We present a measurement of the top-quark width using t{bar t} events produced in p{bar p} collisions at Fermilab's Tevatron collider and collected by the CDF II detector. In the mode where the top quark decays to a W boson and a bottom quark, we select events in which one W decays leptonically and the other hadronically (lepton + jets channel) . From a data sample corresponding to 4.3 fb{sup -1} of integrated luminosity, we identify 756 candidate events. The top-quark mass and the mass of W boson that decays hadronically are reconstructed for each event and compared with templates of different top-quark widths ({Lambda}{sub t}) and deviations from nominal jet energy scale ({Delta}{sub JES}) to perform a simultaneous fit for both parameters, where {Delta}{sub JES} is used for the in situ calibration of the jet energy scale. By applying a Feldman-Cousins approach, we establish an upper limit at 95% confidence level (CL) of {Lambda}{sub t} < 7.6 GeV and a two-sided 68% CL interval of 0.3 GeV < {Lambda}{sub t} < 4.4 GeV for a top-quark mass of 172.5 GeV/c{sup 2}, which are consistant with the standard model prediction. This is the first direct measurement of {Lambda}{sub t} to …

We present a measurement of the WW + WZ production cross section observed in a final state consisting of an identified electron or muon, two jets, and missing transverse energy. The measurement is carried out in a data sample corresponding to up to 4.6 fb{sup -1} of integrated luminosity at {radical}s = 1.96 TeV collected by the CDF II detector. Matrix element calculations are used to separate the diboson signal from the large backgrounds. The WW + WZ cross section is measured to be 17.4 {+-} 3.3 pb, in agreement with standard model predictions. A fit to the dijet invariant mass spectrum yields a compatible cross section measurement.

We report on a search for the flavor-changing neutral-current decay D{sup 0} {yields} {mu}{sup +}{mu}{sup -} in p{bar p} collisions at {radical}s = 1.96TeV using 360 pb{sup -1} of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron collider. A displaced vertex trigger selects long-lived D{sup 0} candidates in the {mu}{sup +}{mu}{sup -}, {pi}{sup +}{pi}{sup -}, and K{sup -}{pi}{sup +} decay modes. We use the Cabibbo-favored D{sup 0} {yields} K{sup -}{pi}{sup +} channel to optimize the selection criteria in an unbiased manner, and the kinematically similar D{sup 0} {yields} {pi}{sup +}{pi}{sup -} channel for normalization. We set an upper limit on the branching fraction {Beta}(D{sup 0} {yields} {mu}{sup +}{mu}{sup -}) < 2.1 x 10{sup -7} (3.0 x 10{sup -7}) at the 90% (95%) confidence level.

We report on a CDF measurement of the total cross section and rapidity distribution, d{sigma}/dy, for q{bar q} {yields} {gamma}{sup *}/Z {yields} e{sup +} e {sup -} events in the Z boson mass region (66 < M {sub ee} < 116 GeV/c {sub 2}) produced in p{bar p} collisions at {radical}s = 1.96 TeV with 2.1 fb{sup -1} of integrated luminosity. The measured cross section of 257 {+-} 16pb and d{sigma}/dy distribution are compared with Next-to-Leading-Order (NLO) and Next-to-Next-to-Leading-Order (NNLO) QCD theory predictions with CTEQ and MRST/MSTW parton distribution functions (PDFs). There is good agreement between the experimental total cross section and d{sigma}/dy measurements with theoretical calcualtion with the most recent NNLO PDFs.

We present evidence for diffractive exclusive dijet production with an invariant dijet mass greater than 100 GeV in data collected with the D0 experiment at the Fermilab Tevatron Collider. A discriminant based on calorimeter information is used to measure a significant number of events with little energy (typically less than 10 GeV) outside the dijet system, consistent with the diffractive exclusive dijet production topology. The probability for these events to be explained by other dijet production processes is 2 x 10{sup -5}, corresponding to a 4.1 standard deviation significance.

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We report the result of a search for the pair production of the lightest supersymmetric partner of the top quark ({tilde t}{sub 1}) in p{bar p} collisions at a center-of-mass energy of 1.96 TeV at the Fermilab Tevatron collider corresponding to an integrated luminosity of 5.4 fb{sup -1}. The scalar top quarks are assumed to decay into a b quark, a charged lepton, and a scalar neutrino ({tilde {nu}}), and the search is performed in the electron plus muon final state. No significant excess of events above the standard model prediction is detected, and improved exclusion limits at the 95% C.L. are set in the (M{sub {tilde t}{sub 1}}, M{sub {tilde {nu}}}) mass plane.

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{sup -1} of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron p{bar p} 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 GeV for the mass of a charged quirk with strong dynamics scale {Lambda} in the range from 10 keV to 1 MeV.

A search for the standard model Higgs boson is presented using events with two charged leptons and large missing transverse energy selected from 5.4 fb{sup -1} of integrated luminosity in p{bar p} collisions at {radical}s = 1.96 TeV collected with the D0 detector at the Fermilab Tevatron Collider. No significant excess of events above background predictions is found, and observed (expected) upper limits at 95% confidence level on the rate of Higgs boson production are derived that are a factor of 1.55 (1.36) above the predicted standard model cross section at m{sub H} = 165 GeV.

We extract the total width of the top quark, {Lambda}{sub t}, from the partial decay width {Lambda}(t {yields} Wb) measured using the t-channel cross section for single top quark production and from the branching fraction B(t {yields} Wb) measured in t{bar t} events using up to 2.3 fb{sup -1} of integrated luminosity collected by the D0 Collaboration at the Tevatron p{bar p} Collider. The result is {Lambda}{sub t} = 1.99{sub -0.55}{sup +0.69} GeV, which translates to a top-quark lifetime of {tau}{sub t} = (3.3{sub -0.9}{sup +1.3}) x 10{sup -25} s. Assuming a high mass fourth generation b{prime} quark and unitarity of the four-generation quark-mixing matrix, we set the first upper limit on |V{sub tb{prime}}| < 0.63 at 95% C.L.

We measure the charge asymmetry A of like-sign dimuon events in 6.1 fb{sup -1} of p{bar p} collisions recorded with the DO detector at a center of mass energy sqrt s = 1.96 TeV at the Fermilab Tevatron Collider.

We present a measurement of the t{bar t} production cross section multiplied by the branching ratio to tau lepton decaying semi-hadronically ({tau}{sub h}) plus jets, {sigma}(p{bar p} {yields} t{bar t} + X) {center_dot} BR(t{bar t} {yields} {tau}{sub h} + jets), at a center of mass energy {radical}s = 1.96 TeV using 1 fb{sup -1} of integrated luminosity collected with the D0 detector. Assuming a top quark mass of 170 GeV, we measure {sigma}{sub t{bar t}} {center_dot} BR{sub {tau}{sub h}+j} = 0.60{sub -0.22}{sup +0.23}(stat){sub -0.14}{sup +0.15}(syst){+-}0.04 (lumi) pb. In addition, we extract the t{bar t} production cross section using the t{bar t} {yields} {tau}{sub h} + jets topology, with the result {sigma}{sub t{bar t}} = 6.9{sub -1.2}{sup +1.2}(stat){sub -0.7}{sup +0.8}(syst) {+-} 0.4 (lumi) pb. These findings are in good agreement with standard model predictions and measurements performed using other top quark decay channels.

We present a new measurement of the Z/{gamma}* transverse momentum distribution in the range 0-330 GeV, in proton anti-proton collisions at {radical}s = 1.96 TeV. The measurement uses 0.97 fb{sup -1} of integrated luminosity recorded by the D0 experiment and is the first using the Z/{gamma}* {yields} {mu}{sup +}{mu}{sup -} + X channel at this center-of-mass energy. This is also the first measurement of the Z/{gamma}* transverse momentum distribution that presents the result at the level of particles entering the detector, minimizing dependence on theoretical models. As any momentum of the Z/{gamma}* in the plane transverse to the incoming beams must be balanced by some recoiling system, primarily the result of QCD radiation in the initial state, this variable is an excellent probe of the underlying process. Tests of the predictions of QCD calculations and current event generators show they have varied success in describing the data. Using this measurement as an input to theoretical predictions will allow for a better description of hadron collider data and hence it will increase experimental sensitivity to rare signals.

The standard model (SM) of particle physics has been extensively tested in the past three decades and is found to be in excellent agreement with experimental observations. It is widely assumed, however, that the SM is only a low energy approximation of a more general theory. Therefore, any significant deviation from the SM predictions yields information on the nature of a more fundamental theory. Production of WZ pairs is the least studied diboson process within the SM, as it is a charged final state and can only be produced at hadron colliders. A detailed study of this process probes the electroweak sector of the SM. In addition, searches for new phenomena in the production of heavy gauge boson pairs are interesting, as many extensions of the SM predict 1-4 additional heavy gauge bosons that can decay into a WZ boson pairs.

Using 7.3 fb{sup -1} of p{bar p} collisions collected by the D0 detector at the Fermilab Tevatron, we measure the distribution of the variable {phi}*{sub {eta}}, which probes the same physical effects as the Z/{gamma}* boson transverse momentum, but is less susceptible to the effects of experimental resolution and efficiency. A QCD prediction is found to describe the general features of the {phi}*{sub {eta}} distribution, but is unable to describe its detailed shape or dependence on boson rapidity. A prediction that includes a broadening of transverse momentum for small values of the parton momentum fraction is strongly disfavored.

We report a search for diphoton events with large missing transverse energy produced in p{bar p} collisions at {radical}s = 1.96 TeV. The data were collected with the D0 detector at the Fermilab Tevatron Collider, and correspond to 6.3 fb{sup -1} of integrated luminosity. The observed missing transverse energy distribution is well described by the standard model prediction, and 95% C.L. limits are derived on two realizations of theories beyond the standard model. In a gauge mediated supersymmetry breaking scenario, the breaking scale {Lambda} is excluded for {Lambda} < 124 TeV. In a universal extra dimension model including gravitational decays, the compactification radius R{sub c} is excluded for R{sub c}{sup -1} < 477 GeV.

We present the first search for pair production of isolated jets of charged leptons in association with a large imbalance in transverse energy in p{bar p} collisions using 5.8 fb{sup -1} of integrated luminosity collected by the D0 detector at the Fermilab Tevatron Collider. No excess is observed above Standard Model background, and the result is used to set upper limits on the production cross section of pairs of supersymmetric chargino and neutralino particles as a function of 'dark-photon' mass, where the dark photon is produced in the decay of the lightest supersymmetric particle.

We present a search for flavor changing neutral currents via quark-gluon couplings in a sample of single top quark final states corresponding to 2.3 fb{sup -1} of integrated luminosity collected with the D0 detector at the Fermilab Tevatron Collider. We select events containing a single top quark candidates with an additional jet, and obtain separation between signal and background using Bayesian neural networks. We find consistency between background expectation and observed data, and set limits on avor changing neutral current gluon couplings of the top quark to up quarks (tgu) and charm quarks (tgc). The cross section limits at the 95% C.L. are {sigma}{sub tgu} < 0.20 pb and {sigma}{sub tgc} < 0.27 pb. These correspond to limits on the top quark decay branching fractions of B(t {yields} gu) < 2.0 x 10{sup -4} and B(t {yields} gc) < 3.9 x 10{sup -3}.

Using 5.4 fb{sup -1} of integrated luminosity from p{bar p} collisions at {radical}s = 1.96 TeV collected by the D0 detector at the Fermilab Tevatron Collider, we search for decays of the lightest Kaluza-Klein mode of the graviton in the Randall-Sundrum model to ee and {gamma}{gamma}. We set 95% C.L. lower limits on the mass of the lightest graviton between 560 GeV and 1050 GeV for values of the coupling k/{bar M}{sub Pl} between 0.01 and 0.1.

We report the results of a search for R parity violating (RPV) interactions leading to the production of supersymmetric sneutrinos decaying into e{mu} final states using 5.3 fb{sup -1} of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider. Having observed no evidence for production of e{mu} resonances, we set direct bounds on the RPV couplings {lambda}{prime}{sub 311} and {lambda}{sub 312} as a function of sneutrino mass.

We present a search for the standard model Higgs boson produced in association with a Z boson in 4.2 fb{sup -1} of p{bar p} collisions, collected with the D0 detector at the Fermilab Tevatron at {radical}s = 1.96 TeV. Selected events contain one reconstructed Z {yields} {ell}{sup +}{ell}{sup -} candidate and at least two jets, including at least one b-tagged jet. In the absence of an excess over the background expected from other standard model processes, limits on the ZH cross section multiplied by the branching ratios are set. The limit at M{sub H} = 115 GeV is a factor of 5.9 larger than the standard model prediction.

The ratio of the cross section for p{bar p} interactions producing a Z boson and at least one b quark jet to the inclusive Z+jet cross section is measured using 4.2 fb{sup -1} of p{bar p} collisions collected with the D0 detector at the Fermilab Tevatron collider at {radical}s = 1.96 TeV. The Z {yields} {ell}{sup +}{ell}{sup -} candidate events with at least one b jet are discriminated from Z+ charm and light jet(s) events by a novel technique that exploits the properties of the tracks associated to the jet. The measured ratio is 0.0193 {+-} 0.0027 for events having a jet with transverse momentum p{sub T} > 20 GeV and pseudorapidity |{eta}| {le} 2.5, which is the most precise to date and is consistent with theoretical predictions.

Data recorded by the D0 experiment at the Fermilab Tevatron Collider are analyzed to search for neutral Higgs bosons produced in association with b quarks. The search is performed in the three-b-quark channel using multijet-triggered events corresponding to an integrated luminosity of 5.2 fb{sup -1}. In the absence of any significant excess above background, limits are set on the cross section multiplied by the branching ratio in the Higgs boson mass range 90 to 300 GeV, extending the excluded regions in the parameter space of the minimal supersymmetric standard model.

We present a search for hypothetical vector-like quarks in p{bar p} collisions at {radical}s = 1.96 TeV. The data were collected by the D0 detector at the Fermilab Tevatron Collider and correspond to an integrated luminosity of 5.4 fb{sup -1}. We select events with a final state composed of a W or Z boson and a jet consistent with a heavy object decay. We observe no significant excess in comparison to the background prediction and set limits on production cross sections for vector-like quarks decaying to W+jet and Z+jet. Assuming a vector-like quark to standard model quark coupling parameter {tilde {kappa}}{sub qQ} of unity, we exclude vector-like quarks with mass below 693 GeV for decays to W+jet and mass below 449 GeV for decays to Z+jet. These are the most stringent mass limits for electroweak single vector-like quark production at hadron colliders.