Search

We have measured the ZZγ and Zγγ couplings by studying 13.1 pb-1 of pp̄ → γ + X data at √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 strineent 95% C.L. limits to date on anomalous couplings: |h30Z| < 0.4, |h40Z| < 0.06 (Λ = 750 GeV).

We present a study of Zγ+X production in pp̄ collisions at √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 |hZ30

We present new results on the search for the top quark in pp̄ collisions at s=1.8 TeV with an integrated luminosity of 13.5±1.6 pb-1. We have considered tt̄ production in the standard model using electron and muon dilepton decay channels (tt̄→eμ + jets, ee + jets, and μμ + jets) and single-lepton decay channels (tt̄→e + jets and μ + jets) with and without tagging of b quark jets. From all channels we have nine events with an expected background of 3.8±0.9. If we assume that the excess is due to tt̄ production, and assume a top mass of 180 GeV/ c2, we obtain a cross section of 8.2±5.1 pb. © 1995 The American Physical Society.

We search for new physics using events with one high transverse energy photon, two or more jets, and an apparent imbalance in transverse energy, in pp̄ collisions at the Fermilab Tevatron at √s = 1.8 TeV. Such events are predicted for production of supersymmetric particles in some models. No excess is observed beyond expected background. For the parameter space of the minimal supersymmetric standard model with branching fraction B(χ̃20 → γχ̃10) = 1 and mχ̃20 - mχ̃10 > 20 GeV, we obtain a 95% confidence level lower mass limit of 310 GeV for equal mass squarks and gluinos. The results are also interpreted in models with low energy gauge mediated supersymmetry breaking.

We present a search for events consistent with the production and decay of the squarks and gluinos of the minimal supersymmetric standard model (MSSM) in the D0 detector at the Fermilab Tevatron pp collider. We examined data for events containing large missing transverse energy and three or more jets. We observed no excess of events above the expected yield from standard model processes. For a choice of MSSM parameter values, we set a lower limit at the 95% confidence level on the mass of the gluino of 144 GeV /c2 for all squark masses and a lower limit of 212 GeV /c2 for equal squark and gluino masses. © 1995 The American Physical Society.

We present results from a search for anomalous WW and WZ production in p p̄ collisions at √s = 1.8 TeV. We used p p̄ →jjX 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 paper presents the first measurement of inclusive J/ψ production cross section in the forward pseudorapidity region 2.5 ≤ |ηJ/ψ| ≤ 3.7 in pp̄ collisions at √s = 1.8 TeV. The results are based on 9.8 pb-1 of data collected using the D0 detector at the Fermilab Tevatron Collider. The inclusive J/ψ cross section for transverse momenta between 1 and 16 GeV/c is compared with theoretical models of charmonium production.

We have searched for first generation scalar leptoquark (LQ) pairs in the ev + jets channel using pp̄ collider data (∫ L dt ≈ 115 pb-1) collected by the DO experiment at the Fermilab Tevatron during 1992-1996. The analysis yields no candidate events. We combine the results with those from the ee + jets and vv + jets channels to obtain 95% confidence level (C.L.) upper limits on the LQ pair production cross section as a function of mass and of β, the branching fraction to a charged lepton. Comparing with the next-to-leading order theory, we set 95% C.L. lower limits on the LQ mass of 225, 204, and 79 GeV/c2 for β= 1, 1/2, and 0, respectively.

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 p̄p collisions at √s = 1.8 TeV and recorded with the DO 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 ≲ ET y ≲ 80 GeV.

We report a measurement of the W boson mass based on an integrated luminosity of 82 pb-1 from pp̄ collisions at √s = 1.8 TeV recorded in 1994-1995 by the D0 detector at the Fermilab Tevatron. We identify W bosons by their decays to e v and extract the mass by fitting the transverse mass spectrum from 28 323 W boson candidates. A sample of 3563 dielectron events, mostly due to Z → ee decays, constrains models of W boson production and the detector. We measure MW = 80.44 ± 0.10(stat) ± 0.07(syst) GeV. By combining this measurement with our result form the 1992-1993 data set, we obtain Mw = 80.43 ± 0.11 GeV.

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 DO 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 results of a search for W boson pair production in pp collisions at √s= 1.8 TeV with subsequent decay to eμ, ee, and μμ channels are presented. Five candidate events are observed with an expected background of 3.1 ±0.4 events for an integrated luminosity of approximately 97 pb-1. Limits on the anomalous couplings are obtained from a maximum likelihood fit of the ET spectra of the leptons in the candidate events. Assuming identical WWγ and WWZ couplings, the 95% C.L. limits are -0.62

A study of the particle multiplicity between jets with large rapidity separation has been performed using the D0 detector at the Fermilab Tevatron pp̄ Collider operating at √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.

We present a measurement of the tt̄ production cross section in pp̄ collisions at √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 tt̄ 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 tt̄ production cross section to be 5.5 ± 1.8 pb.

We have studied J/ψ production in pp̄ collisions at √s = 1.8 TeV with the DØ detector at Fermilab using μ+/μ- data. We have measured the inclusive J/ψ production cross section as a function of J/ψ transverse momentum, pT. For the kinematic range pT > 8 GeV/c and |η\ < 0.6 we obtain σ(pp̄ → J/ψ + X) ·Br(J/ψ → μ+μ-) = 2.08 ± 0.17(stat) ± 0.46(syst) nb. Using the muon impact parameter we have estimated the fraction of J/ψ mesons coming from B meson decays to be fb = 0.35 ± 0.09(stat)±0.10(syst) and inferred the inclusive b production cross section. From the information on the event topology the fraction of nonisolated J/ψ events has been measured to be fnonisol = 0.64 ± 0.08(stat)±0.06(syst). We have also obtained the fraction of J/ψ events resulting from radiative decays of χc states, fχ = 0.32 ± 0.07(stat)±0.07(syst). We discuss the implications of our measurements for charmonium production processes.

We report on a search for second generation scalar leptoquarks with the D0 detector at the Fermilab Tevatron pp̄ collider at √s = 1.8 TeV. This search is based on 12.7 pb-1 of data. Second generation leptoquarks are assumed to be produced in pairs and to decay into a neutrino and quark with branching ratio β or into a neutrino and quark with branching ratio 1 - β. We obtain cross section times branching ratio limits as a function of leptoquark mass and set lower limits on the leptoquark mass at the 95% confidence level.

We performed a direct search for the anomalous ZZγ and Zγγ couplings by studying pp̄ → ℓℓγ + X (ℓ = e, μ) events at √s = 1.8 TeV with the D0 detector at the Fermilab Tevatron Collider. A fit to the transverse energy spectrum of the photon in the signal events, based on the data set corresponding to an integrated luminosity of 14.3 pb-1 (13.7 pb-1) for the electron (muon) channel, yields the following 95% confidence level limits on the anomalous CP-conserving ZZγ couplings: |h30Z| < 1.8 (h40Z = 0) and |h30Z| < 0.5 (h30Z = 0), for a form-factor scale A = 500 GeV. Limits for the Zγγ couplings and CP-violating couplings are also discussed.

A search for new physics has been carried out in the channel pp̄ → γγ + ET, as expected in various supersymmetric models. We require two photons with transverse energy ET > 12 GeV and pseudorapidity |ηγ < 1.1. The distribution of missing transverse energy (ET) is consistent with background and no events have ET > 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(pp̄ → γγET + X), where ETγ > 12 GeV, |ηγ| < 1.1, and ET > 25 GeV.

A measurement of the mass of the W boson is presented which is based on a sample of 5982 W → eν decays observed in pp̄ collisions at √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 MW = 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 have made a precise measurement of the central inclusive jet cross section at √s = 1.8 TeV. The measurement is based on an integrated luminosity of 92 pb-1 collected at the Fermilab Tevatron p̄p Collider with the D0 detector. The cross section, reported as a function of jet transverse energy (ET ≥ 60 GeV) in the pseudorapidity interval |η| ≤ 0.5, is in good agreement with predictions from next-to-leading order quantum chromo dynamics. © 1999 The American Physical Society.

The WW gauge boson couplings were measured using pp» +X (=e,) events at s=1.8 TeV observed with the D0 detector at the Fermilab Tevatron Collider. The signal, obtained from the data corresponding to an integrated luminosity of 13.8 pb-1, agrees well with the standard model prediction. A fit to the photon transverse energy spectrum yields limits at the 95% confidence level on the CP-conserving anomalous coupling parameters of -1.6<

The results of a search for W boson pair production in pp» collisions at s=1.8 TeV with subsequent decay to dilepton (e,ee, and) channels are presented. One event is observed with an expected background of 0.56 0.13 events with an integrated luminosity of approximately 14 pb-1. Assuming equal strengths for the WWZ and WW gauge boson coupling parameters and, limits on the CP-conserving anomalous coupling constants are -2.6<

The ratio of the number of W+1 jet to W+0 jet events is measured with the D0 detector using data from the 1992-93 Tevatron Collider run. For the W→eν channel with a minimum jet ET cutoff of 25 GeV, the experimental ratio is 0.065±0.003stat±0.007syst. Next-to-leading order QCD predictions for various parton distributions agree well with each other and are all over 1 standard deviation below the measurement. Varying the strong coupling constant αs in both the parton distributions and the partonic cross sections simultaneously does not remove this discrepancy. © 1995 The American Physical Society.

The results of a search for W boson pair production in pp̄ collisions at √s = 1.8 TeV with subsequent decay to eμ, ee, and μμ channels are presented. Five candidate events are observed with an expected background of 3.1 ± 0.4 events for an integrated luminosity of approximately 97 pb-1. Limits on the anomalous couplings are obtained from a maximum likelihood fit of the ET spectra of the leptons in the candidate events. Assuming identical WWγ and WWZ couplings, the 95% C.L. limits are -0.62

We present a search for pair produced top squarks, the supersymmetric partners of the top quark, using the D0 detector at the Fermilab Tevatron pp̄ collider. We consider a scenario in which the lighter of the two top squarks t̃ l decays with 100% branching fraction to a charm quark and the lightest neutralino X̃ 1 0 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 mt̃1-mX̃10 parameter space. The highest mt̃1 value we exclude is 93 GeV/c 2 with a corresponding mmX̃10 value of 8 GeV/c 2.

The inclusive cross sections times leptonic branching ratios for W and Z boson production in p p̄ collisions at √s = 1.8 TeV were measured using the D0 detector at the Fermilab Tevatron collider: σWB(W → e ν) = 2.36 ± 0.07 ± 0.13 n b, σWB(W → μν) = 2.09 ± 0.23 ± 0.11 n b, σZB(Z → e+e-) = 0.218 ± 0.011 ± 0.012 n b, and σZB(Z → μ+μ-) = 0.178 ± 0.030 ± 0.009 n b. The first error is the combined statistical and systematic uncertainty, and the second reflects the uncertainty in the luminosity. For the combined electron and muon analyses we find σWB(W → lν)/σZB(Z → l+l-) = 10.90 ± 0.49. Assuming standard model couplings, this result is used to determine the width of the W boson, Γ(W) = 2.044 ± 0.093 GeV.

A search for a heavy charged gauge boson, W′, using the decay channels W′ → eν and W′ → τν → eννν is reported. The data used in the analysis were collected by the DØ experiment at the Fermilab Tevatron during the 1992-93 pp collider run from an integrated luminosity of 13.9 ± 0.8 pb-1 at s=1.8 TeV. Assuming that the neutrino from W′ decay is stable and has a mass significantly less than mW′, an upper limit at the 95% confidence level is set on the cross section times branching ratio for pp → W′ → eν. A W′ with the same couplings to quarks and leptons as the standard model W boson is excluded for mW′ < 610 GeV/c2. © 1995.