Session X11: Top II

Search for Flavor Changing Neutral Currents in Top Quark Decays at CDF

In the CDF~II detector at the Fermilab Tevatron, $t\bar{t}$ pairs are produced in $p\bar{p}$ collisions at 1.96 TeV. We have performed a search for the flavor-changing neutral current (FCNC) decay of the top quark $t \rightarrow Zq$. This decay is extremely rare in the standard model, and any signal at the Tevatron would be an indication of new physics. For the summer 2007 conferences, using 1.12\,fb$^{-1}$ of data we presented the world's best limit on the branching fraction $\mathcal{B}$ $\left (t \rightarrow Zq\right)$ of 10.6\% at 95\% C.L. In this talk, we show the updated analysis with 1.9\,fb$^{-1}$ of data. We discriminate signal from background by exploring kinematic constraints present in FCNC events. We construct a mass $\chi^2$ variable and fit templates to the data, taking into account shape systematic uncertainties of the $\chi^2$ distribution. Using a Feldman-Cousins limit technique, we expect to set an improved limit on the branching fraction $\mathcal{B}$ $\left (t \rightarrow Zq \right)$.   

An upper limit on the total width of the top quark at CDF

We present the first direct experimental upper limit on the total decay width of the top quark using 955 pb$^{-1}$ of the Tevatron's $p\bar p$ collisions taken by the CDF II detector at the Fermilab Tevatron. We identify 253 top-antitop pair candidate events. The distribution of reconstructed top quark mass from the candidate events is fitted to template representing different values of the top quark width. The Feldman-Cousins prescription is used to extract an upper limit on the top quark width at 95$\%$ confidence level for an assumed top quark mass of 175 GeV/c$^2$.   

Search for W' boson resonances decaying to a top quark and a bottom quark

We search for the production of a heavy W' gauge boson that decays to third generation quarks in 0.9 fb$^{-1}$ of p pbar collsions at $\sqrt{s}$ = 1.96 TeV, collected with the D0 detector at the Fermilab Tevatron collider. We find no significant excess in the final-state invariant mass distribution and set upper limits on the production cross section times branching fraction. For a left-handed W' boson with SM couplings, we set a lower mass limit of 731 GeV. For right-handed W' bosons, we set lower mass limits of 739 GeV if the W' boson decays to both leptons and quarks and 768 GeV if the W' boson decays only to quarks. We also set limits on the coupling of the W' boson to fermions as a function of its mass.   

Search for single top quark production at CDF using a matrix element method

We present recent results from searches for single-top-quark production using 2 fb$^{-1}$ of data accumulated with the CDF detector at the Fermilab Tevatron. We select events with one charged lepton, large missing transverse energy, and two jets, where one jet is identified as a b-quark jet using displaced secondary-vertex information from the CDF silicon detector. We employ a matrix-element analysis technique and a neural-network jet-flavor separator to improve separation of signal and background and greatly improve the sensitivity of our search.   

Search for single top quark production at CDF using neural networks at CDF

We present a search for electroweak single top quark production in proton-antiproton collisions using 2~fb$^{-1}$ of data collected by the CDF II detector at the Fermilab Tevatron. Single top quarks are expected to be produced via virtual $W$ boson exchange in t-channel and s-channel processes. We select events with one charged lepton, missing transverse energy, and two or three jets, at least one of which is identified as containing a B hadron. Bayesian neural networks are used to further separate the signal from the backgrounds. Results are presented for s-channel and t- channel single top quark production as well as the combined process.   

Search for single top quark production optimized for s-channel production at CDF

We present a search for single top quark production using 1.7 fb$^{-1}$ of data accumulated with the CDF II detector at the Fermilab Tevatron. We select events with one high-$p_{T}$ lepton, large missing transverse energy and two jets with $b$ tags. One jet must have a $b$ tag using secondary-vertex information and the other jet's $b$ tag may use secondary-vertex or impact-parameter information. The kinematic fitter and the background-separation technique are designed to maximize the sensitivity to s-channel single top production.   

Optimizing the search for single top quark and $WH$ production at CDF

We report new searches for single top quarks and $WH$ production in a lepton+jets channel using 2 fb$^{-1}$ of data accumulated with the CDF detector at the Fermilab Tevatron. We present ways to increase the acceptance of candidate events using complementary trigger paths. The sensitivity of our search is further improved by employing a boosted decision tree together with a neural-network jet flavor separator to better classify signal and background events in the analysis.