Search Results (77 total)

Measurements of the double-differential π^± production cross section in the momentum range 100≤p≤800 MeV/c and angle range 0.35≤θ≤2.15 rad in proton-beryllium, proton-carbon, proton-aluminium, proton-copper, proton-tin, proton-tantalum, and proton-lead collisions are presented. The data were taken with the large-acceptance HARP detector in the T9 beam line of the CERN PS. The pions were produced by proton beams in a momentum range from 3 to 12.9 GeV/c hitting a target with a thickness of 5% of a nuclear interaction length. Tracking and identification of the produced particles was performed by using a small-radius cylindrical Time Projection Chamber (TPC) placed inside a solenoidal magnet. Incident particles were identified by an elaborate system of beam detectors. Results are obtained for the double-differential cross sections d²σ/(dpdθ) at six incident proton beam momenta [3, 5, 8, and 8.9 GeV/c (Be only) and 12 and 12.9 GeV/c (Al only)]. They are based on a complete correction of static and dynamic distortions of tracks in the HARP TPC, which allows the complete statistics of the collected data set to be used. The results include and supersede our previously published results and are compatible with these. Results are compared with the GEANT4 and MARS Monte Carlo simulation.

The observation of neutrino oscillations is clear evidence for physics beyond the standard model. To make precise measurements of this phenomenon, neutrino oscillation experiments, including MiniBooNE, require an accurate description of neutrino charged current quasielastic (CCQE) cross sections to predict signal samples. Using a high-statistics sample of ν_μ CCQE events, MiniBooNE finds that a simple Fermi gas model, with appropriate adjustments, accurately characterizes the CCQE events observed in a carbon-based detector. The extracted parameters include an effective axial mass, M_A^eff=1.23±0.20  GeV, that describes the four-momentum dependence of the axial-vector form factor of the nucleon, and a Pauli-suppression parameter, κ=1.019±0.011. Such a modified Fermi gas model may also be used by future accelerator-based experiments measuring neutrino oscillations on nuclear targets.

The MiniBooNE Collaboration reports first results of a search for ν_e appearance in a ν_μ beam. With two largely independent analyses, we observe no significant excess of events above the background for reconstructed neutrino energies above 475 MeV. The data are consistent with no oscillations within a two-neutrino appearance-only oscillation model.

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.

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.

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.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

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.

We present results of a search for R-parity-violating decay of the neutralino χ ˜₁⁰, taken as the lightest supersymmetric particle, to a muon and two jets. The decay proceeds through a lepton-number violating coupling λ_2jk^′ (j=1,2; k=1,2,3), with R-parity conservation in all other production and decay processes. In the absence of candidate events from 77.5±3.9   pb^-1 of data collected by the D0 experiment at the Fermilab Tevatron in pp̅ collisions at sqrt[s]=1.8  TeV, and with an expected background of 0.18±0.03±0.02 events, we set limits on squark and gluino masses within the framework of the minimal low-energy supergravity-supersymmetry model.

Based on 85 pb^-1 data of pp̅ collisions at sqrt[s]=1.8 TeV collected using the DØ detector at Fermilab during the 1994–1995 run of the Tevatron, we present a direct measurement of the total decay width of the W boson Γ_W. The width is determined from the transverse mass spectrum in the W→e+ν_e decay channel and found to be Γ_W=2.23_-0.14^+0.15(stat)±0.10(syst) GeV, consistent with the expectation from the standard model.

We have measured the W boson mass using the DØ detector and a data sample of 82 pb^-1 from the Fermilab Tevatron collider. This measurement uses W→eν decays, where the electron is close to a boundary of a central electromagnetic calorimeter module. Such “edge” electrons have not been used in any previous DØ analysis, and represent a 14% increase in the W boson sample size. For these electrons, new response and resolution parameters are determined, and revised backgrounds and underlying event energy flow measurements are made. When the current measurement is combined with previous DØ W boson mass measurements, we obtain M_W=80.483±0.084 GeV. The 8% improvement from the previous DØ measurement is primarily due to the improved determination of the response parameters for non-edge electrons using the sample of Z bosons with non-edge and edge electrons.

We present the results of a search for leptoquark (LQ) pairs in (85.2±3.7)  pb^-1 of pp̅ collider data collected by the D0 experiment at the Fermilab Tevatron. We observe no evidence for leptoquark production and set a limit on σ(pp̅ →LQLQ̅ →νν+jets) as a function of the mass of the leptoquark (m_LQ). Assuming the decay LQ→νq, we exclude scalar leptoquarks for m_LQ<98 GeV/c², and vector leptoquarks for m_LQ<200 GeV/c² and coupling which produces the minimum cross section, at a 95% confidence level.

We have performed a search for scalar top quark (stop) pair production in the inclusive electron-muon-missing transverse energy final state, using a sample of pp̅ events corresponding to 108.3 pb^-1 of data collected with the D0 detector at Fermilab. The search is done in the framework of the minimal supersymmetric standard model assuming that the sneutrino is the lightest supersymmetric particle. For the dominant decays of the lightest stop, t̃→bχ̃₁⁺ and t̃→bℓν̃, no evidence for signal is found. We derive cross-section limits as a function of stop ( t̃ ), chargino ( χ̃₁⁺), and sneutrino ( ν̃) masses.

We present a search for charged Higgs bosons in decays of pair-produced top quarks in pp̅ collisions at sqrt[s]  =  1.8 TeV recorded by the D0 detector at the Fermilab Tevatron collider. With no evidence for signal, we exclude most regions of the ( M_H^±,tanβ) parameter space where the decay t→ H⁺b has a branching fraction >0.36 and B(H^±→τν_τ) is large.

The DØ Collaboration has studied for the first time the properties of hadron-collider jets reconstructed with a successive-combination algorithm based on relative transverse momenta (k_⊥) of energy clusters. Using the standard value D=1.0 of the jet-separation parameter in the k_⊥ algorithm, we find that the p_T of such jets is higher than the E_T of matched jets reconstructed with cones of radius R=0.7, by about 5 (8) GeV at p_T≈90 (240) GeV. To examine internal jet structure, the k_⊥ algorithm is applied within D=0.5 jets to resolve any subjets. The multiplicity of subjets in jet samples at sqrt[s]=1800 GeV and 630 GeV is extracted separately for gluons (M_g) and quarks (M_q), and the ratio of average subjet multiplicities in gluon and quark jets is measured as (〈M_g〉-1)/(〈M_q〉-1)=1.84±0.15 (stat)±_0.18^0.22 (syst). This ratio is in agreement with the expectations from the HERWIG Monte Carlo event generator and a resummation calculation, and with observations in e⁺e^- annihilations, and is close to the naive prediction for the ratio of color charges of C_A/C_F=9/4=2.25.

The inclusive cross section for production of isolated photons has been measured in pp̅ collisions at sqrt[s]  =  630 GeV with the D0 detector at the Fermilab Tevatron Collider. The photons span a transverse energy (E_T) range from 7–49 GeV and have pseudorapidity |η|<2.5. This measurement is combined with the previous D0 result at sqrt[s]  =  1800 GeV to form a ratio of the cross sections. Comparison of next-to-leading-order QCD with the measured cross section at 630 GeV and the ratio of cross sections show satisfactory agreement in most of the E_T range.

We describe QUAERO, a method that (i) enables the automatic optimization of searches for physics beyond the standard model, and (ii) provides a mechanism for making high energy collider data generally available. We apply QUAERO to searches for standard model WW, ZZ, and tt̅ production, to searches for these objects produced through a new heavy resonance, and to the first direct search for W^′→WZ. Through this interface, we make three data sets collected by the D0 experiment at sqrt[s]  =  1.8 TeV publicly available.

We describe a search for the pair production of first-generation scalar and vector leptoquarks in the eejj and eνjj channels by the DØ Collaboration. The data are from the 1992–1996 pp̅ run at sqrt[s]=1.8 TeV at the Fermilab Tevatron collider. We find no evidence for leptoquark production; in addition, no kinematically interesting events are observed using relaxed selection criteria. The results from the eejj and eνjj channels are combined with those from a previous DØ analysis of the ννjj channel to obtain 95% confidence level (C.L.) upper limits on the leptoquark pair-production cross section as a function of mass and of β, the branching fraction to a charged lepton. These limits are compared to next-to-leading-order theory to set 95% C.L. lower limits on the mass of a first-generation scalar leptoquark of 225, 204, and 79 GeV/c² for β=1, 1 / 2, and 0, respectively. For vector leptoquarks with gauge (Yang-Mills) couplings, 95% C.L. lower limits of 345, 337, and 206 GeV/c² are set on the mass for β=1, 1 / 2, and 0, respectively. Mass limits for vector leptoquarks are also set for anomalous vector couplings.

We present results of searches for technirho (ρ_T), techniomega (ω_T), and Z^′ particles, using the decay channels ρ_T,ω_T,Z^′→e⁺e^-. The search is based on 124.8 pb^-1 of data collected by the D0 detector at the Fermilab Tevatron during 1992–1996. In the absence of a signal, we set 95% C.L. upper limits on the cross sections for the processes pp̅ →ρ_T,ω_T,Z^′→e⁺e^- as a function of the mass of the decaying particle. For certain model parameters, we exclude the existence of degenerate ρ_T and ω_T states with masses below about 200 GeV. We exclude a Z^′ with mass below 670 GeV, assuming that it has the same couplings to fermions as the Z boson.