Search Results (104 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.

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.

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.

We have performed a search for scalar top quark (stop) pair production in the inclusive electron-muon-missing transverse energy final state, using a sample of pp̅ events corresponding to 108.3 pb^-1 of data collected with the D0 detector at Fermilab. The search is done in the framework of the minimal supersymmetric standard model assuming that the sneutrino is the lightest supersymmetric particle. For the dominant decays of the lightest stop, t̃→bχ̃₁⁺ and t̃→bℓν̃, no evidence for signal is found. We derive cross-section limits as a function of stop ( t̃ ), chargino ( χ̃₁⁺), and sneutrino ( ν̃) masses.

We present a search for charged Higgs bosons in decays of pair-produced top quarks in pp̅ collisions at sqrt[s]  =  1.8 TeV recorded by the D0 detector at the Fermilab Tevatron collider. With no evidence for signal, we exclude most regions of the ( M_H^±,tanβ) parameter space where the decay t→ H⁺b has a branching fraction >0.36 and B(H^±→τν_τ) is large.

The DØ Collaboration has studied for the first time the properties of hadron-collider jets reconstructed with a successive-combination algorithm based on relative transverse momenta (k_⊥) of energy clusters. Using the standard value D=1.0 of the jet-separation parameter in the k_⊥ algorithm, we find that the p_T of such jets is higher than the E_T of matched jets reconstructed with cones of radius R=0.7, by about 5 (8) GeV at p_T≈90 (240) GeV. To examine internal jet structure, the k_⊥ algorithm is applied within D=0.5 jets to resolve any subjets. The multiplicity of subjets in jet samples at sqrt[s]=1800 GeV and 630 GeV is extracted separately for gluons (M_g) and quarks (M_q), and the ratio of average subjet multiplicities in gluon and quark jets is measured as (〈M_g〉-1)/(〈M_q〉-1)=1.84±0.15 (stat)±_0.18^0.22 (syst). This ratio is in agreement with the expectations from the HERWIG Monte Carlo event generator and a resummation calculation, and with observations in e⁺e^- annihilations, and is close to the naive prediction for the ratio of color charges of C_A/C_F=9/4=2.25.

The inclusive cross section for production of isolated photons has been measured in pp̅ collisions at sqrt[s]  =  630 GeV with the D0 detector at the Fermilab Tevatron Collider. The photons span a transverse energy (E_T) range from 7–49 GeV and have pseudorapidity |η|<2.5. This measurement is combined with the previous D0 result at sqrt[s]  =  1800 GeV to form a ratio of the cross sections. Comparison of next-to-leading-order QCD with the measured cross section at 630 GeV and the ratio of cross sections show satisfactory agreement in most of the E_T range.

We describe QUAERO, a method that (i) enables the automatic optimization of searches for physics beyond the standard model, and (ii) provides a mechanism for making high energy collider data generally available. We apply QUAERO to searches for standard model WW, ZZ, and tt̅ production, to searches for these objects produced through a new heavy resonance, and to the first direct search for W^′→WZ. Through this interface, we make three data sets collected by the D0 experiment at sqrt[s]  =  1.8 TeV publicly available.

We describe a search for the pair production of first-generation scalar and vector leptoquarks in the eejj and eνjj channels by the DØ Collaboration. The data are from the 1992–1996 pp̅ run at sqrt[s]=1.8 TeV at the Fermilab Tevatron collider. We find no evidence for leptoquark production; in addition, no kinematically interesting events are observed using relaxed selection criteria. The results from the eejj and eνjj channels are combined with those from a previous DØ analysis of the ννjj channel to obtain 95% confidence level (C.L.) upper limits on the leptoquark pair-production cross section as a function of mass and of β, the branching fraction to a charged lepton. These limits are compared to next-to-leading-order theory to set 95% C.L. lower limits on the mass of a first-generation scalar leptoquark of 225, 204, and 79 GeV/c² for β=1, 1 / 2, and 0, respectively. For vector leptoquarks with gauge (Yang-Mills) couplings, 95% C.L. lower limits of 345, 337, and 206 GeV/c² are set on the mass for β=1, 1 / 2, and 0, respectively. Mass limits for vector leptoquarks are also set for anomalous vector couplings.

We present results of searches for technirho (ρ_T), techniomega (ω_T), and Z^′ particles, using the decay channels ρ_T,ω_T,Z^′→e⁺e^-. The search is based on 124.8 pb^-1 of data collected by the D0 detector at the Fermilab Tevatron during 1992–1996. In the absence of a signal, we set 95% C.L. upper limits on the cross sections for the processes pp̅ →ρ_T,ω_T,Z^′→e⁺e^- as a function of the mass of the decaying particle. For certain model parameters, we exclude the existence of degenerate ρ_T and ω_T states with masses below about 200 GeV. We exclude a Z^′ with mass below 670 GeV, assuming that it has the same couplings to fermions as the Z boson.

Results are presented from analyses of jet data produced in pp̅ collisions at sqrt[s]=630 and 1800 GeV collected with the DØ detector during the 1994–1995 Fermilab Tevatron Collider run. We discuss the details of detector calibration, and jet selection criteria in measurements of various jet production cross sections at sqrt[s]=630 and 1800 GeV. The inclusive jet cross sections, the dijet mass spectrum, the dijet angular distributions, and the ratio of inclusive jet cross sections at sqrt[s]=630 and 1800 GeV are compared to next-to-leading-order QCD predictions. The order α_s³ calculations are in good agreement with the data. We also use the data at sqrt[s]=1800 GeV to rule out models of quark compositeness with a contact interaction scale less than 2.2 TeV at the 95% confidence level.

We apply a quasi-model-independent strategy (“SLEUTH”) to search for new high p_T physics in ≈100 pb^-1 of pp̅ collisions at sqrt[s]=1.8 TeV collected by the DØ experiment during 1992–1996 at the Fermilab Tevatron. Over 32 eμX, W+jets-like, Z+jets-like, and (l/γ)(l/γ)(l/γ)X exclusive final states are systematically analyzed for hints of physics beyond the standard model. Simultaneous sensitivity to a variety of models predicting new phenomena at the electroweak scale is demonstrated by testing the method on a particular signature in each set of final states. No evidence of new high p_T physics is observed in the course of this search, and we find that 89% of an ensemble of hypothetical similar experimental runs would have produced a final state with a candidate signal more interesting than the most interesting observed in these data.

We apply a quasi-model-independent strategy (“Sleuth”) to search for new high p_T physics in ≈100 pb^-1 of pp̅ collisions at sqrt[s]  =  1.8 TeV collected by the D0 experiment during 1992–1996 at the Fermilab Tevatron. We systematically analyze many exclusive final states and demonstrate sensitivity to a variety of models predicting new phenomena at the electroweak scale. No evidence of new high p_T physics is observed.

We report on a search for supersymmetry using the DØ detector. The 1994–1996 data sample of sqrt[s]=1.8 TeV pp̅ collisions was analyzed for events containing two leptons (e or μ), two or more jets, and missing transverse energy. Assuming the minimal supergravity model, with A₀=0 and μ<0, various thresholds were employed to optimize the search. No events were found beyond expectation from the background. We set a lower limit at the 95% C.L. of 255 GeV/c² for equal mass squarks and gluinos for tan β=2, and present exclusion contours in the (m₀,m_1/2) plane for tan β=2–6.

The D0 Collaboration has measured the inclusive jet cross section in p̅ p collisions at sqrt[s]  =  630 GeV. The results for pseudorapidities ||(η)<0.5 are combined with our previous results at sqrt[s]  =  1800 GeV to form a ratio of cross sections with smaller uncertainties than either individual measurement. Next-to-leading-order QCD predictions show excellent agreement with the measurement at 630 GeV; agreement is also satisfactory for the ratio. Specifically, despite a 10% to 15% difference in the absolute magnitude, the dependence of the ratio on jet transverse momentum is very similar for data and theory.

We report on a study of the ratio of inclusive three-jet to inclusive two-jet production cross sections as a function of total transverse energy in pp̅ collisions at a center-of-mass energy sqrt[s]  =  1.8 TeV, using data collected with the D0 detector during the 1992–1993 run of the Fermilab Tevatron Collider. The measurements are used to deduce preferred renormalization scales in perturbative O(α_s³) QCD calculations in modeling soft-jet emission.

We present the first measurement of the electron angular distribution parameter α₂ in W→eν events produced in proton-antiproton collisions as a function of the W boson transverse momentum. Our analysis is based on data collected using the DØ detector during the 1994–1995 Fermilab Tevatron run. We compare our results with next-to-leading order perturbative QCD, which predicts an angular distribution of (1±α₁ cos θ^*+α₂ cos² θ^*), where θ^* is the polar angle of the electron in the Collins-Soper frame. In the presence of QCD corrections, the parameters α₁ and α₂ become functions of p_T^W, the W boson transverse momentum. This measurement provides a test of next-to-leading order QCD corrections which are a non-negligible contribution to the W boson mass measurement.

We report a new measurement of the pseudorapidity ( η) and transverse-energy ( E_T) dependence of the inclusive jet production cross section in pp̅ collisions at sqrt[s]  =  1.8 TeV using 95 pb^-1 of data collected with the D0 detector at the Fermilab Tevatron. The differential cross section d²σ/(dE_Tdη) is presented up to |η|  =  3, significantly extending previous measurements. The results are in good overall agreement with next-to-leading order predictions from QCD and indicate a preference for certain parton distribution functions.

We report a search for effects of large extra spatial dimensions in pp̅ collisions at a center-of-mass energy of 1.8 TeV with the D0 detector, using events containing a pair of electrons or photons. The data are in good agreement with the expected background and do not exhibit evidence for large extra dimensions. We set the most restrictive lower limits to date, at the 95% C.L. on the effective Planck scale between 1.0 and 1.4 TeV for several formalisms and numbers of extra dimensions.

We present a search for electroweak production of single top quarks in the electron+jets and muon+jets decay channels. The measurements use ≈90 pb^-1 of data from Run 1 of the Fermilab Tevatron collider, collected at 1.8 TeV with the DØ detector between 1992 and 1995. We use events that include a tagging muon, implying the presence of a b jet, to set an upper limit at the 95% confidence level on the cross section for the s-channel process pp̅ →tb+X of 39 pb. The upper limit for the t-channel process pp̅ →tqb+X is 58 pb.

Bottom-quark production in p̅ p collisions at sqrt[s]  =  1.8 TeV is studied with 5 pb^-1 of data collected in 1995 by the D0 detector at the Fermilab Tevatron Collider. The differential production cross section for b jets in the central rapidity region ( | y^b|<1) as a function of jet transverse energy is extracted from a muon-tagged jet sample. Within experimental and theoretical uncertainties, D0 results are found to be higher than, but compatible with, next-to-leading-order QCD predictions.

We present a measurement of the W boson mass using data collected by the DØ experiment at the Fermilab Tevatron during 1994–1995. We identify W bosons by their decays to eν final states where the electron is detected in a forward calorimeter. We extract the W boson mass M_W by fitting the transverse mass and transverse electron and neutrino momentum spectra from a sample of 11 089 W→eν decay candidates. We use a sample of 1687 dielectron events, mostly due to Z→ee decays, to constrain our model of the detector response. Using the forward calorimeter data, we measure M_W=80.691±0.227 GeV. Combining the forward calorimeter measurements with our previously published central calorimeter results, we obtain M_W=80.482±0.091 GeV.

We present a quasi-model-independent search for the physics responsible for electroweak symmetry breaking. We define final states to be studied, and construct a rule that identifies a set of relevant variables for any particular final state. A new algorithm (“SLEUTH”) searches for regions of excess in those variables and quantifies the significance of any detected excess. After demonstrating the sensitivity of the method, we apply it to the semi-inclusive channel eμX collected in 108 pb^-1 of pp̅ collisions at sqrt[s]=1.8 TeV at the DØ experiment during 1992–1996 at the Fermilab Tevatron. We find no evidence of new high p_T physics in this sample.

The result of a search for gaugino pair production with a trilepton signature is reinterpreted in the framework of minimal supergravity (MSUGRA) with R-parity violation via leptonic λ Yukawa couplings. The search used 95 pb^-1 of pp̅ collisions at sqrt[s]=1.8 TeV recorded by the DØ detector at the Fermilab Tevatron. A large domain of the MSUGRA parameter space is excluded for λ₁₂₁, λ₁₂₂>~10^-4.

Limits on anomalous WWγ and WWZ couplings are presented from a study of WW/WZ→eνjj events produced in pp̅ collisions at sqrt[s]=1.8 TeV. Results from the analysis of data collected using the DØ detector during the 1993–1995 Tevatron collider run at Fermilab are combined with those of an earlier study from the 1992–1993 run. A fit to the transverse momentum spectrum of the W boson yields direct limits on anomalous WWγ and WWZ couplings. With the assumption that the WWγ and WWZ couplings are equal, we obtain -0.34<λ<0.36 (with Δκ=0) and -0.43<Δκ<0.59 (with λ=0) at the 95% confidence level for a form-factor scale Λ=2.0 TeV.