Search Results (229 total)

The AMANDA-II detector, operating since 2000 in the deep ice at the geographic South Pole, has accumulated a large sample of atmospheric muon neutrinos in the 100 GeV to 10 TeV energy range. The zenith angle and energy distribution of these events can be used to search for various phenomenological signatures of quantum gravity in the neutrino sector, such as violation of Lorentz invariance or quantum decoherence. Analyzing a set of 5511 candidate neutrino events collected during 1387 days of livetime from 2000 to 2006, we find no evidence for such effects and set upper limits on violation of Lorentz invariance and quantum decoherence parameters using a maximum likelihood method. Given the absence of evidence for new flavor-changing physics, we use the same methodology to determine the conventional atmospheric muon neutrino flux above 100 GeV.

A search for muon neutrinos from neutralino annihilations in the Sun has been performed with the IceCube 22-string neutrino detector using data collected in 104.3 days of live time in 2007. No excess over the expected atmospheric background has been observed. Upper limits have been obtained on the annihilation rate of captured neutralinos in the Sun and converted to limits on the weakly interacting massive particle (WIMP) proton cross sections for WIMP masses in the range 250–5000 GeV. These results are the most stringent limits to date on neutralino annihilation in the Sun.

We present a search for point sources of high energy neutrinos using 3.8 yr of data recorded by AMANDA-II during 2000–2006. After reconstructing muon tracks and applying selection criteria designed to optimally retain neutrino-induced events originating in the northern sky, we arrive at a sample of 6595 candidate events, predominantly from atmospheric neutrinos with primary energy 100 GeV to 8 TeV. Our search of this sample reveals no indications of a neutrino point source. We place the most stringent limits to date on E^-2 neutrino fluxes from points in the northern sky, with an average upper limit of E²Φ_νμ+ντ≤5.2×10^-11  TeV cm^-2 s^-1 on the sum of ν_μ and ν_τ fluxes, assumed equal, over the energy range from 1.9 TeV to 2.5 PeV.

A search for TeV-PeV muon neutrinos from unresolved sources was performed on AMANDA-II data collected between 2000 and 2003 with an equivalent live time of 807 days. This diffuse analysis sought to find an extraterrestrial neutrino flux from sources with nonthermal components. The signal is expected to have a harder spectrum than the atmospheric muon and neutrino backgrounds. Since no excess of events was seen in the data over the expected background, an upper limit of E²Φ_90%C.L.<7.4×10^-8  GeV cm^-2 s^-1 sr^-1 is placed on the diffuse flux of muon neutrinos with a Φ∝E^-2 spectrum in the energy range 16 TeV to 2.5 PeV. This is currently the most sensitive Φ∝E^-2 diffuse astrophysical neutrino limit. We also set upper limits for astrophysical and prompt neutrino models, all of which have spectra different from Φ∝E^-2.

The IceCube neutrino detector is a cubic kilometer TeV to PeV neutrino detector under construction at the geographic South Pole. The dominant population of neutrinos detected in IceCube is due to meson decay in cosmic-ray air showers. These atmospheric neutrinos are relatively well understood and serve as a calibration and verification tool for the new detector. In 2006, the detector was approximately 10% completed, and we report on data acquired from the detector in this configuration. We observe an atmospheric neutrino signal consistent with expectations, demonstrating that the IceCube detector is capable of identifying neutrino events. In the first 137.4 days of live time, 234 neutrino candidates were selected with an expectation of 211±76.1(syst)±14.5(stat) events from atmospheric neutrinos.

We report the results of a five-year survey of the northern sky to search for point sources of high energy neutrinos. The search was performed on the data collected with the AMANDA-II neutrino telescope in the years 2000 to 2004, with a live time of 1001 days. The sample of selected events consists of 4282 upward going muon tracks with high reconstruction quality and an energy larger than about 100 GeV. We found no indication of point sources of neutrinos and set 90% confidence level flux upper limits for an all-sky search and also for a catalog of 32 selected sources. For the all-sky search, our average (over declination and right ascension) experimentally observed upper limit Φ⁰=(E / 1  TeV)^γ·dΦ / dE to a point source flux of muon and tau neutrino (detected as muons arising from taus) is Φ_{νμ+ν̅ μ}⁰+Φ_{ντ+ν̅ τ}⁰=11.1×  10^-11  TeV^-1 cm^-2 s^-1, in the energy range between 1.6 TeV and 2.5 PeV for a flavor ratio Φ_{νμ+ν̅ μ}⁰/Φ_{ντ+ν̅ τ}⁰=1 and assuming a spectral index γ=2. It should be noticed that this is the first time we set upper limits to the flux of muon and tau neutrinos. In previous papers we provided muon neutrino upper limits only neglecting the sensitivity to a signal from tau neutrinos, which improves the limits by 10% to 16%. The value of the average upper limit presented in this work corresponds to twice the limit on the muon neutrino flux Φ_{νμ+ν̅ μ}⁰=5.5×10^-11  TeV^-1 cm^-2 s^-1. A stacking analysis for preselected active galactic nuclei and a search based on the angular separation of the events were also performed. We report the most stringent flux upper limits to date, including the results of a detailed assessment of systematic uncertainties.

On 27 December 2004, a giant γ flare from the Soft Gamma-Ray Repeater 1806-20 saturated many satellite gamma-ray detectors, being the brightest transient event ever observed in the Galaxy. AMANDA-II was used to search for down-going muons indicative of high-energy gammas and/or neutrinos from this object. The data revealed no significant signal, so upper limits (at 90% C.L.) on the normalization constant were set: 0.05(0.5)  TeV^-1 m^-2 s^-1 for γ=-1.47 (-2) in the gamma flux and 0.4(6.1)  TeV^-1 m^-2 s^-1 for γ=-1.47 (-2) in the high-energy neutrino flux.

We measure the masses of b hadrons in exclusively reconstructed final states containing a J/ψ→μ^-μ⁺ decay using 220  pb^-1 of data collected by the CDF II experiment. We find: m(B⁺)=5279.10±0.41_(stat.)±0.36_(sys.)  MeV/c², m(B⁰)=5279.63±0.53_(stat.)±0.33_(sys.)  MeV/c², m(B_s⁰)=5366.01±0.73_(stat.)±0.33_(sys.)  MeV/c², m(Λ_b⁰)=5619.7±1.2_(stat.)±1.2_(sys.)  MeV/c². m(B⁺)-m(B⁰)=-0.53±0.67_(stat.)±0.14_(sys.)  MeV/c², m(B_s⁰)-m(B⁰)=86.38±0.90_(stat.)±0.06_(sys.)  MeV/c², m(Λ_b⁰)-m(B⁰)=339.2±1.4_(stat.)±0.1_(sys.)  MeV/c². The measurements of the B_s⁰, Λ_b⁰ mass, m(B_s⁰)-m(B⁰) and m(Λ_b⁰)-m(B⁰) mass difference are of better precision than the current world averages.

We present the first measurement of the A₂ and A₃ angular coefficients of the W boson produced in proton-antiproton collisions. We study W→eν_e and W→μν_μ candidate events produced in association with at least one jet at CDF, during Run Ia and Run Ib of the Tevatron at sqrt[s]=1.8  TeV. The corresponding integrated luminosity was 110  pb^-1. The jet balances the transverse momentum of the W and introduces QCD effects in W boson production. The extraction of the angular coefficients is achieved through the direct measurement of the azimuthal angle of the charged lepton in the Collins-Soper rest-frame of the W boson. The angular coefficients are measured as a function of the transverse momentum of the W boson. The electron, muon, and combined results are in good agreement with the standard model prediction, up to order α_s² in QCD.

We present a study of the dijet invariant mass distribution for the reaction p̅ p→two   jets+γ+X, at a center of mass energy of 1.8 TeV, using data collected by the CDF experiment. We compare the data to predictions for the production of a photon with two jets, together with the resonant processes p̅ p→W/Z+γ+X, in which the W and Z bosons decay hadronically. A fit is made to the dijet invariant mass distribution combining the nonresonant background and resonant processes. We use the result to establish a limit for the inclusive production cross section of W/Z+γ with hadronic decay of the W and Z bosons.

A search for the direct production of Higgs bosons in the di-tau decay mode is performed with 86.3±3.5  pb^-1 of data collected with the Collider Detector at Fermilab during the 1994–1995 data taking period of the Tevatron. We search for events where one tau decays to an electron plus neutrinos and the other tau decays hadronically. We perform a counting experiment and set limits on the cross section for supersymmetric Higgs boson production where tan⁡β is large and m_A is small. For a benchmark parameter space point where m_A⁰=100  GeV/c² and tan⁡β=50, we limit the production cross section multiplied by the branching ratio to be less than 77.9 pb at the 95% confidence level compared to the theoretically predicted value of 11.0 pb. This is the first search for Higgs bosons decaying to tau pairs at a hadron collider.

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 203   pb^-1 collected at the Fermilab Tevatron collider by the CDF experiment. We observe no evidence for LQ production in the topologies arising from LQLQ̅ →eqeq and LQLQ̅ →eqνq, and derive 95% C.L. upper limits on the LQ production cross section. The results are combined with those obtained from a separately reported CDF search in the topology arising from LQLQ̅ →νqνq and 95% C.L. lower limits on the LQ mass as a function of β=BR(LQ→eq) are derived. The limits are 236, 205 and 145  GeV/c² for β=1, β=0.5 and β=0.1, respectively.

We present the results of a search for anomalous resonant production of tau lepton pairs with large invariant mass, the first such search using the CDF II Detector in Run II of the Tevatron pp̅ collider. Such anomalous production could arise from various new physics processes. In a data sample corresponding to 195  pb^-1 of integrated luminosity we predict 2.8±0.5 events from standard model background processes and observe 4. We use this result to set limits on the production of heavy scalar and vector particles decaying to tau lepton pairs.

We report on a search for Λ_b⁰→pπ^- and Λ_b⁰→pK^- (and charge conjugate) decays in pp̅ collisions at sqrt[s]=1.96  TeV using 193  pb^-1 of data collected by the CDF II experiment at the Fermilab Tevatron Collider. Data were collected using a track trigger that has been optimized to select tracks belonging to a secondary vertex that is typical of two-body charmless decays of b-flavored hadrons, including Λ_b⁰ baryons. As no Λ_b⁰ signal was observed, we set the upper limits on the branching fraction B(Λ_b⁰→ph^-), where h is K or π, of 2.3×10^-5 at 90% C.L. and 2.9×10^-5 at 95% C.L.

We present a measurement of the top pair production cross section in pp̅ collisions at sqrt[s]=1.96  TeV. We collect a data sample with an integrated luminosity of 194±11  pb^-1 with the CDF II detector at the Fermilab Tevatron. We use an artificial neural network technique to discriminate between top pair production and background processes in a sample of 519 lepton+jets events, which have one isolated energetic charged lepton, large missing transverse energy and at least three energetic jets. We measure the top pair production cross section to be σ_tt̅ =6.6±1.1±1.5  pb, where the first uncertainty is statistical and the second is systematic.

We present a study of the production of K_S⁰ and Λ⁰ in inelastic pp̅ collisions at sqrt[s]=1800 and 630 GeV using data collected by the CDF experiment at the Fermilab Tevatron. Analyses of K_S⁰ and Λ⁰ multiplicity and transverse momentum distributions, as well as of the dependencies of the average number and ⟨p_T⟩ of K_S⁰ and Λ⁰ on charged particle multiplicity, are reported. Systematic comparisons are performed for the full sample of inelastic collisions, and for the low and high momentum transfer subsamples, at the two energies. The p_T distributions extend above 8  GeV/c, showing a ⟨p_T⟩ higher than previous measurements. The dependence of the mean K_S⁰(Λ⁰) p_T on the charged particle multiplicity for the three samples shows a behavior analogous to that of charged primary tracks.

We present a measurement of the ratio of top-quark branching fractions R=B(t→Wb)/B(t→Wq), where q can be a b, s, or a d quark, using lepton-plus-jets and dilepton data sets with an integrated luminosity of ∼162  pb^-1 collected with the Collider Detector at Fermilab during Run II of the Tevatron. The measurement is derived from the relative numbers of tt̅ events with different multiplicity of identified secondary vertices. We set a lower limit of R>0.61 at 95% confidence level.

We present a search for long-lived doubly charged Higgs bosons (H^±±), with signatures of high ionization energy loss and muonlike penetration. We use 292  pb^-1 of data collected in pp̅ collisions at sqrt[s]=1.96  TeV by the CDF II detector at the Fermilab Tevatron. Observing no evidence of long-lived doubly charged particle production, we exclude H_L^±± and H_R^±± bosons with masses below 133  GeV/c² and 109  GeV/c², respectively. In the degenerate case we exclude H^±± mass below 146  GeV/c². All limits are quoted at the 95% confidence level.

We present a measurement of the tt̅ production cross section using 194   pb^-1 of CDF II data using events with a high transverse momentum electron or muon, three or more jets, and missing transverse energy. The measurement assumes 100% t→Wb branching fraction. Events consistent with tt̅ decay are found by identifying jets containing heavy-flavor semileptonic decays to muons. The dominant backgrounds are evaluated directly from the data. Based on 20 candidate events and an expected background of 9.5±1.1 events, we measure a production cross section of 5.3±3.3_-1.0^+1.3   pb, in agreement with the standard model.

We present a new search for H⁰V production, where H⁰ is a scalar Higgs boson decaying into bb̅ with branching ratio β, and V is a Z⁰ boson decaying into e⁺e^-, μ⁺μ^-, or νν̅ . This search is then combined with previous searches for H⁰V where V is a W^± boson or a hadronically decaying Z⁰. The data sample consists of 106±4  pb^-1 of pp̅ collisions at sqrt[s]=1.8  TeV accumulated by the Collider Detector at Fermilab. Observing no evidence of a signal, we set 95% Bayesian credibility level upper limits on σ(pp̅ →H⁰V)×β. For H⁰ masses of 90, 110, and 130  GeV/c², the limits are 7.8, 7.2, and 6.6 pb, respectively.

We present the first evidence of charmless decays of the B_s⁰ meson, the decay B_s⁰→ϕϕ, and a measurement of the branching ratio BR(B_s⁰→ϕϕ) using 180  pb^-1 of data collected by the CDF II experiment at the Fermilab Tevatron collider. In addition, the BR and direct CP asymmetry for the B⁺→ϕK⁺ decay are measured. We obtain BR(B_s⁰→ϕϕ)=[14_-5⁺⁶(stat)±6(syst)]×10^-6, BR(B⁺→ϕK⁺)=[7.6±1.3(stat)±0.6(syst)]×10^-6, and A_CP(B⁺→ϕK⁺)=-0.07±0.17(stat)+0.03 / -0.02(syst). Both decays are governed in the standard model by second order (penguin) b→ss̅ s amplitudes.

We report a measurement of the rate of prompt diphoton production in pp̅ collisions at sqrt[s]=1.96  TeV using a data sample of 207  pb^-1 collected with the upgraded Collider Detector at Fermilab. The background from nonprompt sources is determined using a statistical method based on differences in the electromagnetic showers. The cross section is measured as a function of the diphoton mass, the transverse momentum of the diphoton system, and the azimuthal angle between the two photons and is found to be consistent with perturbative QCD predictions.

We report on a search for anomalous kinematics of tt̅ dilepton events in pp̅ collisions at sqrt[s]=1.96  TeV using 193  pb^-1 of data collected with the CDF II detector. We developed a new a priori technique designed to isolate the subset in a data sample revealing the largest deviation from standard model (SM) expectations and to quantify the significance of this departure. In the four-variable space considered, no particular subset shows a significant discrepancy, and we find that the probability of obtaining a data sample less consistent with the SM than what is observed is 1.0%–4.5%.