Search for Pair Production of Light Scalar Top Quarks in \u3ci\u3epp̅\u3c/i\u3e Collisions at √\u3ci\u3es\u3c/i\u3e = 1.8 TeV

Using 85.2 ± 3.6 pb1 of pp̅ collisions collected at √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 (t̃), 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 t̃ and neutralino phase space. In particular, we exclude the t̃ mass mt̃ \u3c 122 GeV/c2 for a neutralino mass of 45 GeV/c2

Search for Narrow \u3ci\u3ett̅\u3c/i\u3e Resonances in \u3ci\u3epp̅\u3c/i\u3e Collisions at √\u3ci\u3es\u3c/i\u3e = 1.8 TeV

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 √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 Mx. These limits are used to exclude the existence of a leptophobic top-color particle with mass Mx \u3c 560 GeV/c2, using a theoretical cross section for a width Γx = 0.012Mx

Measurement of the \u3ci\u3eWW\u3c/i\u3e Production Cross Section in \u3ci\u3epp̅\u3c/i\u3e Collisions at √\u3ci\u3es\u3c/i\u3e = 1.96 TeV

We present a measurement of the W boson pair-production cross section in pp̅ collisions at a center-of- mass energy of √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 x 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

Search for New Physics Using QUAERO: A General Interface to D0 Event Data

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 √s =1.8 TeV publicly available

Multiple jet production at low transverse energies in \u3ci\u3epp̅\u3c/i\u3e collisions at √\u3ci\u3es\u3c/i\u3e = 1.8 TeV

We present data on multiple production of jets with transverse energies near 20 GeV in pp̅ collisions at √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

Direct Search for Charged Higgs Bosons in Decays of Top Quarks

We present a search for charged Higgs bosons in decays of pair-produced top quarks in pp̅ collisions at√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 (MH±, tan β) parameter space where the decay t → H+b has a branching fraction \u3e0.36 and B(H± → τντ is large

\u3ci\u3ett̅\u3c/i\u3e production cross section in \u3ci\u3epp̅\u3c/i\u3e collisions at √\u3ci\u3es\u3c/i\u3e = 1.8 TeV

Results are presented on a measurement of the tt̅ pair production cross section in pp̅ collisions at √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

Search for first-generation scalar leptoquarks in (\u3ci\u3epp̅\u3c/i\u3e collisions at√\u3ci\u3es\u3c/i\u3e = 1.96 TeV

We report on a search for pair production of first-generation scalar leptoquarks (LQ) in pp̅ collisions at √s = 1.96 TeV using an integrated luminosity of 252 pb-1 collected at the Fermilab Tevatron collider by the D0 detector. We observe no evidence for LQ production in the topologies arising from LQL̅Q̅ → ℯqℯq and LQL̅Q̅ → ℯqνq, and derive 95% C.L. lower limits on the LQ mass as a function of β, where β is the branching fraction for LQ → ℯq. The limits are 241 and 218 GeV/c2 for β = 1 and 0.5, respectively. These results are combined with those obtained by √s = 1.8 TeV, which increases these LQ mass limits to 256 and 234 GeV/c2