Search Results (159 total)

We present a search for electroweak production of single top quarks in the s-channel (pp̅ →tb̅ +X) and t-channel (pp̅ →tqb̅ +X) modes. We have analyzed 230  pb^-1 of data collected with the D0 detector at the Fermilab Tevatron Collider at a center-of-mass energy of sqrt[s]=1.96  TeV. No evidence for a single top quark signal is found. We set 95% confidence level upper limits on the production cross sections, based on binned likelihoods formed from a neural network output. The observed (expected) limits are 6.4 pb (4.5 pb) in the s-channel and 5.0 pb (5.8 pb) in the t-channel.

We report a measurement of the B_s⁰ lifetime in the semileptonic decay channel B_s⁰→D_s^-μ⁺νX (and its charge conjugate), using approximately 0.4  fb^-1 of data collected with the D0 detector during 2002–2004. Using 5176 reconstructed D_s^-μ⁺ signal events, we have measured the B_s⁰ lifetime to be τ(B_s⁰)=1.398±0.044(stat)_-0.025^+0.028(syst)  ps. This is the most precise measurement of the B_s⁰ lifetime to date.

A search for the production of neutral Higgs bosons Φ decaying into τ⁺τ^- final states in pp̅ collisions at a center-of-mass energy of 1.96 TeV is presented. The data, corresponding to an integrated luminosity of approximately 325  pb^-1, were collected by the D0 experiment at the Fermilab Tevatron Collider. Since no excess compared to the expectation from standard model processes is found, limits on the production cross section times branching ratio are set. The results are combined with those obtained from the D0 search for Φb(b̅ )→bb̅ b(b̅ ) and are interpreted in the minimal supersymmetric standard model.

We present a search for supersymmetry in the R-parity violating resonant production and decay of smuons and muon sneutrinos in the channels μ˜→χ˜₁⁰μ, μ˜→χ˜_2,3,4⁰μ, and ν˜_μ→χ˜_1,2^±μ. We analyzed 0.38  fb^-1 of integrated luminosity collected between April 2002 and August 2004 with the D0 detector at the Fermilab Tevatron Collider. The observed number of events is in agreement with the standard model expectation, and we calculate 95% C.L. limits on the slepton production cross section times branching fraction to gaugino plus muon, as a function of slepton and gaugino masses. In the framework of minimal supergravity, we set limits on the coupling parameter λ₂₁₁^′, extending significantly previous results obtained in Run I of the Tevatron and at the CERN LEP collider.

We present a search for the flavor-changing neutral current decay B_s⁰→ϕμ⁺μ^- using about 0.45  fb^-1 of data collected in pp̅ collisions at sqrt[s]=1.96  TeV with the D0 detector at the Fermilab Tevatron Collider. We find an upper limit on the branching ratio of this decay normalized to B_s⁰→J/ψϕ of B(B_s⁰→ϕμ⁺μ^-) / B(B_s⁰→J/ψϕ)<4.4×10^-3 at the 95% C.L. Using the central value of the world average branching fraction of B_s⁰→J/ψϕ, the limit corresponds to B(B_s⁰→ϕμ⁺μ^-)<4.1×10^-6 at the 95% C.L., the most stringent upper bound to date.

We have searched for a heavy resonance decaying into a Z+jet final state in pp̅ collisions at a center of mass energy of 1.96  TeV at the Fermilab Tevatron collider using the D0 detector. No indication for such a resonance was found in a data sample corresponding to an integrated luminosity of 370  pb^-1. We set upper limits on the cross section times branching fraction for heavy resonance production at the 95% C.L. as a function of the resonance mass and width. The limits are interpreted within the framework of a specific model of excited quark production.

We report results of a study of the B_s⁰ oscillation frequency using a large sample of B_s⁰ semileptonic decays corresponding to approximately 1  fb^-1 of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider in 2002–2006. The amplitude method gives a lower limit on the B_s⁰ oscillation frequency at 14.8  ps^-1 at the 95% C.L. At Δm_s=19  ps^-1, the amplitude deviates from the hypothesis A=0 (1) by 2.5 (1.6) standard deviations, corresponding to a two-sided C.L. of 1% (10%). A likelihood scan over the oscillation frequency, Δm_s, gives a most probable value of 19  ps^-1 and a range of 17<Δm_s<21  ps^-1 at the 90% C.L., assuming Gaussian uncertainties. This is the first direct two-sided bound measured by a single experiment. If Δm_s lies above 22  ps^-1, then the probability that it would produce a likelihood minimum similar to the one observed in the interval 16–22  ps^-1 is (5.0±0.3)%.

We present the results of a search for the production of an excited state of the muon, μ^*, in proton antiproton collisions at sqrt[s]=1.96  TeV. The data have been collected with the D0 experiment at the Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 380  pb^-1. We search for μ^* in the process pp̅ →μ^*μ, with the μ^* subsequently decaying to a muon plus photon. No excess above the standard model expectation is observed in data. Interpreting our data in the context of a model that describes μ^* production by four-fermion contact interactions and μ^* decay via electroweak processes, we set a 95% confidence level production cross section upper limit ranging from 0.057 to 0.112 pb, depending on the mass of the excited muon. Choosing the scale for contact interactions to be Λ=1  TeV, excited muon masses below 618 GeV are excluded.

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).