Search Results (275 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 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 ℓ+jets and dilepton decay channels corresponding to an integrated luminosity of 370  pb^-1 collected by the D0 detector at the Fermilab Tevatron pp̅ Collider at sqrt[s]=1.96  TeV. We reconstruct the decay angle θ^* for each lepton. By comparing the cos⁡θ^* distribution from the data with that for the expected background and signal for various values of f₊ (where we assume that the fraction of longitudinally-polarized W bosons has the standard model value of 0.70), we find f₊=0.056±0.080(stat)±0.057(syst) (f₊<0.23 at 95% C.L.), consistent with the standard model prediction of f₊=3.6×10^-4.

We present the first experimental discrimination between the 2e/3 and 4e/3 top quark electric charge scenarios, using top quark pairs (tt̅ ) produced in pp̅ collisions at sqrt[s]=1.96  TeV by the Fermilab Tevatron Collider. We use 370  pb^-1 of data collected by the D0 experiment and select events with at least one high transverse momentum electron or muon, high transverse energy imbalance, and four or more jets. We discriminate between b- and b̅ -quark jets by using the charge and momenta of tracks within the jet cones. The data are consistent with the expected electric charge, |q|=2e/3. We exclude, at the 92% C.L., that the sample is solely due to the production of exotic quark pairs QQ̅ with |q|=4e/3. We place an upper limit on the fraction of QQ̅ pairs ρ<0.80 at the 90% C.L.

We report a new measurement of the tt̅ production cross section in pp̅ collisions at a center-of-mass energy of 1.96 TeV using events with one charged lepton (electron or muon), missing transverse energy, and jets. Using 425  pb^-1 of data collected using the D0 detector at the Fermilab Tevatron Collider, and enhancing the tt̅ content of the sample by tagging b jets with a secondary vertex tagging algorithm, the tt̅ production cross section is measured to be σ_{pp̅ →tt̅ +X}=6.6±0.9(stat+syst)±0.4(lum)  pb. This cross section is the most precise D0 measurement to date for tt̅ production and is in good agreement with standard model expectations.

We report on a measurement of the B_d⁰ mixing frequency and the calibration of an opposite-side flavor tagger in the D0 experiment. Various properties associated with the b quark on the opposite side of the reconstructed B meson are combined using a likelihood-ratio method into a single variable with enhanced tagging power. Its performance is tested with data, using a large sample of reconstructed semileptonic B→μD⁰X and B→μD^*X decays, corresponding to an integrated luminosity of approximately 1  fb^-1. The events are divided into groups depending on the value of the combined tagging variable, and an independent analysis is performed in each group. Combining the results of these analyses, the overall effective tagging power is found to be εD²=(2.48±0.21_-0.06^+0.08)%. The measured B_d⁰ mixing frequency Δm_d=0.506±0.020(stat)±0.016(syst)  ps^-1 is in good agreement with the world average value.

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.

We present a measurement of the top quark mass with the matrix element method in the lepton+jets final state. As the energy scale for calorimeter jets represents the dominant source of systematic uncertainty, the matrix element likelihood is extended by an additional parameter, which is defined as a global multiplicative factor applied to the standard energy scale. The top quark mass is obtained from a fit that yields the combined statistical and systematic jet energy scale uncertainty. Using a data set of 0.4  fb^-1 taken with the D0 experiment at Run II of the Fermilab Tevatron Collider, the mass of the top quark is measured using topological information to be: m_top^ℓ+jets(topo)=169.2_-7.4^+5.0(stat+JES)_-1.4^+1.5(syst)  GeV, and when information about identified b jets is included: m_top^ℓ+jets(b-tag)=170.3_-4.5^+4.1(stat+  JES)_-1.8^+1.2(syst)  GeV. The measurements yield a jet energy scale consistent with the reference scale.

We measure the dimuon charge asymmetry A in pp̅ collisions at a center of mass energy sqrt[s]=1960  GeV. The data was recorded with the D0 detector and corresponds to an integrated luminosity of approximately 1.0  fb^-1. Assuming that the asymmetry A is due to asymmetric B⁰↔B̅ ⁰ mixing and decay, we extract the CP-violation parameter of B⁰ mixing and decay: ℜ(ϵ_B⁰) / 1+|ϵ_B⁰|²=A_B⁰ / 4=  -0.0023±0.0011(stat)±0.0008(syst).A_B⁰ is the dimuon charge asymmetry from decays of B⁰B̅ ⁰ pairs. The general case, with CP violation in both B⁰ and B_s⁰ systems, is also considered. Finally we obtain the forward-backward asymmetry that quantifies the tendency of μ⁺ to go in the proton direction and μ^- to go in the antiproton direction. The results are consistent with the standard model and constrain new physics.

A search for direct production of scalar bottom quarks (b˜) is performed with 310  pb^-1 of data collected by the D0 experiment in pp̅ collisions at sqrt[s]=1.96  TeV at the Fermilab Tevatron Collider. The topology analyzed consists of two b jets and an imbalance in transverse momentum due to undetected neutralinos (χ˜₁⁰), with χ˜₁⁰ assumed to be the lightest supersymmetric particle. We find the data consistent with standard model expectations, and set a 95% C.L. exclusion domain in the (m_b˜, m_χ˜1⁰) mass plane, improving significantly upon the results from run I of the Tevatron.

We report a search for the standard model (SM) Higgs boson based on data collected by the D0 experiment at the Fermilab Tevatron Collider, corresponding to an integrated luminosity of 260  pb^-1. We study events with missing transverse energy and two acoplanar b jets, which provide sensitivity to the ZH production cross section in the νν̅ bb̅ channel, and to WH production when the lepton from the W→ℓν decay is undetected. The data are consistent with the SM background expectation, and we set 95% C.L. upper limits on σ(pp̅ →ZH/WH)×B(H→bb̅ ) from 3.4/8.3 to 2.5/6.3  pb, for Higgs-boson masses between 105 and 135 GeV.

We present a search for a neutral particle, pair produced in pp̅ collisions at sqrt[s]=1.96  TeV, which decays into two muons and lives long enough to travel at least 5 cm before decaying. The analysis uses ≈380  pb^-1 of data recorded with the D0 detector. The background is estimated to be about one event. No candidates are observed, and limits are set on the pair-production cross section times branching fraction into dimuons + X for such particles. For a mass of 10 GeV and lifetime of 4×10^-11  s, we exclude values greater than 0.14 pb (95% C.L.). These results are used to limit the interpretation of NuTeV’s excess of dimuon events.

We present a search for associated Higgs boson production in the process pp̅ →WH→WWW^*→l^±νl^′±ν^′+X in final states containing two like-sign isolated electrons or muons (e^±e^±, e^±μ^±, or μ^±μ^±). The search is based on D0 run II data samples corresponding to integrated luminosities of 360–380  pb^-1. No excess is observed over the predicted standard model background. We set 95% C.L. upper limits on σ(pp̅ →WH)×Br(H→WW^*) between 3.2 and 2.8 pb for Higgs boson masses from 115 to 175 GeV.

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.

Limits are set on anomalous WWγ and WWZ trilinear gauge couplings using W⁺W^-→e⁺ν_ee^-ν̅ _e, W⁺W^-→e^±ν_eμ^∓ν_μ, and W⁺W^-→μ⁺ν_μμ^-ν̅ _μ events. The data set was collected by the Run II D0 detector at the Fermilab Tevatron Collider and corresponds to approximately 250  pb^-1 of integrated luminosity at sqrt[s]=1.96  TeV. Under the assumption that the WWγ couplings are equal to the WWZ couplings and using a form factor scale of Λ=2.0  TeV, the combined 95% C.L. one-dimensional coupling limits from all three channels are -0.32<Δκ<0.45 and -0.29<λ<0.30.

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.