Measurement of the differential cross section for the production of an isolated photon with associated jet in \u3ci\u3epp\u3c/i\u3e collisions at √s = 1.96 TeV

The process pp → γ + jet + X is studied using 1.0 fb−1 of data collected by the D0 detector at the Fermilab Tevatron pp collider at a center-of-mass energy √s = 1.96 TeV. Photons are reconstructed in the central rapidity region |yγ | \u3c 1.0 with transverse momenta in the range 30 \u3c pγT \u3c 400 GeV while jets are reconstructed in either the central |yjet| \u3c 0.8 or forward 1.5 \u3c |yjet| \u3c 2.5 rapidity intervals with pjetT \u3e 15 GeV. The differential cross section d3σ/dpγT dyγ dyjet is measured as a function of pγT in four regions, differing by the relative orientations of the photon and the jet in rapidity. Ratios between the differential cross sections in each region are also presented. Next-to-leading order QCD predictions using different parameterizations of parton distribution functions and theoretical scale choices are compared to the data. The predictions do not simultaneously describe the measured normalization and pγT dependence of the cross section in the four measured regions

Search for Large Extra Dimensions via Single Photon plus Missing Energy Final States at √\u3ci\u3es\u3c/i\u3e = 1.96 TeV

We report on a search for large extra dimensions in a data sample of approximately 1 fb-1 of pp collisions at √s = 1.96 TeV.We investigate Kaluza-Klein graviton production with a photon and missing transverse energy in the final state. At the 95% C.L. we set limits on the fundamental mass scale MD from 884 to 778 GeV for two to eight extra dimensions

Evidence for production of single top quarks

We present first evidence for the production of single top quarks in the D0 detector at the Fermilab Tevatron pp collider. The standard model predicts that the electroweak interaction can produce a top quark together with an anti-bottom quark or light quark, without the antiparticle top-quark partner that is always produced from strong-coupling processes. Top quarks were first observed in pair production in 1995, and since then, single top-quark production has been searched for in ever larger data sets. In this analysis, we select events from a 0.9 fb-1 data set that have an electron or muon and missing transverse energy from the decay of a W boson from the top-quark decay, and two, three, or four jets, with one or two of the jets identified as originating from a b hadron decay. The selected events are mostly backgrounds such as W + jets and tt events, which we separate from the expected signals using three multivariate analysis techniques: boosted decision trees, Bayesian neural networks, and matrix-element calculations. A binned likelihood fit of the signal cross section plus background to the data from the combination of the results from the three analysis methods gives a cross section for single top-quark production of σ(pp Y→ tb + X, tqb + X) = 4.7 ± 1:3 pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.014%, corresponding to a 3.6 standard deviation significance. The measured cross section value is compatible at the 10% level with the standard model prediction for electroweak top-quark production. We use the cross section measurement to directly determine the Cabibbo-Kobayashi- Maskawa quark mixing matrix element that describes the Wtb coupling and find |VtbƒL1 | = 1.31-0.21+0.25 , where ƒL1 is a generic vector coupling. This model-independent measurement translates into 0.68 \u3c |Vtb| ≤ 1 at the 95% C.L. in the standard model

Evidence for production of single top quarks

We present first evidence for the production of single top quarks in the D0 detector at the Fermilab Tevatron pp collider. The standard model predicts that the electroweak interaction can produce a top quark together with an anti-bottom quark or light quark, without the antiparticle top-quark partner that is always produced from strong-coupling processes. Top quarks were first observed in pair production in 1995, and since then, single top-quark production has been searched for in ever larger data sets. In this analysis, we select events from a 0.9 fb-1 data set that have an electron or muon and missing transverse energy from the decay of a W boson from the top-quark decay, and two, three, or four jets, with one or two of the jets identified as originating from a b hadron decay. The selected events are mostly backgrounds such as W + jets and tt events, which we separate from the expected signals using three multivariate analysis techniques: boosted decision trees, Bayesian neural networks, and matrix-element calculations. A binned likelihood fit of the signal cross section plus background to the data from the combination of the results from the three analysis methods gives a cross section for single top-quark production of σ(pp Y→ tb + X, tqb + X) = 4.7 ± 1:3 pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.014%, corresponding to a 3.6 standard deviation significance. The measured cross section value is compatible at the 10% level with the standard model prediction for electroweak top-quark production. We use the cross section measurement to directly determine the Cabibbo-Kobayashi- Maskawa quark mixing matrix element that describes the Wtb coupling and find |VtbƒL1 | = 1.31-0.21+0.25 , where ƒL1 is a generic vector coupling. This model-independent measurement translates into 0.68 \u3c |Vtb| ≤ 1 at the 95% C.L. in the standard model

Precision measurements of the top quark mass from the Tevatron in the pre-LHC era

The top quark is the heaviest of the six quarks of the Standard Model. Precise knowledge of its mass is important for imposing constraints on a number of physics processes, including interactions of the as yet unobserved Higgs boson. The Higgs boson is the only missing particle of the Standard Model, central to the electroweak symmetry breaking mechanism and generation of particle masses. In this Review, experimental measurements of the top quark mass accomplished at the Tevatron, a proton-antiproton collider located at the Fermi National Accelerator Laboratory, are described. Topologies of top quark events and methods used to separate signal events from background sources are discussed. Data analysis techniques used to extract information about the top mass value are reviewed. The combination of several most precise measurements performed with the two Tevatron particle detectors, CDF and \D0, yields a value of \Mt = 173.2 \pm 0.9 GeV/$c^2$.Comment: This version contains the most up-to-date top quark mass averag

Limits on scalar leptoquark interactions and consequences for GUTs

A colored weak singlet scalar state with hypercharge 4/3 is one of the possible candidates for the explanation of the unexpectedly large forward-backward asymmetry in t tbar production as measured by the CDF and D0 experiments. We investigate the role of this state in a plethora of flavor changing neutral current processes and precision observables of down-quarks and charged leptons. Our analysis includes tree- and loop-level mediated observables in the K and B systems, the charged lepton sector, as well as the Z to b bbar decay width. We perform a global fit of the relevant scalar couplings. This approach can explain the (g-2)_mu anomaly while tensions among the CP violating observables in the quark sector, most notably the nonstandard CP phase (and width difference) in the Bs system cannot be fully relaxed. The results are interpreted in a class of grand unified models which allow for a light colored scalar with a mass below 1TeV. We find that the renormalizable SU(5) scenario is not compatible with our global fit, while in the SO(10) case the viability requires the presence of both the 126- and 120-dimensional representations.Comment: 26 pages, 7 figures; version as publishe

Search for first generation scalar and vector leptoquarks

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 p¯p run at √s=1.8TeV 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, 12, 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, 12, and 0, respectively. Mass limits for vector leptoquarks are also set for anomalous vector couplings

Measurement of the Inclusive Jet Cross Section in \u3ci\u3epp\u3c/i\u3e Collisions at √\u3ci\u3es\u3c/i\u3e = 1.96 TeV

We report on a measurement of the inclusive jet cross section in pp collisions at a center-of-mass energy √s = 1.96 TeV using data collected by the D0 experiment at the Fermilab Tevatron Collider corresponding to an integrated luminosity of 0.70 fb-1. The data cover jet transverse momenta from 50 to 600 GeV and jet rapidities in the range -2.4 to 2.4. Detailed studies of correlations between systematic uncertainties in transverse momentum and rapidity are presented, and the cross section measurements are found to be in good agreement with next-to-leading order QCD calculations

Search for Decay of a Fermiophobic Higgs Boson \u3ci\u3eh\u3c/i\u3e\u3csub\u3eƒ\u3c/sub\u3e→ γγ with the D0 Detector at √\u3ci\u3es\u3c/i\u3e = 1.96 TeV

We report the results of a search for a narrow resonance decaying into two photons in 1.1 fb-1 of data collected by the D0 experiment at the Fermilab Tevatron Collider during the period 2002–2006. We find no evidence for such a resonance and set a lower limit on the mass of a fermiophobic Higgs boson of mhƒ \u3e 100 GeV at the 95% C.L. This exclusion limit exceeds those obtained in previous searches at the Fermilab Tevatron and covers a significant region of the parameter space B(hƒ→ γγ) vs mhƒ which was not accessible at the CERN Large Electron-Positron Collider