Search Results (218 total)

We present a search for the standard model Higgs boson in H→WW^(*) decays with e⁺e^-, e^±μ^∓, and μ⁺μ^- final states in pp̅ collisions at a center-of-mass energy of sqrt[s]=1.96  TeV. The data, collected from April 2002 to June 2004 with the D0 detector, correspond to an integrated luminosity of 300–325  pb^-1, depending on the final state. The number of events observed is consistent with the expectation from backgrounds. Limits from the combination of all three channels on the Higgs boson production cross section times branching ratio σ×BR(H→WW^(*)) are presented.

Using the data accumulated in 2002–2004 with the D0 detector in proton-antiproton collisions at the Fermilab Tevatron collider with a center-of-mass energy of 1.96 TeV, the branching fractions of the decays B→D̅ ₁⁰(2420)μ⁺ν_μX and B→D̅ ₂^*0(2460)μ⁺ν_μX and their ratio have been measured: B(b̅ →B)×B(B→D̅ ₁⁰μ⁺ν_μX)×B(D̅ ₁⁰→D^*-π⁺)=[0.087±0.007(stat)±0.014(syst)]%; B(b̅ →B)×B(B→D̅ ₂^*0μ⁺ν_μX)×B(D̅ ₂^*0→D^*-π⁺)=[0.035±0.007(stat)±0.008(syst)]% and [B(B→D̅ ₂^*0μ⁺ν_μX)×B(D̅ ₂^*0→D^*-π⁺)]/[B(B→D̅ ₁⁰μ⁺ν_μX)×B(D̅ ₁⁰→D^*-π⁺)]=0.39±0.09(stat)±0.12(syst), where the charge conjugated states are always implied.

We present a study of the decay B_s⁰→J/ψϕ. We obtain the CP-odd fraction in the final state at time zero, R_⊥=0.16±0.10(stat)±0.02(syst), the average lifetime of the (B_s⁰, B̅ _s⁰) system, τ̅ (B_s⁰)=1.39_-0.16^+0.13(stat)_-0.02^+0.01(syst)  ps, and the relative width difference between the heavy and light mass eigenstates, ΔΓ/Γ̅ ≡(Γ_L-Γ_H)/Γ̅ =0.24_-0.38^+0.28(stat)_-0.04^+0.03(syst). With the additional constraint from the world average of the B_s⁰ lifetime measurements using semileptonic decays, we find τ̅ (B_s⁰)=1.39±0.06  ps and ΔΓ/Γ̅ =0.25_-0.15^+0.14. For the ratio of the B_s⁰ and B⁰ lifetimes we obtain τ̅ (B_s⁰) / τ(B⁰)=0.91±0.09(stat)±0.003(syst).

We present the results of a search for the effects of large extra spatial dimensions in pp̅ collisions at sqrt[s]=1.96  TeV in events containing a pair of energetic muons. The data correspond to 246  pb^-1 of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider. Good agreement with the expected background was found, yielding no evidence for large extra dimensions. We set 95% C.L. lower limits on the fundamental Planck scale between 0.85 and 1.27 TeV within several formalisms. These are the most stringent limits achieved in the dimuon channel to date.

A search for associated production of charginos and neutralinos is performed using data recorded with the D0 detector at a pp̅ center-of-mass energy of 1.96 TeV at the Fermilab Tevatron Collider. This analysis considers final states with missing transverse energy and three charged leptons, of which at least two are electrons or muons. No evidence for supersymmetry is found in a data set corresponding to an integrated luminosity of 320  pb^-1. Limits on the product of the production cross section and leptonic branching fraction are set. For the minimal supergravity model, a chargino lower mass limit of 117 GeV at the 95% C.L. is derived in regions of parameter space with enhanced leptonic branching fractions.

We have performed a search for neutral Higgs bosons produced in association with bottom quarks in pp̅ collisions, using 260  pb^-1 of data collected with the D0 detector in Run II of the Fermilab Tevatron Collider. The cross sections for these processes are enhanced in many extensions of the standard model (SM), such as in its minimal supersymmetric extension at large tan⁡β. The results of our analysis agree with expectations from the SM, and we use our measurements to set upper limits on the production of neutral Higgs bosons in the mass range of 90 to 150 GeV.

We present results from a search for WZ production with subsequent decay to ℓνℓ^′ℓ̅ ^′ (ℓ and ℓ^′=e or μ) using 0.30  fb^-1 of data collected by the D0 experiment between 2002 and 2004 at the Fermilab Tevatron. Three events with WZ decay characteristics are observed. With an estimated background of 0.71±0.08 events, we measure the WZ production cross section to be 4.5_-2.6^+3.8  pb, with a 95% C.L. upper limit of 13.3 pb. The 95% C.L. limits for anomalous WWZ couplings are found to be -2.0<Δκ_Z<2.4 for form factor scale Λ=1  TeV, and -0.48<λ_Z<0.48 and -0.49<Δg₁^Z<0.66 for Λ=1.5  TeV.

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 pp̅ 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 ≈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.

We present a measurement of the fraction f₊ of right-handed W bosons produced in top quark decays, based on a candidate sample of tt̅ events in the lepton+jets decay mode. These data correspond to an integrated luminosity of 230  pb^-1, collected by the D0 detector at the Fermilab Tevatron pp̅ Collider at sqrt[s]=1.96  TeV. We use a constrained fit to reconstruct the kinematics of the tt̅ and decay products, which allows for the measurement of the leptonic decay angle θ^* for each event. By comparing the cos⁡θ^* distribution from the data with those for the expected background and signal for various values of f₊, we find f₊=0.00±0.13(stat)±0.07(syst). This measurement is consistent with the standard model prediction of f₊=3.6×10^-4.

We present a measurement of the Zγ production cross section and limits on anomalous ZZγ and Zγγ couplings for form-factor scales of Λ=750 and 1000 GeV. The measurement is based on 138 (152) candidates in the eeγ (μμγ) final state using 320(290)  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV. The 95% C.L. limits on real and imaginary parts of individual anomalous couplings are |h_10,30^Z|<0.23, |h_20,40^Z|<0.020, |h_10,30^γ|<0.23, and |h_20,40^γ|<0.019 for Λ=1000  GeV.

We present measurements of the inclusive production cross sections of the Υ(1S) bottomonium state in pp̅ collisions at sqrt[s]=1.96   TeV. Using the Υ(1S)→μ⁺μ^- decay mode for a data sample of 159±10   pb^-1 collected by the D0 detector at the Fermilab Tevatron collider, we determine the differential cross sections as a function of the Υ(1S) transverse momentum for three ranges of the Υ(1S) rapidity: 0<|y^Υ|≤0.6, 0.6<|y^Υ|≤1.2, and 1.2<|y^Υ|≤1.8.

Correlations in the azimuthal angle between the two largest transverse momentum jets have been measured using the D0 detector in pp̅ collisions at a center-of-mass energy sqrt[s]=1.96  TeV. The analysis is based on an inclusive dijet event sample in the central rapidity region corresponding to an integrated luminosity of 150  pb^-1. Azimuthal correlations are stronger at larger transverse momenta. These are well described in perturbative QCD at next-to-leading order in the strong coupling constant, except at large azimuthal differences where contributions with low transverse momentum are significant.

The WWγ triple gauge boson coupling parameters are studied using pp̅ →ℓνγ+X(ℓ=e,μ) events at sqrt[s]=1.96 TeV. The data were collected with the D0 detector from an integrated luminosity of 162  pb^-1 delivered by the Fermilab Tevatron Collider. The cross section times branching fraction for pp̅ →W(γ)+X→ℓνγ+X with E_T^γ>8 GeV and ΔR_ℓγ> 0.7 is 14.8±1.6(stat)±1.0(syst)±1.0(lum)  pb. The one-dimensional 95% confidence level limits on anomalous couplings are -0.88<Δκ_γ<0.96 and -0.20<λ_γ<0.20.

The ratio of the B⁺ and B⁰ meson lifetimes was measured using data collected in 2002–2004 by the D0 experiment in Run II of the Fermilab Tevatron Collider. These mesons were reconstructed in B→μ⁺νD^*-X decays, which are dominated by B⁰ and B→μ⁺νD̅ ⁰X decays, which are dominated by B⁺. The ratio of lifetimes is measured to be τ⁺/τ⁰=1.080±0.016(stat)±0.014(syst).

We report on a search for pair production of first-generation scalar leptoquarks (LQ) in pp̅ collisions at sqrt[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 LQLQ̅ →eqeq and LQLQ̅ →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 D0 at sqrt[s]=1.8  TeV, which increases these LQ mass limits to 256 and 234  GeV/c².

Using the data collected with the D0 detector at sqrt[s]=1.96   TeV, for integrated luminosities of about 180   pb^-1, we have measured the ratio of inclusive cross sections for pp̅ →Z+b   jet to pp̅ →Z+jet production. The inclusive Z+b-jet reaction is an important background to searches for the Higgs boson in associated ZH production at the Fermilab Tevatron collider. Our measurement is the first of its kind, and relies on the Z→e⁺e^- and Z→μ⁺μ^- modes. The combined measurement of the ratio yields 0.021±0.005 for hadronic jets with transverse momenta p_T>20  GeV/c and pseudorapidities |η|<2.5, consistent with next-to-leading-order predictions of the standard model.

We search for anomalous production of heavy-flavor quark jets in association with W bosons at the Fermilab Tevatron pp̅ 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 μ+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.

We present a measurement of the W boson pair-production cross section in pp̅ collisions at a center-of-mass energy of sqrt[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 2.3×10^-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.

We present a measurement of the cross section for Z production times the branching fraction to τ leptons, σ·Br(Z→τ⁺τ^-), in pp̅ collisions at sqrt[s]=1.96  TeV in the channel in which one τ decays into μν_μν_τ, and the other into hadrons+ν_τ or eν_eν_τ. The data sample corresponds to an integrated luminosity of 226  pb^-1 collected with the D0 detector at the Fermilab Tevatron collider. The final sample contains 2008 candidate events with an estimated background of 55%. From this we obtain σ·Br(Z→τ⁺τ^-)=237±15(stat)±18(sys)±15(lum)pb, in agreement with the standard model prediction.

We present measurements of the Λ_b⁰ lifetime in the exclusive decay channel Λ_b⁰→J/ψΛ⁰, with J/ψ→μ⁺μ^- and Λ⁰→pπ^-, the B⁰ lifetime in the decay B⁰→J/ψK_S⁰ with J/ψ→μ⁺μ^- and K_S⁰→π⁺π^-, and the ratio of these lifetimes. The analysis is based on approximately 250  pb^-1 of data recorded with the D0 detector in pp̅ collisions at sqrt[s]=1.96  TeV. The Λ_b⁰ lifetime is determined to be τ(Λ_b⁰)=1.22_-0.18^+0.22(stat)±0.04(syst)  ps, the B⁰ lifetime τ(B⁰)=1.40_-0.10^+0.11(stat)±0.03(syst)  ps, and the ratio τ(Λ_b⁰)/τ(B⁰)=0.87_-0.14^+0.17(stat)±0.03(syst). In contrast with previous measurements using semileptonic decays, this is the first determination of the Λ_b⁰ lifetime based on a fully reconstructed decay channel.

We present a search for Wbb̅ production in pp̅ collisions at sqrt[s]=1.96  TeV in events containing one electron, an imbalance in transverse momentum, and two b-tagged jets. Using 174  pb^-1 of integrated luminosity accumulated by the D0 experiment at the Fermilab Tevatron collider, and the standard-model description of such events, we set a 95% C.L. upper limit on Wbb̅ production of 6.6 pb for b quarks with transverse momenta p_T^b>20  GeV and bb̅ separation in pseudorapidity-azimuth space ΔR_bb>0.75. Restricting the search to optimized bb̅ mass intervals provides upper limits on WH production of 9.0–12.2 pb for Higgs-boson masses of 105–135 GeV.

We present the results of a search for the flavor-changing neutral current decay B_s⁰→μ⁺μ^- using a data set with integrated luminosity of 240   pb^-1 of pp̅ collisions at sqrt[s]=1.96   TeV collected with the D0 detector in run II of the Fermilab Tevatron collider. We find the upper limit on the branching fraction to be B(B_s⁰→μ⁺μ^-)≤5.0×10^-7 at the 95% C.L. assuming no contributions from the decay B_d⁰→μ⁺μ^- in the signal region. This limit is the most stringent upper bound on the branching fraction B_s⁰→μ⁺μ^- to date.