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Register a new userThe Significance of the Heavy Top Quark
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E.H. Simmons
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Experiment shows that the top quark is far heavier than the other elementary fermions. This finding has stimulated research on theories of electroweak and flavor symmetry breaking that include physics beyond the standard model. Efforts to accommodate a heavy top quark within existing frameworks have revealed constraints on model-building. Other investigations have started from the premise that a large top quark mass could signal a qualitative difference between the top quark and other fermions, perhaps in the form of new interactions peculiar to the top quark. Such new dynamics may also help answer existing questions about electroweak and flavor physics. This talk explores the implications of the heavy top quark in the context of weakly-coupled (e.g. SUSY) and strongly-coupled (e.g. technicolor) theories of electroweak symmetry breaking.
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On the twentieth anniversary of the observation of the top quark, we trace our understanding of this heaviest of all known particles from the prediction of its existence, through the searches and discovery, to the current knowledge of its production mechanisms and properties. We also discuss the central role of the top quark in the Standard Model and the windows that it opens for seeking new physics beyond the Standard Model.
The single top quark production has an electroweak nature and provides an additional to the top pair production source of the top quarks. The processes involving single top have unique properties, they are very interesting from both theoretical and experimental view points. Short review of the single top quark production processes is given in the paper.
We calculate the dominant three body Higgs decays, $H to W^+W^-(Z^0, gamma)$ and $H to toverline{t}(Z^0,gamma ,g)$, in the Standard Model. We find that the branching ratios of these decays are of the order of few percent for large Higgs masses. We comment on the behaviour of the partial decay width $Gamma (H to toverline{b}W^-)$ below the $toverline{t}$ threshold. Numerical results of the following three body top decays, $t to W^+b(gamma ,g,Z^0)$ and $t to W^+bH$, are also given. We discuss the feasibility of observing these Higgs and top decays at future high energy colliders.
We discuss the rare decay of the top quark into a pair of same charge leptons (with identical or different flavors), a b quark and a (real or virtual) W-. The above process proceeds only if the exchanged neutrino N is of the Majorana type. This decay is the neutrinoless double b decay of the top. We find measurable values for its rate at the LHC with luminosity of 100 inverse fb. Furthermore, we consider an interaction of charged Higgs bosons with N which leads to lepton number violating processes such as pp to l+ N to l+ l+ H-, exhibiting spectacular events of the type: l+ l+ b b(bar) + 2 jets.
The asymmetries ${cal A}_k equiv [Gamma(twddec) -Gamma(tbwddec)] /[Gamma(twddec) + Gamma(tbwddec)]$ in the partial widths of the top quark decays are discussed within the Standard Model (SM), the Two-Higgs-Doublet Model (2HDM) and supersymmetric extensions of the SM (SSM). The leading contributions to these asymmetries in the SM and in the 2HDM are induced by the up-type quark self-energy diagrams and are found to be very small. However, in the SSM, the asymmetry ${cal A}_b$ can be substantial, ${cal O}(alpha_{QCD})$, provided the CP-violating phase of gluino-top-stop couplings is not suppressed. Within the SSM ${cal A}_b$ is generated by the vertex corrections.
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