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Register a new userATLAS sensitivity to top quark and W boson polarization in $tbar{t}$ events
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F. Hubaut
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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.
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The latest ATLAS results for processes with a top quark pair and an associated vector boson are presented here. The measurement of the production cross sections for these processes is important for the direct determination of the top quark couplings to gauge bosons and for constraints on new physics models, in particular for models which go beyond the Standard Model regarding the mechanism for the mass generation.
The effect of anomalous chromomagnetic (mu) and chromoelectric couplings (d) of the gluon to the top quark are considered in e+ e- --> t tbar, with unpolarized and longitudinally polarized electron beams. The total cross section, as well as t and tbar polarizations are calculated to order alpha_s in the presence of the anomalous couplings. One of the two linear combinations of t and tbar polarizations is CP even, while the other is CP odd. The limits that could be obtained at a typical future linear collider with an integrated luminosity of 50 1/fb and a total c.m. energy of 500 GeV on the most sensitive CP-even combination of anomalous couplings are estimated as -3 < Re(mu) < 2 for Im(mu) = 0 = d, and sqrt{Im(mu)^2 + |d|^2} < 2.25 for Re(mu) = 0. There is an improvement by roughly a factor of 2 at 1000 GeV. On the other hand, from the CP-odd combination, we derive the possible complementary bounds as -3.6 < Im(mu^* d) < 3.6 for Im(d) = 0, and -10 < Im(d) < 10 for Im(mu^* d) = 0, at a c.m. energy of 500 GeV. The corresponding limit for 1000 GeV is almost an order of magnitude better for Im(mu^* d), though somewhat worse for Im(d). Results for the c.m. energies 500 GeV and 1000 GeV, if combined, would yield independen limits on the two CP-violating parameters of -0.8 < Im(mu^* d) < 0.8 and -11 < Im(d) < 11.
The ATLAS experiment sensitivity to top quark Flavour Changing Neutral Current (FCNC) decays was studied at LHC using ttbar events. While one of the top quarks is expected to follow the dominant Standard Model decay t->bW, the other decays through a FCNC channel, i.e. t-> Z u(c), t-> gamma u(c) or t-> g u(c). Different types of analyses, applied to each FCNC decay mode, were compared. The FCNC branching ratio sensitivity (assuming a 5sigma signal significance) and 95% confidence level limits on the branching ratios (in the hypothesis of signal absence) were obtained.
We study the fusion processes $W^-W^+to tbar t$ and $ZZto tbar t$ observable at a future $e^-e^+$ collider and we discuss their sensitivity to an $Htt$ form factor which may be due to compositeness, in particular when the $H$ and the top quark have common constituents. We make an amplitude analysis and illustrate which helicity amplitudes and cross sections for specific final $tbar t$ polarizations are especially sensitive to this form factor.
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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