Search Results (36 total)

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.

A measurement of the mass of the W boson is presented which is based on a sample of 5982 W→eν decays observed in pp̅ collisions at sqrt[s]=1.8 TeV with the D∅ detector during the 1992–1993 run. From a fit to the transverse mass spectrum, combined with measurements of the Z boson mass, the W boson mass is measured to be M_W=80.350±0.140(stat)±0.165(syst)±0.160(scale) GeV/c². Detailed discussions of the determination of the absolute energy scale, the measured efficiencies, and all systematic uncertainties are presented.

We present a study of Zγ+X production in pp̅ collisions at sqrt[s]=1.8 TeV from 97 (87) pb^-1 of data collected in the eeγ (μμγ) decay channel with the D0 detector at Fermilab. The event yield and kinematic characteristics are consistent with the standard model predictions. We obtain limits on anomalous ZZγ and Zγγ couplings for form factor scales Λ=500 GeV and Λ=750 GeV. Combining this analysis with our previous results yields 95% C.L. limits |h₃₀^Z|<0.36, |h₄₀^Z|<0.05, |h₃₀^γ|<0.37, and |h₄₀^γ|<0.05 for a form factor scale Λ=750 GeV.

The D0 Collaboration has performed a measurement of the top quark mass m_t based on six candidate events for the process tt̅ →bW⁺b̅ W^-, where the W bosons decay to eν or μν. This sample was collected during an exposure of the D0 detector to an integrated luminosity of 125 pb^-1 of sqrt[s]  =  1.8  TeV pp̅ collisions. We obtain m_t  =  168.4±12.3(stat)±3.6(syst)  GeV/c², consistent with the measurement obtained using single-lepton events. Combination of the single-lepton and dilepton results yields m_t  =  172.0±7.5 GeV/c².

We have measured the dijet angular distribution in sqrt[s]  =  1.8  TeV pp̅ collisions using the D0 detector. Order α_s³ QCD predictions are in good agreement with the data. At 95% confidence limit the data exclude models of quark compositeness in which the contact interaction scale is below 2 TeV.

We search for inclusive high E_T diphoton events with large missing transverse energy in pp̅ collisions at sqrt[s]  =  1.8  TeV. Such events are expected from pair production of charginos and neutralinos within the framework of the minimal supersymmetric standard model with a light gravitino. No excess of events is observed. In that model, and assuming gaugino mass unification at the GUT scale, we obtain a 95% C.L. exclusion region in the supersymmetry parameter space and lower mass bounds of 150 GeV/c² for the lightest chargino and 77 GeV/c² for the lightest neutralino.

This report describes the first search for top squark pair production in the channel t₁t̃̅ ₁→bb̅ χ̃₁⁺χ̃₁^-→ee+jets+E_T using 74.9±8.9 pb^-1 of data collected using the DØ detector. A 95% confidence level upper limit on σ⋅B is presented. The limit is above the theoretical expectation for σ⋅B for this process, but does show the sensitivity of the current DØ data set to a particular topology for new physics.

We have searched for the pair production of first generation scalar leptoquarks in the eejj channel using the full data set (123 pb^-1) collected with the D0 detector at the Fermilab Tevatron during 1992–1996. We observe no candidates with an expected background of approximately 0.4 events. Comparing the experimental 95% confidence level upper limit to theoretical calculations of the cross section with the assumption of a 100% branching fraction of eq, we set a lower limit on the mass of a first generation scalar leptoquark of 225 GeV/c². The results of this analysis rule out the interpretation of the excess of high Q² events at DESY HERA as leptoquarks which decay exclusively to eq.

The gauge boson pair production processes Wγ, WW, WZ, and Zγ were studied using pp̅ collisions corresponding to an integrated luminosity of approximately 14 pb^-1 at a center-of-mass energy of sqrt[s]=1.8 TeV. Analysis of Wγ production with subsequent W boson decay to lν (l=e,μ) is reported, including a fit to the p_T spectrum of the photons which leads to limits on anomalous WWγ couplings. A search for WW production with subsequent decay to ll¯νν̅ (l=e,μ) is presented, leading to an upper limit on the WW production cross section and limits on anomalous WWγ and WWZ couplings. A search for high p_T W bosons in WW and WZ production is described, where one W boson decays to an electron and a neutrino and the second W boson or the Z boson decays to two jets. A maximum likelihood fit to the p_T spectrum of W bosons resulted in limits on anomalous WWγ and WWZ couplings. A combined fit to the three data sets which provided the tightest limits on anomalous WWγ and WWZ couplings is also described. Limits on anomalous ZZγ and Zγγ couplings are presented from an analysis of the photon E_T spectrum in Zγ events in the decay channels (ee, μμ, and νν) of the Z boson.

We present limits on anomalous WWZ and WWγ couplings from a search for WW and WZ production in pp̅ collisions at sqrt[s]  =  1.8 TeV. We use pp̅ →eνjjX events recorded with the D0 detector at the Fermilab Tevatron Collider during the 1992–1995 run. The data sample corresponds to an integrated luminosity of 96.0±5.1 pb^-1. Assuming identical WWZ and WWγ coupling parameters, the 95% C.L. limits on the CP-conserving couplings are -0.33<λ<0.36 (Δκ  =  0) and -0.43<Δκ<0.59 (λ  =  0), for a form factor scale Λ  =  2.0 TeV. Limits based on other assumptions are also presented.

We present a measurement of the tt̅ production cross section in pp̅ collisions at sqrt[s]  =  1.8 TeV by the D0 experiment at the Fermilab Tevatron. The measurement is based on data from an integrated luminosity of approximately 125 pb^-1 accumulated during the 1992–1996 collider run. We observe 39 tt̅ candidate events in the dilepton and lepton+jets decay channels with an expected background of 13.7±2.2 events. For a top quark mass of 173.3 GeV/c², we measure the tt̅ production cross section to be 5.5±1.8 pb.

We measure the top quark mass m_t using tt̅ pairs produced in the D0 detector by sqrt[s]  =  1.8 TeVpp̅ collisions in a 125 pb^-1 exposure at the Fermilab Tevatron. We make a two constraint fit to m_t in tt̅ →bW⁺b̅ W^- final states with one W decaying to qq̅ and the other to eν or μν. Events are binned in fit mass versus a measure of probability for events to be signal rather than background. Likelihood fits to the data yield m_t  =  173.3±5.6(stat)±6.2(syst) GeV/c².

We report on a search for pair production of a fourth generation charge -1/3 quark ( b^′) in pp̅ collisions at sqrt[s]  =  1.8 TeV by the DØ experiment at the Fermilab Tevatron using an integrated luminosity of 93 pb^-1. Both b^′ quarks are assumed to decay via flavor changing neutral currents (FCNC). The search uses the signatures γ+3 jets +μ-tag and 2γ+2 jets. We see no significant excess of events over the expected background. We place an upper limit on the production cross section times branching fraction that is well below theoretical expectations for a b^′ decaying exclusively via FCNC for b^′ masses up to m_Z+m_b.

We have measured the ZZγ and Zγγ couplings by studying 13.1 pb^-1 of pp̅ →E_tγ+X data at sqrt[s]  =  1.8 TeV with the DØ detector at the Fermilab Tevatron Collider. This is the first study of hadronic Zγ production in the neutrino decay channel. Combining this measurement with our previous results using Z→ee and μμ yields the most stringent 95% C.L. limits to date on anomalous couplings: |h₃₀^Z|<0.4, |h₄₀^Z|<0.06 ( Λ  =  750 GeV).

We have measured the WWγ gauge boson coupling parameters using pp̅ →ℓνγ+X ( ℓ  =  e,μ) events at sqrt[s]  =  1.8 TeV. The data, corresponding to an integrated luminosity of 92.8 pb^-1, were collected using the DØ detector at the Fermilab Tevatron Collider. The measured cross section times branching ratio for pp̅ →Wγ+X with p_T^γ>10 GeV/c and R_ℓγ>0.7 is 11.3_-1.5^+1.7±1.5 pb, in agreement with the standard model prediction. The 1 degree of freedom 95% confidence level limits on individual CP-conserving parameters are -0.93<Δκ<0.94 and -0.31<λ<0.29. Similar limits are set on the CP-violating coupling parameters.

A search for new physics has been carried out in the channel pp̅ →γγ+E_T, as expected in various supersymmetric models. We require two photons with transverse energy E_T^γ>12 GeV and pseudorapidity |η^γ|<1.1. The distribution of missing transverse energy ( E_T) is consistent with background and no events have E_T>25 GeV. We set limits on production cross sections for selectron, sneutrino, and neutralino pairs, decaying into photons, which range from about 400 fb to 1 pb depending on the sparticle masses. A general limit of 185 fb (95% C.L.) is set on σ(pp̅ →γγE_T+X), where E_T^γ>12 GeV, |η^γ|<1.1, and E_T>25 GeV.

A measurement of the cross section for production of single, isolated photons is reported for transverse energies in the range of 10–125 GeV for two regions of pseudorapidity, |η|<0.9 and 1.6<|η|<2.5. The data represent 12.9 pb^-1 of integrated luminosity accumulated in p̅ p collisions at sqrt[s]  =  1.8 TeV and recorded with the D0 detector at the Fermilab Tevatron Collider. The background, predominantly from jets which fragment to neutral mesons, is estimated using the longitudinal shower shape in the calorimeter. In both pseudorapidity regions the cross section is found to agree with the next-to-leading order QCD prediction for 30≲E_T^γ≲80 GeV.

A measurement of the mass of the W boson is presented based on a sample of 5982 W→eν decays observed in pp̅ collisions at sqrt[s]  =  1.8 TeV with the D0 detector during the 1992–1993 run. From a fit to the transverse mass spectrum, combined with measurements of the Z boson mass, the W boson mass is measured to be M_W  =  80.350±0.140(stat)±0.165(syst)±0.160(scale) GeV/c².

We present results from a search for anomalous WW and WZ production in pp̅ collisions at sqrt[s]  =  1.8  TeV. We used pp̅ →eνjjX events observed during the 1992–1993 run of the Fermilab Tevatron collider, corresponding to an integrated luminosity of 13.7±0.7 pb^-1. A fit to the transverse momentum spectrum of the W boson yields direct limits on the CP-conserving anomalous WWγ and WWZ coupling parameters of -0.9<Δκ<1.1 (with λ  =  0) and -0.6<λ<0.7 (with Δκ  =  0) at the 95% confidence level, for a form factor scale Λ  =  1.5  TeV, assuming that the WWγ and WWZ coupling parameters are equal.

This study reports the first measurement of the azimuthal decorrelation between jets with pseudorapidity separation up to five units. The data were accumulated using the D0 detector during the 1992–1993 collider run of the Fermilab Tevatron at sqrt[s]  =  1.8 TeV. These results are compared to next-to-leading order (NLO) QCD predictions and to two leading-log approximations (LLA) where the leading-log terms are resummed to all orders in α_S. The final state jets as predicted by NLO QCD show less azimuthal decorrelation than the data. The parton showering LLA Monte Carlo HERWIG describes the data well; an analytical LLA prediction based on Balitsky-Fadin-Kuraev-Lipatov resummation shows more decorrelation than the data.

The global topologies of inclusive three- and four-jet events produced in p̅ p interactions are described. The three- and four-jet events are selected from data recorded by the D0 detector at the Fermilab Tevatron Collider operating at a center-of-mass energy of sqrt[s]=1800 GeV. The measured, normalized distributions of various topological variables are compared with parton-level predictions of tree-level QCD calculations. The parton-level QCD calculations are found to be in good agreement with the data. The studies also show that the topological distributions of the different subprocesses involving different numbers of quarks are very similar and reproduce the measured distributions well. The parton-shower Monte Carlo generators provide a less satisfactory description of the topologies of the three- and four-jet events.

We report on a search for right-handed W bosons (W_R) with mass greater than 200 GeV/c². We used data collected with the D0 detector at the Fermilab Tevatron pp̅ collider at sqrt[s]  =  1.8 TeV to search for W_R decays into an electron and a massive right-handed neutrino W_R^±→e^±N_R. Using the inclusive electron data, we set mass limits independent of the N_R decay: m_WR>650 GeV/c² and m_WR>720 GeV/c² at the 95% confidence level, valid for m_NR<1 / 2 m_WR and m_NR≪m_WR, respectively. The latter also represents a new lower limit on the mass of a heavy left-handed W boson (W^′) decaying into eν. In addition, limits on m_WR valid for larger values of the N_R mass are obtained assuming that N_R decays to an electron and two jets.

We have searched for associated production of the lightest chargino W̃₁ and next-to-lightest neutralino Z̃₂ of the minimal supersymmetric standard model in pp̅ collisions at sqrt[s]  =  1.8 TeV using the D0 detector at the Fermilab Tevatron collider. Data corresponding to an integrated luminosity of 12.5±0.7 pb^-1 were examined for events containing three isolated leptons. No evidence for W̃₁Z̃₂ pair production was found. Limits on σ(W̃₁Z̃₂)B(W̃₁→lνZ̃₁)B(Z̃₂→ll̅ Z̃₁) are presented.

We present a search for pair produced top squarks, the supersymmetric partners of the top quark, using the D0 detector at the Fermilab Tevatron pp̅ collider. We consider a scenario in which the lighter of the two top squarks t̃₁ decays with 100% branching fraction to a charm quark and the lightest neutralino χ̃₁⁰ yielding a signal of two acollinear jets with missing transverse energy. We observe 3 events while we expect 3.5±1.2 events from the known standard model processes. We exclude at the 95% confidence level a significant region of the m_t̃1- m_χ̃1⁰ parameter space. The highest m_t̃1 value we exclude is 93 GeV/c² with a corresponding m_χ̃1⁰ value of 8 GeV/c².

A study of the particle multiplicity between jets with large rapidity separation has been performed using the D0 detector at the Fermilab Tevatron pp̅ Collider operating at sqrt[s]  =  1.8 TeV. A significant excess of low-multiplicity events is observed above the expectation for color-exchange processes. The measured fractional excess is 1.07±0.10(stat)_-0.13^+0.25(syst)%, which is consistent with a strongly interacting color-singlet (colorless) exchange process and cannot be explained by electroweak exchange alone. A lower limit of 0.80% (95% C.L.) is obtained on the fraction of dijet events with color-singlet exchange, independent of the rapidity gap survival probability.