 = 0.682 pm 0.057 [pm 0.035 (stat.) pm 0.046 (syst.)] and f+ = -0.015pm0.035 [pm 0.018 (stat.) pm 0.030 (syst.)]. The results are consistent with standard model expectations.
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