Fermilab Tevatron Pp Collider Research Articles

Search for the Standard Model Higgs Boson in Tau Final States

We present a search for the standard model Higgs boson using hadronically decaying tau leptons, in 1 fb(-1) of data collected with the D0 detector at the Fermilab Tevatron pp collider. We select two final states: tau+/- plus missing transverse energy and b jets, and tau+ tau- plus jets. These final states are sensitive to a combination of associated W/Z boson plus Higgs boson, vector boson fusion, and gluon-gluon fusion production processes. The observed ratio of the combined limit on the Higgs production cross section at the 95% C.L. to the standard model expectation is 29 for a Higgs boson mass of 115 GeV.

Search for Resonant Diphoton Production with the D0 Detector

We present a search for a narrow resonance in the inclusive diphoton final state using approximately 2.7 fb(-1) of data collected with the D0 detector at the Fermilab Tevatron pp Collider. We observe good agreement between the data and the background prediction, and set the first 95% C.L. upper limits on the production cross section times the branching ratio for decay into a pair of photons for resonance masses between 100 and 150 GeV. This search is also interpreted in the context of several models of electroweak symmetry breaking with a Higgs boson decaying into two photons.

Model-Independent Measurement of theW-Boson Helicity in Top-Quark Decays at D0

We present the first model-independent measurement of the helicity of W bosons produced in top quark decays, based on a 1 fb(-1) sample of candidate tt events in the dilepton and lepton plus jets channels collected by the D0 detector at the Fermilab Tevatron pp Collider. We reconstruct the angle theta(*) between the momenta of the down-type fermion and the top quark in the W boson rest frame for each top quark decay. A fit of the resulting costheta(*) distribution finds that the fraction of longitudinal W bosons f(0)=0.425+/-0.166(stat)+/-0.102(syst) and the fraction of right-handed W bosons f(+)=0.119+/-0.090(stat)+/-0.053(syst), which is consistent at the 30% C.L. with the standard model.

NLO QCD corrections toWboson production with a massiveb-quark jet pair at the Fermilab Tevatronpp¯collider

We calculate the next-to-leading order (NLO) QCD corrections to Wbb production including full bottom-quark mass effects. We study the impact of NLO QCD corrections on the total cross section and invariant mass distribution of the bottom-quark jet pair at the Fermilab Tevatron pp collider. We perform a detailed comparison with a calculation that considers massless bottom quarks. We find that neglecting bottom-quark mass effects overestimates the NLO total cross section for Wbb production at the Tevatron by about 8% independent of the choice of renormalization and factorization scale.

Search for Randall-Sundrum Gravitons in Dilepton and Diphoton Final States

We report the first direct search for the Kaluza-Klein (KK) modes of Randall-Sundrum gravitons using dielectron, dimuon, and diphoton events observed with the D0 detector operating at the Fermilab Tevatron pp(-) Collider at sqrt[s]=1.96 TeV. No evidence for resonant production of gravitons has been found in the data corresponding to an integrated luminosity of approximately equal to 260 pb(-1). Lower limits on the mass of the first KK mode at the 95% C.L. have been set between 250 and 785 GeV, depending on its coupling to standard model particles.

Search for Anomalous Heavy-Flavor Quark Production in Association withWBosons

We search for anomalous production of heavy-flavor quark jets in association with W bosons at the Fermilab Tevatron pp Collider in final states in which the heavy-flavor quark content is enhanced by requiring at least one tagged jet in an event. Jets are tagged using one algorithm based on semileptonic decays of b/c hadrons, and another on their lifetimes. We compare e+jets (164 pb(-1)) and mu+jets (145 pb(-1)) channels collected with the D0 detector at sqrt[s]=1.96 TeV to expectations from the standard model and set upper limits on anomalous production of such events.

Central pseudorapidity gaps in events with a leading antiproton at the fermilab tevatron pp collider.

We report a measurement of the fraction of events with a large pseudorapidity gap deltaeta within the pseudorapidity region available to the proton dissociation products X in p+p-->p+X. For a final state p of fractional momentum loss xi(p) and 4-momentum transfer squared t(p) within 0.06<xi(p)<0.09 and |t(p)|<1.0 [0.2] GeV2 at sqrt[s]=1800 [630] GeV, the fraction of events with deltaeta>3 is found to be 0.246+/-0.001 (stat)+/-0.042 (syst) [0.184+/-0.001 (stat)+/-0.043 (syst)]. Our results are compared with gap fractions measured in minimum bias pp collisions and with theoretical expectations.

Inclusive jet cross sections in [formula omitted] collisions at √ s = 630 and 1800 GeV

We have made a precise measurement of the inclusive jet cross section at √ s=1800 GeV. The result is based on an integrated luminosity of 92 pb −1 collected at the Fermilab Tevatron pp Collider with the DØ detector. The measurement is reported as a function of jet transverse energy (60 GeV ⩽ E T ⩽ 500 GeV), and in the pseudo-rapidity intervals φηφ ⩽ 0.5 and 0.1 ⩽ φηφ ⩽0.7. A preliminary measurement of the pseudorapidity dependence of inclusive jet production (φηφ < 1.5) is also discussed. The results are in good agreement with predictions from next-to-leading order (NLO) quantum chromodynamics (QCD). DØ has also determined the ratio of jet cross sections at √s=630 GeV and √s=1800 GeV (φη v ̊ b ⩽ 0.5) . This preliminary measurement differes from NLO QCD predictions.

Search for right-handed W bosons and heavy W' in pp-bar collisions at sqrt s=1.8 TeV.

We report on a search for right-handed W bosons (W R ) with mass greater than 200GeV/c 2 . We used data collected with the D0 detector at the Fermilab Tevatron pp collider at √s=1.8TeV 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 W R >650GeV/c 2 and m W R >720GeV/c 2 at the 95% confidence level, valid for m N R W R and mN R «m W R , respectively. The latter also represents a new lower limit on the mass of a heavy left-handed W boson (W') decaying into ev. In addition, limits on mWR valid for larger values of the N R mass are obtained assuming that NR decays to an electron and two jets.

Search for light top squarks in pp-bar collisions at sqrt s=1.8 TeV.

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