Measurement of $WZ$ production and searches for anomalous top quark decays and Higgs boson production using tri-lepton final states in $p\bar{p}$ collisions at $\sqrt{s} = 1.96~\rm{TeV}$
We present the results of three analyses; a $WZ$ production cross section measurement, a search for new physics in anomalous top quark decays, and the search for the standard model Higgs boson, all with final states of three or more leptons -- either electrons or muons -- plus an imbalance of transverse momentum using Tevatron proton and anti-proton collisions at a center--of--mass energy of $\sqrt{s}$ = 1.96 TeV with the D0 detector at the Fermi National Accelerator Laboratory in Chicago, IL. The first analysis reports a measurement of the $WZ \rightarrow \ell^{\prime}\nu\ell\bar{\ell}$ cross section. Using 4.1 $fb^{-1}$ of integrated luminosity, we measure a cross section of 3.90$_{-0.85}^{+1.01}$($stat+syst$) $\pm$ 0.31($lumi$) pb, which is found to be in good agreement with the standard model prediction. The second analysis is an extension of the first, in which we use the same dataset and look for the flavor changing neutral current decay of $t \rightarrow Zq$ in $p\bar {p} \rightarrow t\bar{t} \rightarrow WbZq \rightarrow \ell^{\prime}\nu\ell\bar{\ell} + \rm{jets}$ decays. Here $q$ is considered to be either a $u$ or $c$ quark, and both the $q$ and $b$ quarks decay hadronically. We find no evidence of flavor changing neutral current production and set upper limits on the branching ratio of $BR(t \rightarrow Zq) < 3.2\%$ observed, with an expected limit of $< 3.8\%$, at the 95\% confidence level. Finally, we present a search for the standard model Higgs boson, using associated $WH$ and $ZH$ production, in the $ee\mu$ final state. Now using 9.7 $fb^{-1}$, the full dataset available at D0, we observe no excess above the standard model background prediction and extract limits on the Higgs boson production cross section times branching ratio for a Higgs boson mass range of $m_{H}$ = 100 -- 200 GeV/c$^2$, in intervals of 5 GeV/c$^2$.