Search Results (61 total)

The longitudinal charge distribution of electron bunches in the Fermilab/NICADD photoinjector was determined using the coherent transition radiation produced by electrons passing through a thin metallic foil. The autocorrelation of the transition radiation signal was measured with a Michelson-type interferometer. The response function of the interferometer was determined from measured and simulated intensity spectra for low electron bunch charge and maximum longitudinal compression. Both pyroelectric and Golay detectors were used for these measurements. A Kramers-Kronig technique was used to determine longitudinal charge distribution. Measurements were performed for electron bunch lengths in the range from 0.3 to 2 ps (rms). To test the accuracy of this interferometric method, the longitudinal charge distribution was measured for double-peaked electron bunches with known distance between the two pulses. The agreement between measured bunch length and simulation is within 30%.

We have developed a two-macroparticle bunch to explore the longitudinal beam dynamics through various components of the Fermilab/NICADD photoinjector. Such a two-macroparticle bunch is generated by splitting the ultraviolet pulse from the photocathode drive laser. The presented method allows the exploration of radio-frequency-induced compression in the 1.625 cell radio frequency gun and the booster cavity. It also allows a direct measurement of the momentum compaction of the magnetic bunch compressor. The measurements are compared with analytical and numerical models.

A model-independent partial-wave analysis of the S-wave component of the Kπ system from decays of D⁺ mesons to the three-body K^-π⁺π⁺ final state is described. Data come from the Fermilab E791 experiment. Amplitude measurements are made independently for ranges of K^-π⁺ invariant mass, and results are obtained below 825  MeV/c², where previous measurements exist only in two mass bins. This method of parametrizing a three-body decay amplitude represents a new approach to analyzing such decays. Though no model is required for the S-wave, a parametrization of the relatively well-known reference P- and D-waves, optimized to describe the data used, is required. In this paper, a Breit-Wigner model is adopted to describe the resonances in these waves. The observed phase variation for the S-, P-, and D-waves do not match existing measurements of I=1 / 2 K^-π⁺ scattering in the invariant mass range in which scattering is predominantly elastic. If the data are mostly I=1 / 2, this observation indicates that the Watson theorem, which requires these phases to have the same dependence on invariant mass, may not apply to these decays without allowing for some interaction with the other pion. The production rate of K^-π⁺ from these decays, if assumed to be predominantly I=1 / 2, is also found to have a significant dependence on invariant mass in the region above 1.25  GeV/c². These measurements can provide a relatively model-free basis for future attempts to determine which strange scalar amplitudes contribute to the decays.

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 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).

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 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 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.

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.

A search for narrow resonances that decay into tt̅ pairs has been performed using 130   pb^-1 of data in the lepton + jets channel collected by the DØ detector in pp̅ collisions at sqrt[s]=1.8   TeV. There is no significant deviation observed from the standard-model predictions at a top-quark mass of 175   GeV/c². We therefore present upper limits at the 95% confidence level on the product of the production cross section and branching fraction to tt̅ for narrow resonances as a function of the resonance mass M_X. These limits are used to exclude the existence of a leptophobic top-color particle with mass M_X<560   GeV/c², using a theoretical cross section for a width Γ_X=0.012M_X.

We present the results of a search for the production of new particles decaying into two jets in p̅ p collisions at sqrt[s]=1.8 TeV, using the DØ 1992–1995 data set corresponding to 109 pb^-1. We exclude at the 95% confidence level the production of excited quarks (q^*) with masses below 775 GeV/c², the most restrictive limit to date. We also exclude standard-model-like W^′ (Z^′) bosons with masses between 300 and 800 GeV/c² (400 and 640 GeV/c²). A W^′ boson with mass <786 GeV/c² has been excluded by previous measurements, and our lower limit is therefore the most stringent to date.

We present a search for large extra dimensions (ED) in pp̅ collisions at a center-of-mass energy of 1.8 TeV using data collected by the DØ detector at the Fermilab Tevatron in 1994–1996. Data corresponding to 78.8±3.9   pb^-1 are examined for events with large missing transverse energy, one high-p_T jet, and no isolated muons. There is no excess observed beyond expectation from the standard model, and we place lower limits on the fundamental Planck scale of 1.0 and 0.6 TeV for 2 and 7 ED, respectively.

We present data on multiple production of jets with transverse energies near 20 GeV in pp̅ collisions at sqrt[s]=1.8 TeV. QCD calculations in the parton-shower approximation of PYTHIA and HERWIG and the next-to-leading order approximation of JETRAD are compared to the data for one, two, three, and four jet inclusive production. Transverse energy spectra and multiple jet angular and summed transverse-energy distributions are adequately described by the shower approximation while next-to-leading order calculations describe the data poorly.

Results are presented on a measurement of the tt̅ pair production cross section in pp̅ collisions at sqrt[s]=1.8 TeV from nine independent decay channels. The data were collected by the DØ experiment during the 1992–1996 run of the Fermilab Tevatron Collider. A total of 80 candidate events is observed with an expected background of 38.8±3.3 events. For a top quark mass of 172.1 GeV/c², the measured cross section is 5.69±1.21(stat)±1.04(syst) pb.

We describe a search for evidence of minimal supergravity (MSUGRA) in 92.7 pb^-1 of data collected with the DØ detector at the Fermilab Tevatron pp̅ collider at sqrt[s]=1.8 TeV. Events with a single electron, four or more jets, and large missing transverse energy were used in this search. The major backgrounds are from W+jets, misidentified multijet, tt̅ , and WW production. We observe no excess above the expected number of background events in our data. A new limit in terms of MSUGRA model parameters is obtained.

We report the first search for supersymmetric particles via s-channel production and decay of smuons or muon sneutrinos at hadronic colliders. The data for the two-muon and two-jets final states were collected by the D0 experiment and correspond to an integrated luminosity of 94±5   pb^-1. Assuming that R parity is violated via the single coupling λ₂₁₁^′, the number of candidate events is in agreement with expectation from the standard model. Exclusion contours are given in the (m₀,m_1/2) and (m_{χ ˜},m_{ν ˜}) planes for λ₂₁₁^′=0.09, 0.08, and 0.07.