Measurement of WZ production and searches for anomalous top quark decays and Higgs boson production using tri-lepton final states in ppbar collisions at √s = 1.96 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 DZero detector at the Fermi National Accelerator Laboratory in Chicago, IL. The first analysis reports a measurement of the WZ → lνl+l− cross section. Using 4.1 fb−1 of integrated luminosity, we measure a cross section of 3.90−0.85+1.01(stat+syst) ± 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 → Zq in ppbar → ttbar → WbZq → l'νl+l− + 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 → Zq) −1, the full dataset available at DZero, 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 mH = 100 - 200 GeV/c2, in intervals of 5 GeV/c2

Evidence for an anomalous like-sign dimuon charge asymmetry

This is the publisher's version, also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.82.032001

Search for the Standard Model Higgs Boson in Tau Final States

This is the publisher's version, also available electronically from http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.102.251801.We present a search for the standard model Higgs boson using hadronically decaying tau leptons, in 1  fb(−1) of data collected with the D0 detector at the Fermilab Tevatron pp-bar collider. We select two final states: τ± plus missing transverse energy and b jets, and τ(+)τ(−) plus jets. These final states are sensitive to a combination of associated W/Z boson plus Higgs boson, vector boson fusion, and gluon-gluon fusion production processes. The observed ratio of the combined limit on the Higgs production cross section at the 95% C.L. to the standard model expectation is 29 for a Higgs boson mass of 115 GeV

Search for Dark Photons from Supersymmetric Hidden Valleys

This is the publisher's version, also available electronically from http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.103.081802.We search for a new light gauge boson, a dark photon, with the D0 experiment. In the model we consider, supersymmetric partners are pair produced and cascade to the lightest neutralinos that can decay into the hidden sector state plus either a photon or a dark photon. The dark photon decays through its mixing with a photon into fermion pairs. We therefore investigate a previously unexplored final state that contains a photon, two spatially close leptons, and large missing transverse energy. We do not observe any evidence for dark photons and set a limit on their production

Combination of Tevatron Searches for the Standard Model Higgs Boson in the W(+)W(−) Decay Mode

This is the publisher's version, also available electronically from http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.104.061802.We combine searches by the CDF and D0 Collaborations for a Higgs boson decaying to W(+)W(−). The data correspond to an integrated total luminosity of 4.8 (CDF) and 5.4 (D0) fb(−1) of pp-bar collisions at s√=1.96  TeV at the Fermilab Tevatron collider. No excess is observed above background expectation, and resulting limits on Higgs boson production exclude a standard model Higgs boson in the mass range 162–166 GeV at the 95% C.L

Measurement of the top quark mass in final states with two leptons

This is the publisher's version, also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.80.092006.We present measurements of the top quark mass (mt) in tt-bar candidate events with two final state leptons using 1  fb(−1) of data collected by the D0 experiment. Our data sample is selected by requiring two fully identified leptons or by relaxing one lepton requirement to an isolated track if at least one jet is tagged as a b jet. The top quark mass is extracted after reconstructing the event kinematics under the tt-bar hypothesis using two methods. In the first method, we integrate over expected neutrino rapidity distributions, and in the second we calculate a weight for the possible top quark masses based on the observed particle momenta and the known parton distribution functions. We analyze 83 candidate events in the data and obtain mt=176.2±4.8(stat)±2.1(sys)  GeV and mt=173.2±4.9(stat)±2.0(sys)  GeV for the two methods, respectively. Accounting for correlations between the two methods, we combine the measurements to obtain mt=174.7±4.4(stat)±2.0(sys)  GeV

Search for charged Higgs bosons in decays of top quarks

This is the publisher's version, also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.80.051107.We present a search for charged Higgs bosons in decays of top quarks, in the mass range 800.24 for m(H±)=80  GeV and B(t→H(+b))>0.19 for mH±=155  GeV at the 95% C.L

Direct Measurement of the W Boson Width

This is the publisher's version, also available electronically from http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.103.231802.We present a direct measurement of the width of the W boson using the shape of the transverse mass distribution of W→eν candidate events. Data from approximately 1  fb(−1) of integrated luminosity recorded at s√=1.96  TeV by the D0 detector at the Fermilab Tevatron pp-bar collider are analyzed. We use the same methods and data sample that were used for our recently published W boson mass measurement, except for the modeling of the recoil, which is done with a new method based on a recoil library. Our result, 2.028±0.072  GeV, is in agreement with the predictions of the standard model

Search for New Fermions (“Quirks”) at the Fermilab Tevatron Collider

This is the publisher's version, also available electronically from http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.105.211803.We report results of a search for particles with anomalously high ionization in events with a high transverse energy jet and large missing transverse energy in 2.4  fb(−1) of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron pp-bar collider. Production of such particles (quirks) is expected in scenarios with extra QCD-like SU(N) sectors, and this study is the first dedicated search for such signatures. We find no evidence of a signal and set a lower mass limit of 107, 119, and 133 GeV for the mass of a charged quirk with strong dynamics scale Λ in the range from 10 keV to 1 MeV and N=2, 3, and 5, respectively

Measurement of the tt-bar cross section using high-multiplicity jet events

This is the publisher's version, also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.82.032002.We present a measurement of the tt-bar cross section using high-multiplicity jet events produced in pp-bar collisions at s√=1.96  TeV. These data were recorded at the Fermilab Tevatron Collider with the D0 detector. Events with at least six jets, two of them identified as b jets, were selected from a 1  fb(−1) data set. The measured cross section, assuming a top quark mass of 175  GeV/c2, is 6.9±2.0  pb, in agreement with theoretical expectations