Search Results (188 total)

We report a study of the decay B_s⁰→D_s^(*)D_s^(*) using a data sample corresponding to 1.3  fb^-1 of integrated luminosity collected by the D0 experiment in 2002–2006 during run II of the Fermilab Tevatron collider. One D_s^(*) meson was partially reconstructed in the decay D_s→ϕμν, and the other D_s^(*) meson was identified using the decay D_s→ϕπ where no attempt was made to distinguish D_s and D_s^* states. For the branching fraction Br(B_s⁰→D_s^(*)D_s^(*)) we obtain a 90% C.L. range [0.002,0.080] and central value 0.039_-0.017^+0.019(stat)_-0.015^+0.016(syst). This was subsequently used to make the most precise estimate of the width difference ΔΓ_s^CP in the B_s⁰-B̅ _s⁰ system: ΔΓ_s^CP/Γ_s=0.079_-0.035^+0.038(stat)_-0.030^+0.031(syst).

We present a measurement of the top quark pair production cross section in pp̅ collisions at sqrt[s]=1.96  TeV utilizing 425  pb^-1 of data collected with the D0 detector at the Fermilab Tevatron Collider. We consider the final state of the top quark pair containing one high-p_T electron or muon and at least four jets. We exploit specific kinematic features of tt̅ events to extract the cross section. For a top quark mass of 175 GeV, we measure σ_tt̅ =6.4_-1.2^+1.3(stat)±0.7(syst)±0.4(lum)  pb, in good agreement with the standard model prediction.

We search for the production of single top quarks via flavor-changing-neutral-current couplings of a gluon to the top quark and a charm (c) or up (u) quark. We analyze 230  pb^-1 of lepton+jets data from pp̅ collisions at a center of mass energy of 1.96 TeV collected by the D0 detector at the Fermilab Tevatron Collider. We observe no significant deviation from standard model predictions, and hence set upper limits on the anomalous coupling parameters κ_g^c/Λ and κ_g^u/Λ, where κ_g define the strength of tcg and tug couplings, and Λ defines the scale of new physics. The limits at 95% C.L. are κ_g^c/Λ<0.15  TeV^-1 and κ_g^u/Λ<0.037  TeV^-1.

We report a measurement of the Λ_b⁰ lifetime using a sample corresponding to 1.3  fb^-1 of data collected by the D0 experiment in 2002–2006 during run II of the Fermilab Tevatron collider. The Λ_b⁰ baryon is reconstructed via the decay Λ_b⁰→μν̅ Λ_c⁺X. Using 4437±329 signal candidates, we measure the Λ_b⁰ lifetime to be τ(Λ_b⁰)=1.290_-0.110^+0.119(stat)_-0.091^+0.087(syst)  ps, which is among the most precise measurements in semileptonic Λ_b⁰ decays. This result is in good agreement with the world average value.

This Letter presents the first strong evidence for the resolution of the excited B mesons B₁ and B₂^* as two separate states in fully reconstructed decays to B^+(*)π^-. The mass of B₁ is measured to be 5720.6±2.4±1.4  MeV/c² and the mass difference ΔM between B₂^* and B₁ is 26.2±3.1±0.9  MeV/c², giving the mass of the B₂^* as 5746.8±2.4±1.7  MeV/c². The production rate for B₁ and B₂^* mesons is determined to be a fraction (13.9±1.9±3.2)% of the production rate of the B⁺ meson.

A measurement of the top quark pair production cross section in proton antiproton collisions at an interaction energy of sqrt[s]=1.96  TeV is presented. This analysis uses 405±25  pb^-1 of data collected with the D0 detector at the Fermilab Tevatron Collider. Fully hadronic tt̅ decays with final states of six or more jets are separated from the multijet background using secondary vertex tagging and a neural network. The tt̅ cross section is measured as σ_tt̅ =4.5_-1.9^+2.0(stat)_-1.1^+1.4(syst)±0.3(lumi)  pb for a top quark mass of m_t=175  GeV/c².

We have measured the Λ_b lifetime using the exclusive decay Λ_b→J/ψΛ, based on 1.2  fb^-1 of data collected with the D0 detector during 2002–2006. From 171 reconstructed Λ_b decays, where the J/ψ and Λ are identified via the decays J/ψ→μ⁺μ^- and Λ→pπ, we measured the Λ_b lifetime to be τ(Λ_b)=1.218_-0.115^+0.130(stat)±0.042(syst)  ps. We also measured the B⁰ lifetime in the decay B⁰→J/ψ(μ⁺μ^-)K_S⁰(π⁺π^-) to be τ(B⁰)=1.501_-0.074^+0.078(stat)±0.050(syst)  ps, yielding a lifetime ratio of τ(Λ_b)/τ(B⁰)=0.811_-0.087^+0.096(stat)±0.034(syst).

We present a measurement of the tt̅ pair production cross section in pp̅ collisions at sqrt[s]=1.96  TeV utilizing approximately 425  pb^-1 of data collected with the D0 detector. We consider decay channels containing two high p_T charged leptons (either e or μ) from leptonic decays of both top-daughter W bosons. These were gathered using four sets of selection criteria, three of which required that a pair of fully identified leptons (i.e., eμ, ee, or μμ) be found. The fourth approach imposed less restrictive criteria on one of the lepton candidates and required that at least one hadronic jet in each event be tagged as containing a b quark. For a top quark mass of 175 GeV, the measured cross section is 7.4±1.4(stat)±1.0(syst)  pb and for the current Tevatron average top quark mass of 170.9 GeV, the resulting value of the cross section is 7.8±1.8(stat+syst)  pb.

Long-lived, heavy particles are predicted in a number of models beyond the standard model of particle physics. We present the first direct search for such particles’ decays, occurring up to 100 h after their production and not synchronized with an accelerator bunch crossing. We apply the analysis to the gluino (g˜), predicted in split supersymmetry, which after hadronization can become charged and lose enough momentum through ionization to come to rest in dense particle detectors. Approximately 410  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected with the D0 detector during Run II of the Fermilab Tevatron collider are analyzed in search of such “stopped gluinos” decaying into a gluon and a neutralino (χ˜₁⁰). Limits are placed on the (gluino cross section)×(probability to stop)×[BR(g˜→gχ˜₁⁰)] as a function of the gluino and χ˜₁⁰ masses, for gluino lifetimes from 30  μs–100  h.

We combine the D0 measurement of the width difference between the light and heavy B_s⁰ mass eigenstates and of the CP-violating mixing phase determined from the time-dependent angular distributions in the B_s⁰→J/ψϕ decays along with the charge asymmetry in semileptonic decays also measured with the D0 detector. With the additional constraint from the world average of the flavor-specific B_s⁰ lifetime, we obtain ΔΓ_s≡(Γ_L-Γ_H)=0.13±0.09  ps^-1 and |ϕ_s|=0.70_-0.47^+0.39 or ΔΓ_s=-0.13±0.09  ps^-1 and |ϕ_s|=2.44_-0.39^+0.47. The data sample corresponds to an integrated luminosity of 1.1  fb^-1 accumulated with the D0 detector at the Fermilab Tevatron Collider.

We report on a search for charge-1/3 third-generation leptoquarks (LQ) produced in pp̅ collisions at sqrt[s]=1.96  TeV using the D0 detector at Fermilab. Third-generation leptoquarks are assumed to be produced in pairs and to decay to a tau neutrino and a b quark with branching fraction B. We place upper limits on σ(pp̅ →LQLQ̅ )B² as a function of the leptoquark mass M_LQ. Assuming B=1, we exclude at the 95% confidence level third-generation scalar leptoquarks with M_LQ<229  GeV.

We report the first direct observation of the strange b baryon Ξ_b^-(Ξ̅ _b⁺). We reconstruct the decay Ξ_b^-→J/ψΞ^-, with J/ψ→μ⁺μ^-, and Ξ^-→Λπ^-→pπ^-π^- in pp̅ collisions at sqrt[s]=1.96  TeV. Using 1.3  fb^-1 of data collected by the D0 detector, we observe 15.2±4.4(stat)_-0.4^+1.9(syst) Ξ_b^- candidates at a mass of 5.774±0.011(stat)±0.015(syst)  GeV. The significance of the observed signal is 5.5σ, equivalent to a probability of 3.3×10^-8 of it arising from a background fluctuation. Normalizing to the decay Λ_b→J/ψΛ, we measure the relative rate σ(Ξ_b^-)×B(Ξ_b^-→J/ψΞ^-) / σ(Λ_b)×B(Λ_b→J/ψΛ)=0.28±0.09(stat)_-0.08^+0.09(syst).

We present a measurement of the shape of the boson rapidity distribution for pp̅ →Z/γ^*→e⁺e^-+X events at a center-of-mass energy of 1.96 TeV. The measurement is made for events with electron-positron mass 71<M_ee<111  GeV and uses 0.4  fb^-1 of data collected at the Fermilab Tevatron collider with the D0 detector. This measurement significantly reduces the uncertainties on the rapidity distribution in the forward region compared with previous measurements. Predictions of next-to-next-to-leading order (NNLO) QCD are found to agree well with the data over the full rapidity range.

We search for the technicolor process pp̅ →ρ_T/ω_T→Wπ_T in events containing one electron and two jets, in data corresponding to an integrated luminosity of 390  pb^-1, recorded by the D0 experiment at the Fermilab Tevatron. Technicolor predicts that technipions π_T decay dominantly into bb̅ , bc̅ , or b̅ c, depending on their charge. In these events b and c quarks are identified by their secondary decay vertices within jets. Two analysis methods based on topological variables are presented. Since no excess above the standard model prediction was found, the result is presented as an exclusion in the π_T vs ρ_T mass plane for a given set of model parameters.

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.

A measurement of the top quark mass using events with one charged lepton, missing transverse energy, and jets in the final state, collected by the D0 detector from pp̅ collisions at sqrt[s]=1.96  TeV at the Fermilab Tevatron collider, is presented. A constrained fit is used to fully reconstruct the kinematics of the events. For every event a top quark mass likelihood is calculated taking into account all possible jet assignments and the probability that an event is signal or background. Lifetime-based identification of b jets is employed to enhance the separation between tt̅ signal and background from other physics processes and to improve the assignment of the observed jets to the quarks in the tt̅ hypothesis. We extract a multiplicative jet energy scale (JES) factor in situ, greatly reducing the systematic effect related to the jet energy measurement. In a data sample with an integrated luminosity of 425  pb^-1, we observe 230 candidate events, with an estimated background of 123 events, and measure m_t=173.7±4.4(stat+JES)_-2.0^+2.1(syst)  GeV. This result represents the first application of the ideogram technique to the measurement of the top quark mass in lepton+jets events.

The D0 Collaboration presents first evidence for the production of single top quarks at the Fermilab Tevatron pp̅ collider. Using a 0.9  fb^-1 dataset, we apply a multivariate analysis to separate signal from background and measure σ(pp̅ →tb+X,tqb+X)=4.9±1.4  pb. The probability to measure a cross section at this value or higher in the absence of a signal is 0.035%, corresponding to a 3.4 standard deviation significance. We use the cross section measurement to directly determine the Cabibbo-Kobayashi-Maskawa matrix element that describes the Wtb coupling and find 0.68<|V_tb|≤1 at 95% C.L. within the standard model.

We have performed the first direct measurement of the time-integrated flavor untagged charge asymmetry in semileptonic B_s⁰ decays A_SL^s,unt by comparing the decay rate of B_s⁰→μ⁺D_s^-νX, where D_s^-→ϕπ^- and ϕ→K⁺K^-, with the charge-conjugate B̅ _s⁰ decay rate. This sample was selected from 1.3  fb^-1 of data collected by the D0 experiment in run II of the Fermilab Tevatron collider. We obtain A_SL^s,unt=[1.23±0.97(stat)±0.17(syst)]×10^-2. Assuming that Δm_s/Γ̅ _s≫1, this result can be translated into a measurement of the CP-violating phase in B_s⁰ mixing: ΔΓ_s/Δm_stan⁡ϕ_s=  [2.45±1.93(stat)±0.35(syst)]×10^-2.

From an analysis of the decay B_s⁰→J/ψϕ, we obtain the width difference between the light and heavy mass eigenstates ΔΓ≡(Γ_L-Γ_H)=0.17±0.09(stat)±0.02(syst)  ps^-1 and the CP-violating phase ϕ_s=-0.79±0.56(stat)_-0.01^+0.14(syst). Under the hypothesis of no CP violation (ϕ_s≡0), we obtain 1/Γ̅ =  τ̅ (B_s⁰)=1.52±0.08(stat)_-0.03^+0.01(syst)  ps and ΔΓ=0.12_-0.10^+0.08(stat)±0.02(syst)  ps^-1. The data sample corresponds to an integrated luminosity of about 1.1  fb^-1 accumulated with the D0 detector at the Fermilab Tevatron collider. This is the first direct measurement of the CP-violating mixing phase in the B_s⁰ system.

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.