Search Results (50 total)

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.

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

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 ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.

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.

Using the exclusive decay B_s⁰→J/ψ(μ⁺μ^-)ϕ(K⁺K^-), we report the most precise single measurement of the B_s⁰ lifetime. The data sample corresponds to an integrated luminosity of approximately 220  pb^-1 collected with the D0 detector at the Fermilab Tevatron Collider in 2002–2004. We reconstruct 337 signal candidates, from which we extract the B_s⁰ lifetime, τ(B_s⁰)=1.444_-0.090^+0.098(stat)±0.020(sys)  ps. We also report a measurement for the lifetime of the B⁰ meson using the exclusive decay B⁰→J/ψ(μ⁺μ^-)K^*0(892)(K⁺π^-). We reconstruct 1370 signal candidates, obtaining τ(B⁰)=1.473_-0.050^+0.052(stat)±0.023(sys)  ps, and the ratio of lifetimes, τ(B_s⁰)/τ(B⁰)=0.980_-0.071^+0.076(stat)±0.003(sys).

We report the results of a search for supersymmetry (SUSY) with gauge-mediated breaking in the missing transverse energy distribution of inclusive diphoton events using 263  pb^-1 of data collected by the D0 experiment at the Fermilab Tevatron Collider in 2002–2004. No excess is observed above the background expected from standard model processes, and lower limits on the masses of the lightest neutralino and chargino of about 108 and 195 GeV, respectively, are set at the 95% confidence level. These are the most stringent limits to date for models with gauge-mediated SUSY breaking with a short-lived neutralino as the next-to-lightest SUSY particle.

The results based on 1992–95 data (Run 1) from the CDF and D0 experiments on the measurements of the W boson mass and width are presented, along with the combined results. We report a Tevatron collider average M_W=80.456±0.059  GeV. We also report the Tevatron collider average of the directly measured W boson width Γ_W=2.115±0.105  GeV. We describe a new joint analysis of the direct W mass and width measurements. Assuming the validity of the standard model, we combine the directly measured W boson width with the width extracted from the ratio of W and Z boson leptonic partial cross sections. This combined result for the Tevatron is Γ_W=2.135±0.050  GeV. Finally, we use the measurements of the direct total W width and the leptonic branching ratio to extract the leptonic partial width Γ(W→eν)=224±13  MeV.

We report the observation of the X(3872) in the J/ψπ⁺π^- channel, with J/ψ decaying to μ⁺μ^-, in pp̅ collisions at sqrt[s]=1.96  TeV. Using approximately 230  pb^-1 of data collected with the Run II D0 detector, we observe 522±100 X(3872) candidates. The mass difference between the X(3872) state and the J/ψ is measured to be 774.9±3.1(stat)±3.0(syst)  MeV/c². We have investigated the production and decay characteristics of the X(3872) and find them to be similar to those of the ψ(2S) state.

A search for pair production of doubly charged Higgs bosons in the process pp̅ →H⁺⁺H^--→μ⁺μ⁺μ^-μ^- is performed with the D0 run II detector at the Fermilab Tevatron. The analysis is based on a sample of inclusive dimuon data collected at an energy of sqrt[s]=1.96  TeV, corresponding to an integrated luminosity of 113  pb^-1. In the absence of a signal, 95% confidence level mass limits of M(H_L^±±)>118.4  GeV/c² and M(H_R^±±)>98.2  GeV/c² are set for left-handed and right-handed doubly charged Higgs bosons, respectively, assuming 100% branching into muon pairs.

The physics program in Hall A at Jefferson Lab commenced in the summer of 1997 with a detailed investigation of the ¹⁶O(e,e^′p) reaction in quasielastic, constant (q,ω) kinematics at Q²≈0.8  (GeV∕c)², q≈1  GeV∕c, and ω≈445  MeV. Use of a self-calibrating, self-normalizing, thin-film waterfall target enabled a systematically rigorous measurement. Five-fold differential cross-section data for the removal of protons from the 1p-shell have been obtained for 0<p_miss<350  MeV∕c. Six-fold differential cross-section data for 0<E_miss<120  MeV were obtained for 0<p_miss<340  MeV∕c. These results have been used to extract the A_LT asymmetry and the R_L, R_T, R_LT, and R_L+TT effective response functions over a large range of E_miss and p_miss. Detailed comparisons of the 1p-shell data with Relativistic Distorted-Wave Impulse Approximation (RDWIA), Relativistic Optical-Model Eikonal Approximation (ROMEA), and Relativistic Multiple-Scattering Glauber Approximation (RMSGA) calculations indicate that two-body currents stemming from meson-exchange currents (MEC) and isobar currents (IC) are not needed to explain the data at this Q². Further, dynamical relativistic effects are strongly indicated by the observed structure in A_LT at p_miss≈300  MeV∕c. For 25<E_miss<50  MeV and p_miss≈50  MeV∕c, proton knockout from the 1s_1∕2-state dominates, and ROMEA calculations do an excellent job of explaining the data. However, as p_miss increases, the single-particle behavior of the reaction is increasingly hidden by more complicated processes, and for 280<p_miss<340  MeV∕c, ROMEA calculations together with two-body currents stemming from MEC and IC account for the shape and transverse nature of the data, but only about half the magnitude of the measured cross section. For 50<E_miss<120  MeV and 145<p_miss<340  MeV∕c, (e,e^′pN) calculations which include the contributions of central and tensor correlations (two-nucleon correlations) together with MEC and IC (two-nucleon currents) account for only about half of the measured cross section. The kinematic consistency of the 1p-shell normalization factors extracted from these data with respect to all available ¹⁶O(e,e^′p) data is also examined in detail. Finally, the Q²-dependence of the normalization factors is discussed.

We report a virtual Compton scattering study of the proton at low c.m. energies. We have determined the structure functions P_LL-P_TT/ϵ and P_LT, and the electric and magnetic generalized polarizabilities (GPs) α_E(Q²) and β_M(Q²) at momentum transfer Q²=0.92 and 1.76  GeV². The electric GP shows a strong falloff with Q², and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a falloff; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q², compensated at higher Q² by a paramagnetic contribution from πN intermediate states.

Using 85.2±3.6   pb^-1 of pp̅ collisions collected at sqrt[s]=1.8  TeV with the D0 detector at Fermilab’s Tevatron Collider, we present the results of a search for direct pair production of scalar top quarks (t˜), the supersymmetric partners of the top quark. We examined events containing two or more jets and missing transverse energy, the signature of light scalar top quark decays to charm quarks and neutralinos. After selections, we observe 27 events while expecting 31.1±6.4 events from known standard model processes. Comparing these results to next-to-leading-order production cross sections, we exclude a significant region of t˜ and neutralino phase space. In particular, we exclude the t˜ mass m_t˜<122  GeV/c² for a neutralino mass of 45  GeV/c².