497 research outputs found

    Quasiperiodic ∼5–60 s fluctuations of VLF signals propagating in the Earth-ionosphere waveguide: a result of pulsating auroral particle precipitation?

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    Subionospheric very low frequency and low-frequency (VLF/LF) transmitter signals received at middle-latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag-like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi-static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum

    Relationship between cortisol and physical performance in older persons

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    Objective: Hypercortisolism is associated with muscle weakness. This study examines the relationship between cortisol and physical performance in older persons. Design/patients: The study was conducted within the Longitudinal Aging Study Amsterdam (LASA), an ongoing cohort study in a population-based sample of healthy older persons in the Netherlands. Data from the second (1995/1996) and fourth (2001/2002) cycle were used pertaining to 1172 (65-88 years) and 884 (65-94 years) men and women, respectively. Measurements: Physical performance was measured by adding up scores on the chair stands, tandem stand and walk test (range 0-12). In the second cycle serum total and calculated free cortisol were assessed; in the fourth cycle evening salivary cortisol was assessed. Regression analysis (stratified for sex, adjusted for age, body mass index, alcohol use, physical activity and region) was performed to examine the cross-sectional relationship between cortisol and physical performance. Results: Women with higher calculated free cortisol scored less well on physical performance (b = -0.28 per SD higher cortisol, P = 0.016), which was mainly explained by poorer performance on the tandem stand (OR = 1.32 for a lower score per SD higher cortisol, P = 0.003). Men with higher salivary cortisol scored less well on physical performance (b = -0.90 in the highest vs. the lowest quartile, P = 0.008), which was mainly explained by poorer performance on the chair stands and walk test (OR = 1.88, P = 0.020 and OR = 1.81, P = 0.027, respectively, in the highest vs. the lowest quartile). Conclusion: Physical performance is negatively associated with high cortisol levels in older persons. © 2007 The Authors

    Cosmology at the Millennium

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    One hundred years ago we did not know how stars generate energy, the age of the Universe was thought to be only millions of years, and our Milky Way galaxy was the only galaxy known. Today, we know that we live in an evolving and expanding Universe comprising billions of galaxies, all held together by dark matter. With the hot big-bang model, we can trace the evolution of the Universe from the hot soup of quarks and leptons that existed a fraction of a second after the beginning to the formation of galaxies a few billion years later, and finally to the Universe we see today 13 billion years after the big bang, with its clusters of galaxies, superclusters, voids, and great walls. The attractive force of gravity acting on tiny primeval inhomogeneities in the distribution of matter gave rise to all the structure seen today. A paradigm based upon deep connections between cosmology and elementary particle physics -- inflation + cold dark matter -- holds the promise of extending our understanding to an even more fundamental level and much earlier times, as well as shedding light on the unification of the forces and particles of nature. As we enter the 21st century, a flood of observations is testing this paradigm.Comment: 44 pages LaTeX with 14 eps figures. To be published in the Centennial Volume of Reviews of Modern Physic

    Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events

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    The B0B^0-Bˉ0\bar B^0 oscillation frequency has been measured with a sample of 23 million \B\bar B pairs collected with the BABAR detector at the PEP-II asymmetric B Factory at SLAC. In this sample, we select events in which both B mesons decay semileptonically and use the charge of the leptons to identify the flavor of each B meson. A simultaneous fit to the decay time difference distributions for opposite- and same-sign dilepton events gives Δmd=0.493±0.012(stat)±0.009(syst)\Delta m_d = 0.493 \pm 0.012{(stat)}\pm 0.009{(syst)} ps1^{-1}.Comment: 7 pages, 1 figure, submitted to Physical Review Letter

    Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment

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    This paper describes an analysis of the angular distribution of W->enu and W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and the missing transverse energy, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw > 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour, are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables, revised author list, matches European Journal of Physics C versio

    Observation of a new chi_b state in radiative transitions to Upsilon(1S) and Upsilon(2S) at ATLAS

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    The chi_b(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider (LHC) at sqrt(s) = 7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4 fb^-1, these states are reconstructed through their radiative decays to Upsilon(1S,2S) with Upsilon->mu+mu-. In addition to the mass peaks corresponding to the decay modes chi_b(1P,2P)->Upsilon(1S)gamma, a new structure centered at a mass of 10.530+/-0.005 (stat.)+/-0.009 (syst.) GeV is also observed, in both the Upsilon(1S)gamma and Upsilon(2S)gamma decay modes. This is interpreted as the chi_b(3P) system.Comment: 5 pages plus author list (18 pages total), 2 figures, 1 table, corrected author list, matches final version in Physical Review Letter

    Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS

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    We present the results of a search for new, heavy particles that decay at a significant distance from their production point into a final state containing charged hadrons in association with a high-momentum muon. The search is conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS detector operating at the Large Hadron Collider. Production of such particles is expected in various scenarios of physics beyond the standard model. We observe no signal and place limits on the production cross-section of supersymmetric particles in an R-parity-violating scenario as a function of the neutralino lifetime. Limits are presented for different squark and neutralino masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final version to appear in Physics Letters

    Measurement of the inclusive isolated prompt photon cross-section in pp collisions at sqrt(s)= 7 TeV using 35 pb-1 of ATLAS data

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    A measurement of the differential cross-section for the inclusive production of isolated prompt photons in pp collisions at a center-of-mass energy sqrt(s) = 7 TeV is presented. The measurement covers the pseudorapidity ranges |eta|<1.37 and 1.52<=|eta|<2.37 in the transverse energy range 45<=E_T<400GeV. The results are based on an integrated luminosity of 35 pb-1, collected with the ATLAS detector at the LHC. The yields of the signal photons are measured using a data-driven technique, based on the observed distribution of the hadronic energy in a narrow cone around the photon candidate and the photon selection criteria. The results are compared with next-to-leading order perturbative QCD calculations and found to be in good agreement over four orders of magnitude in cross-section.Comment: 7 pages plus author list (18 pages total), 2 figures, 4 tables, final version published in Physics Letters

    Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector

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    A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results

    Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC

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    Measurements of inclusive jet suppression in heavy ion collisions at the LHC provide direct sensitivity to the physics of jet quenching. In a sample of lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the anti-kt algorithm with values for the distance parameter that determines the nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp. Jet production is found to be suppressed by approximately a factor of two in the 10% most central collisions relative to peripheral collisions. Rcp varies smoothly with centrality as characterized by the number of participating nucleons. The observed suppression is only weakly dependent on jet radius and transverse momentum. These results provide the first direct measurement of inclusive jet suppression in heavy ion collisions and complement previous measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables, submitted to Physics Letters B. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
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