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Register a new userHigh p_T Top Quarks at the Large Hadron Collider
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Authors
U. Baur
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Many new physics models predict resonances with masses in the TeV range which decay into a pair of top quarks. With its large cross section, tbar t production at the Large Hadron Collider (LHC) offers an excellent opportunity to search for such particles. The identification of very energetic top quarks is crucial in such an analysis. We consider in detail the tbar ttoell^pm u bbar bqbar q (ell=e, mu) final state for high p_T top quarks. In this phase space region, two or more of the final state quarks can merge into a single jet due to the large Lorentz boost of the parent top quark. As a result, a large fraction of tbar ttoell^pm u bbar bqbar q events with an invariant mass in the TeV region contains less than four observable jets. Requiring one or two tagged b-quarks, we calculate the W+jets, Wb+jets, Wbbar b+jets, Wbt, and single top plus jets backgrounds for these final states, and identify cuts which help to suppress them. In particular, we discuss whether a cut on the jet invariant mass may be useful in reducing the background in the ell u+2 jets channel. We also investigate how next-to-leading order QCD corrections affect high p_T top quark production at the LHC. We find that the ell u+2 jets and ell u+3 jets final states with one or two $b$-tags will significantly improve the chances for discovering new heavy particles in the tbar t channel at the LHC.
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In this talk, I review the T-odd correlations induced by CP violating anomalous top-quark couplings at both production and decay level in the process gg --> t t_bar --> (b mu+ nu_mu) (b_bar mu-nu_mu_bar). In addition I will also focus on experimental sensitivities corresponding to the anomalous couplings at the LHC.
New physics at the TeV scale is highly anticipated at the LHC. New particles with color, if within the LHC energy reach, will be copiously produced. One such particle is a diquark, having the quantum numbers of two quarks, and can be a scalar or a vector. It will decay to two light quarks, or two top quarks, or a top and a light quark, (up or down type depending on the quantum number of the produced diquark). If singly produced, it can be looked for as a dijet resonance, or as giving extra contribution to the single top production or tt production. In this work, we consider a color sextet vector diquark having the quantum number of (ud) type, its resonance production, and the subsequent decay to tb, giving rise to excess contribution to the single top production. Even though the diquark mass is large, its strong resonance production dominate the weak production of tb for a wide range of the diquark mass. Also its subsequent decay to tb produce a very hard b-jet compared to the usual electroweak production. In addition, the missing energy in the final state event is much larger from the massive diquark decays. Thus, with suitable cuts, the final state with b, bar{b} and a charged lepton together with large missing energy stands out compared to the Standard Model background. We make a detailed study of both the signal and the background. We find that such a diquark is accessible at the 7 TeV LHC upto a mass of about 3.3 TeV with the luminosity 1 fb^{-1}, while the reach goes up to about 4.3 TeV with a luminosity of 10 fb^{-1}.
Vector-Like Quarks (VLQs) are predicted by several theoretical scenarios of new physics and, having colour quantum numbers, can copiously be produced at the Large Hadron Collider (LHC), so long that their mass is in the testable kinematic regime of such a machine. While it would be convenient to assume that such objects are narrow and can be treated in the so-called Narrow Width Approximation (NWA), this is not always possible, owing to the fact that couplings and particle content of such new physics scenarios are not generally constrained, so that a large value of the former and/or a large variety of VLQ decay channels into the latter can contribute to generate a large decay width for such extra quarks. We have addressed here the issue of how best to tackle in LHC analysis the presence of large (and model-dependent) interference effects between different VLQ production and decay channels as well as between these and the corresponding irreducible background. We have confined ourselves to the case of single production of VLQs, which is rapidly becoming a channel of choice in experimental searches owing to the ever increasing limits on their mass, in turn depleting the yield of the historically well-established double production channel. Indeed, this poses a further challenge, as the former is model-dependent while the latter is essentially not. Despite these conditions, we show here that an efficient approach is possible, which retains to a large extent a degree of model independence in phenomenological studies of such VLQ dynamics at the LHC.
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