Search Results (33 total)

The Sudbury Neutrino Observatory (SNO) used an array of ³He proportional counters to measure the rate of neutral-current interactions in heavy water and precisely determined the total active (ν_x) ⁸B solar neutrino flux. This technique is independent of previous methods employed by SNO. The total flux is found to be 5.54_-0.31^+0.33(stat)_-0.34^+0.36(syst)×10⁶  cm^-2 s^-1, in agreement with previous measurements and standard solar models. A global analysis of solar and reactor neutrino results yields Δm²=7.59_-0.21^+0.19×10^-5  eV² and θ=34.4_-1.2^+1.3 degrees. The uncertainty on the mixing angle has been reduced from SNO’s previous results.

This article provides the complete description of results from the Phase I data set of the Sudbury Neutrino Observatory (SNO). The Phase I data set is based on a 0.65 kiloton-year exposure of ²H₂O (in the following denoted as D₂O) to the solar ⁸B neutrino flux. Included here are details of the SNO physics and detector model, evaluations of systematic uncertainties, and estimates of backgrounds. Also discussed are SNO's approach to statistical extraction of the signals from the three neutrino reactions (charged current, neutral current, and elastic scattering) and the results of a search for a day-night asymmetry in the ν_e flux. Under the assumption that the ⁸B spectrum is undistorted, the measurements from this phase yield a solar ν_e flux of ϕ(ν_e)=1.76_-0.05^+0.05(stat.)_-0.09^+0.09(syst.)×10⁶ cm^-2 s^-1 and a non-ν_e component of ϕ(ν_μτ)=3.41_-0.45^+0.45(stat.)_-0.45^+0.48(syst.)×10⁶ cm^-2 s^-1. The sum of these components provides a total flux in excellent agreement with the predictions of standard solar models. The day-night asymmetry in the ν_e flux is found to be A_e=7.0±4.9(stat.)_-1.2^+1.3%(syst.), when the asymmetry in the total flux is constrained to be zero.

Results are reported from the complete salt phase of the Sudbury Neutrino Observatory experiment in which NaCl was dissolved in the ²H₂O (“D₂O”) target. The addition of salt enhanced the signal from neutron capture as compared to the pure D₂O detector. By making a statistical separation of charged-current events from other types based on event-isotropy criteria, the effective electron recoil energy spectrum has been extracted. In units of 10⁶cm^-2s^-1, the total flux of active-flavor neutrinos from ⁸B decay in the Sun is found to be 4.94_-0.21^+0.21(stat)_-0.34^+0.38(syst) and the integral flux of electron neutrinos for an undistorted ⁸B spectrum is 1.68_-0.06^+0.06(stat)_-0.09^+0.08(syst); the signal from (ν_x,e) elastic scattering is equivalent to an electron-neutrino flux of 2.35_-0.22^+0.22(stat)_-0.15^+0.15(syst). These results are consistent with those expected for neutrino oscillations with the so-called large mixing angle parameters and also with an undistorted spectrum. A search for matter-enhancement effects in the Earth through a possible day-night asymmetry in the charged-current integral rate is consistent with no asymmetry. Including results from other experiments, the best-fit values for two-neutrino mixing parameters are Δm²=(8.0_-0.4^+0.6)×10^-5 eV² and θ=33.9_-2.2^+2.4 degrees.

A search has been made for sinusoidal periodic variations in the ⁸B solar neutrino flux using data collected by the Sudbury Neutrino Observatory over a 4-year time interval. The variation at a period of 1 yr is consistent with modulation of the ⁸B neutrino flux by the Earth’s orbital eccentricity. No significant sinusoidal periodicities are found with periods between 1 d and 10 years with either an unbinned maximum likelihood analysis or a Lomb-Scargle periodogram analysis. The data are inconsistent with the hypothesis that the results of the recent analysis by Sturrock et al., based on elastic scattering events in Super-Kamiokande, can be attributed to a 7% sinusoidal modulation of the total ⁸B neutrino flux.

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.

Upper limits on the ν̅ _e flux at the Sudbury Neutrino Observatory have been set based on the ν̅ _e charged-current reaction on deuterium. The reaction produces a positron and two neutrons in coincidence. This distinctive signature allows a search with very low background for ν̅ _e’s from the Sun and other potential sources. Both differential and integral limits on the ν̅ _e flux have been placed in the energy range from 4–14.8 MeV. For an energy-independent ν_e→ν̅ _e conversion mechanism, the integral limit on the flux of solar ν̅ _e’s in the energy range from 4–14.8 MeV is found to be Φ_{ν̅ e}≤3.4×10⁴  cm^-2 s^-1 (90% C.L.), which corresponds to 0.81% of the standard solar model ⁸B ν_e flux of 5.05×10⁶  cm^-2 s^-1, and is consistent with the more sensitive limit from KamLAND in the 8.3–14.8 MeV range of 3.7×10²  cm^-2 s^-1 (90% C.L.). In the energy range from 4–8 MeV, a search for ν̅ _e’s is conducted using coincidences in which only the two neutrons are detected. Assuming a ν̅ _e spectrum for the neutron induced fission of naturally occurring elements, a flux limit of Φ_{ν̅ e}≤2.0×10⁶  cm^-2 s^-1 (90% C.L.) is obtained.

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.

The Sudbury Neutrino Observatory has precisely determined the total active (ν_x) ⁸B solar neutrino flux without assumptions about the energy dependence of the ν_e survival probability. The measurements were made with dissolved NaCl in heavy water to enhance the sensitivity and signature for neutral-current interactions. The flux is found to be 5.21±0.27(stat)±0.38(syst)×10⁶   cm^-2  s^-1, in agreement with previous measurements and standard solar models. A global analysis of these and other solar and reactor neutrino results yields Δm²=7.1_-0.6^+1.2×10^-5   eV² and θ=32.5_-2.3^+2.4 degrees. Maximal mixing is rejected at the equivalent of 5.4 standard deviations.

Data from the Sudbury Neutrino Observatory have been used to constrain the lifetime for nucleon decay to “invisible” modes, such as n→3ν. The analysis was based on a search for γ rays from the deexcitation of the residual nucleus that would result from the disappearance of either a proton or neutron from ¹⁶O. A limit of τ_inv>2×10²⁹  yr is obtained at 90% confidence for either neutron- or proton-decay modes. This is about an order of magnitude more stringent than previous constraints on invisible proton-decay modes and 400 times more stringent than similar neutron modes.

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.