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Register a new userSearch for a heavy top t to Wq in top events
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David Cox
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We present a search for a massive quark (t) decaying to Wq and thus mimicking the top quark decay signature in data collected by the CDF II detector corresponding to 2.8 fb^-1. We use the reconstructed mass of the t quark and the scalar sum of the transverse energies in the event to discriminate possible new physics from Standard Model processes, and set limits on a standard 4th generation t quark.
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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 present a search for top-antitop events with a tau lepton in the final state. The data sample corresponds to an integrated luminosity of 194/pb collected with the CDF II detector from proton-antiproton collisions at a center of mass energy of 1.96 TeV. We observe two events with an expected signal of 1.0+/-0.2 events and a background of 1.3+/-0.3 events. We determine a 95% confidence level upper limit on r_tau, the ratio of the measured rate of top->tau nu quark to the expectation, of 5.2.
We present searches for massive top and bottom quark partners at CMS using LHC pp collision data collected at centre-of-mass energy $sqrt{s} = 8$ TeV. Such partners can be found in models predicting vector-like quarks to solve the hierarchy problem and stabilize the Higgs boson mass. The searches span a range of final states containing several lepton and jet multiplicities, and limits were set on mass and production cross sections as a function of branching ratios.
We present the results of a search for pair production of a heavy top-like (t) quark decaying to Wq final states using data corresponding to an integrated luminosity of 5.6 fb-1 collected by the CDF II detector in pp collisions at {surd}s = 1.96 TeV. We perform parallel searches for t to Wb and t to Wq (where q is a generic down-type quark) in events containing a lepton and four or more jets. By performing a fit to the two-dimensional distribution of total transverse energy versus reconstructed t quark mass, we set upper limits on the t bar{t} production cross section and exclude a standard model fourth-generation t quark decaying to W b (W q) with mass below 358 (340) GeV/c2 at 95% CL.
We present details of a search for electroweak production of single top quarks in the electron+jets and muon+jets decay channels. The measurements use ~90 pb^-1 of data from Run 1 of the Fermilab Tevatron collider, collected at 1.8 TeV with the DZero detector. We use events that include a tagging muon, implying the presence of a b jet, to set an upper limit at the 95% confidence level on the cross section for the s-channel process ppbar->tb+X of 39 pb. The upper limit for the t-channel process ppbar->tqb+X is 58 pb.
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