Search Results (111 total)

Correlations in the azimuthal angle between the two largest transverse momentum jets have been measured using the D0 detector in pp̅ collisions at a center-of-mass energy sqrt[s]=1.96  TeV. The analysis is based on an inclusive dijet event sample in the central rapidity region corresponding to an integrated luminosity of 150  pb^-1. Azimuthal correlations are stronger at larger transverse momenta. These are well described in perturbative QCD at next-to-leading order in the strong coupling constant, except at large azimuthal differences where contributions with low transverse momentum are significant.

The ratio of the B⁺ and B⁰ meson lifetimes was measured using data collected in 2002–2004 by the D0 experiment in Run II of the Fermilab Tevatron Collider. These mesons were reconstructed in B→μ⁺νD^*-X decays, which are dominated by B⁰ and B→μ⁺νD̅ ⁰X decays, which are dominated by B⁺. The ratio of lifetimes is measured to be τ⁺/τ⁰=1.080±0.016(stat)±0.014(syst).

Using the data collected with the D0 detector at sqrt[s]=1.96   TeV, for integrated luminosities of about 180   pb^-1, we have measured the ratio of inclusive cross sections for pp̅ →Z+b   jet to pp̅ →Z+jet production. The inclusive Z+b-jet reaction is an important background to searches for the Higgs boson in associated ZH production at the Fermilab Tevatron collider. Our measurement is the first of its kind, and relies on the Z→e⁺e^- and Z→μ⁺μ^- modes. The combined measurement of the ratio yields 0.021±0.005 for hadronic jets with transverse momenta p_T>20  GeV/c and pseudorapidities |η|<2.5, consistent with next-to-leading-order predictions of the standard model.

We present a measurement of the W boson pair-production cross section in pp̅ collisions at a center-of-mass energy of sqrt[s]=1.96  TeV. The data, collected with the Run II D0 detector at Fermilab, correspond to an integrated luminosity of 224–252  pb^-1 depending on the final state (ee, eμ, or μμ). We observe 25 candidates with a background expectation of 8.1±0.6(stat)±0.6(syst)±0.5(lum) events. The probability for an upward fluctuation of the background to produce the observed signal is 2.3×10^-7, equivalent to 5.2 standard deviations. The measurement yields a cross section of 13.8_-3.8^+4.3(stat)_-0.9^+1.2(syst)±0.9(lum)  pb, in agreement with predictions from the standard model.

We present measurements of the Λ_b⁰ lifetime in the exclusive decay channel Λ_b⁰→J/ψΛ⁰, with J/ψ→μ⁺μ^- and Λ⁰→pπ^-, the B⁰ lifetime in the decay B⁰→J/ψK_S⁰ with J/ψ→μ⁺μ^- and K_S⁰→π⁺π^-, and the ratio of these lifetimes. The analysis is based on approximately 250  pb^-1 of data recorded with the D0 detector in pp̅ collisions at sqrt[s]=1.96  TeV. The Λ_b⁰ lifetime is determined to be τ(Λ_b⁰)=1.22_-0.18^+0.22(stat)±0.04(syst)  ps, the B⁰ lifetime τ(B⁰)=1.40_-0.10^+0.11(stat)±0.03(syst)  ps, and the ratio τ(Λ_b⁰)/τ(B⁰)=0.87_-0.14^+0.17(stat)±0.03(syst). In contrast with previous measurements using semileptonic decays, this is the first determination of the Λ_b⁰ lifetime based on a fully reconstructed decay channel.

We present a search for Wbb̅ production in pp̅ collisions at sqrt[s]=1.96  TeV in events containing one electron, an imbalance in transverse momentum, and two b-tagged jets. Using 174  pb^-1 of integrated luminosity accumulated by the D0 experiment at the Fermilab Tevatron collider, and the standard-model description of such events, we set a 95% C.L. upper limit on Wbb̅ production of 6.6 pb for b quarks with transverse momenta p_T^b>20  GeV and bb̅ separation in pseudorapidity-azimuth space ΔR_bb>0.75. Restricting the search to optimized bb̅ mass intervals provides upper limits on WH production of 9.0–12.2 pb for Higgs-boson masses of 105–135 GeV.

We present the results of a search for the flavor-changing neutral current decay B_s⁰→μ⁺μ^- using a data set with integrated luminosity of 240   pb^-1 of pp̅ collisions at sqrt[s]=1.96   TeV collected with the D0 detector in run II of the Fermilab Tevatron collider. We find the upper limit on the branching fraction to be B(B_s⁰→μ⁺μ^-)≤5.0×10^-7 at the 95% C.L. assuming no contributions from the decay B_d⁰→μ⁺μ^- in the signal region. This limit is the most stringent upper bound on the branching fraction B_s⁰→μ⁺μ^- to date.

Using the exclusive decay B_s⁰→J/ψ(μ⁺μ^-)ϕ(K⁺K^-), we report the most precise single measurement of the B_s⁰ lifetime. The data sample corresponds to an integrated luminosity of approximately 220  pb^-1 collected with the D0 detector at the Fermilab Tevatron Collider in 2002–2004. We reconstruct 337 signal candidates, from which we extract the B_s⁰ lifetime, τ(B_s⁰)=1.444_-0.090^+0.098(stat)±0.020(sys)  ps. We also report a measurement for the lifetime of the B⁰ meson using the exclusive decay B⁰→J/ψ(μ⁺μ^-)K^*0(892)(K⁺π^-). We reconstruct 1370 signal candidates, obtaining τ(B⁰)=1.473_-0.050^+0.052(stat)±0.023(sys)  ps, and the ratio of lifetimes, τ(B_s⁰)/τ(B⁰)=0.980_-0.071^+0.076(stat)±0.003(sys).

We report the results of a search for supersymmetry (SUSY) with gauge-mediated breaking in the missing transverse energy distribution of inclusive diphoton events using 263  pb^-1 of data collected by the D0 experiment at the Fermilab Tevatron Collider in 2002–2004. No excess is observed above the background expected from standard model processes, and lower limits on the masses of the lightest neutralino and chargino of about 108 and 195 GeV, respectively, are set at the 95% confidence level. These are the most stringent limits to date for models with gauge-mediated SUSY breaking with a short-lived neutralino as the next-to-lightest SUSY particle.

The results based on 1992–95 data (Run 1) from the CDF and D0 experiments on the measurements of the W boson mass and width are presented, along with the combined results. We report a Tevatron collider average M_W=80.456±0.059  GeV. We also report the Tevatron collider average of the directly measured W boson width Γ_W=2.115±0.105  GeV. We describe a new joint analysis of the direct W mass and width measurements. Assuming the validity of the standard model, we combine the directly measured W boson width with the width extracted from the ratio of W and Z boson leptonic partial cross sections. This combined result for the Tevatron is Γ_W=2.135±0.050  GeV. Finally, we use the measurements of the direct total W width and the leptonic branching ratio to extract the leptonic partial width Γ(W→eν)=224±13  MeV.

We report the observation of the X(3872) in the J/ψπ⁺π^- channel, with J/ψ decaying to μ⁺μ^-, in pp̅ collisions at sqrt[s]=1.96  TeV. Using approximately 230  pb^-1 of data collected with the Run II D0 detector, we observe 522±100 X(3872) candidates. The mass difference between the X(3872) state and the J/ψ is measured to be 774.9±3.1(stat)±3.0(syst)  MeV/c². We have investigated the production and decay characteristics of the X(3872) and find them to be similar to those of the ψ(2S) state.

A search for pair production of doubly charged Higgs bosons in the process pp̅ →H⁺⁺H^--→μ⁺μ⁺μ^-μ^- is performed with the D0 run II detector at the Fermilab Tevatron. The analysis is based on a sample of inclusive dimuon data collected at an energy of sqrt[s]=1.96  TeV, corresponding to an integrated luminosity of 113  pb^-1. In the absence of a signal, 95% confidence level mass limits of M(H_L^±±)>118.4  GeV/c² and M(H_R^±±)>98.2  GeV/c² are set for left-handed and right-handed doubly charged Higgs bosons, respectively, assuming 100% branching into muon pairs.

Using 85.2±3.6   pb^-1 of pp̅ collisions collected at sqrt[s]=1.8  TeV with the D0 detector at Fermilab’s Tevatron Collider, we present the results of a search for direct pair production of scalar top quarks (t˜), the supersymmetric partners of the top quark. We examined events containing two or more jets and missing transverse energy, the signature of light scalar top quark decays to charm quarks and neutralinos. After selections, we observe 27 events while expecting 31.1±6.4 events from known standard model processes. Comparing these results to next-to-leading-order production cross sections, we exclude a significant region of t˜ and neutralino phase space. In particular, we exclude the t˜ mass m_t˜<122  GeV/c² for a neutralino mass of 45  GeV/c².

A search for narrow resonances that decay into tt̅ pairs has been performed using 130   pb^-1 of data in the lepton + jets channel collected by the DØ detector in pp̅ collisions at sqrt[s]=1.8   TeV. There is no significant deviation observed from the standard-model predictions at a top-quark mass of 175   GeV/c². We therefore present upper limits at the 95% confidence level on the product of the production cross section and branching fraction to tt̅ for narrow resonances as a function of the resonance mass M_X. These limits are used to exclude the existence of a leptophobic top-color particle with mass M_X<560   GeV/c², using a theoretical cross section for a width Γ_X=0.012M_X.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We present a search for large extra dimensions (ED) in pp̅ collisions at a center-of-mass energy of 1.8 TeV using data collected by the DØ detector at the Fermilab Tevatron in 1994–1996. Data corresponding to 78.8±3.9   pb^-1 are examined for events with large missing transverse energy, one high-p_T jet, and no isolated muons. There is no excess observed beyond expectation from the standard model, and we place lower limits on the fundamental Planck scale of 1.0 and 0.6 TeV for 2 and 7 ED, respectively.

We present data on multiple production of jets with transverse energies near 20 GeV in pp̅ collisions at sqrt[s]=1.8 TeV. QCD calculations in the parton-shower approximation of PYTHIA and HERWIG and the next-to-leading order approximation of JETRAD are compared to the data for one, two, three, and four jet inclusive production. Transverse energy spectra and multiple jet angular and summed transverse-energy distributions are adequately described by the shower approximation while next-to-leading order calculations describe the data poorly.

Results are presented on a measurement of the tt̅ pair production cross section in pp̅ collisions at sqrt[s]=1.8 TeV from nine independent decay channels. The data were collected by the DØ experiment during the 1992–1996 run of the Fermilab Tevatron Collider. A total of 80 candidate events is observed with an expected background of 38.8±3.3 events. For a top quark mass of 172.1 GeV/c², the measured cross section is 5.69±1.21(stat)±1.04(syst) pb.

We describe a search for evidence of minimal supergravity (MSUGRA) in 92.7 pb^-1 of data collected with the DØ detector at the Fermilab Tevatron pp̅ collider at sqrt[s]=1.8 TeV. Events with a single electron, four or more jets, and large missing transverse energy were used in this search. The major backgrounds are from W+jets, misidentified multijet, tt̅ , and WW production. We observe no excess above the expected number of background events in our data. A new limit in terms of MSUGRA model parameters is obtained.

We report the first search for supersymmetric particles via s-channel production and decay of smuons or muon sneutrinos at hadronic colliders. The data for the two-muon and two-jets final states were collected by the D0 experiment and correspond to an integrated luminosity of 94±5   pb^-1. Assuming that R parity is violated via the single coupling λ₂₁₁^′, the number of candidate events is in agreement with expectation from the standard model. Exclusion contours are given in the (m₀,m_1/2) and (m_{χ ˜},m_{ν ˜}) planes for λ₂₁₁^′=0.09, 0.08, and 0.07.

We present results of a search for R-parity-violating decay of the neutralino χ ˜₁⁰, taken as the lightest supersymmetric particle, to a muon and two jets. The decay proceeds through a lepton-number violating coupling λ_2jk^′ (j=1,2; k=1,2,3), with R-parity conservation in all other production and decay processes. In the absence of candidate events from 77.5±3.9   pb^-1 of data collected by the D0 experiment at the Fermilab Tevatron in pp̅ collisions at sqrt[s]=1.8  TeV, and with an expected background of 0.18±0.03±0.02 events, we set limits on squark and gluino masses within the framework of the minimal low-energy supergravity-supersymmetry model.

Based on 85 pb^-1 data of pp̅ collisions at sqrt[s]=1.8 TeV collected using the DØ detector at Fermilab during the 1994–1995 run of the Tevatron, we present a direct measurement of the total decay width of the W boson Γ_W. The width is determined from the transverse mass spectrum in the W→e+ν_e decay channel and found to be Γ_W=2.23_-0.14^+0.15(stat)±0.10(syst) GeV, consistent with the expectation from the standard model.

We have measured the W boson mass using the DØ detector and a data sample of 82 pb^-1 from the Fermilab Tevatron collider. This measurement uses W→eν decays, where the electron is close to a boundary of a central electromagnetic calorimeter module. Such “edge” electrons have not been used in any previous DØ analysis, and represent a 14% increase in the W boson sample size. For these electrons, new response and resolution parameters are determined, and revised backgrounds and underlying event energy flow measurements are made. When the current measurement is combined with previous DØ W boson mass measurements, we obtain M_W=80.483±0.084 GeV. The 8% improvement from the previous DØ measurement is primarily due to the improved determination of the response parameters for non-edge electrons using the sample of Z bosons with non-edge and edge electrons.

We present the results of a search for leptoquark (LQ) pairs in (85.2±3.7)  pb^-1 of pp̅ collider data collected by the D0 experiment at the Fermilab Tevatron. We observe no evidence for leptoquark production and set a limit on σ(pp̅ →LQLQ̅ →νν+jets) as a function of the mass of the leptoquark (m_LQ). Assuming the decay LQ→νq, we exclude scalar leptoquarks for m_LQ<98 GeV/c², and vector leptoquarks for m_LQ<200 GeV/c² and coupling which produces the minimum cross section, at a 95% confidence level.