11,133 research outputs found

    Similar phenomena at different scales: Black Holes, the Sun, Gamma-ray Bursts, Supernovae, Galaxies and Galaxy Clusters

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    Many similar phenomena occur in astrophysical systems with spatial and mass scales different by many orders of magnitudes. For examples, collimated outflows are produced from the Sun, proto-stellar systems, gamma-ray bursts, neutron star and black hole X-ray binaries, and supermassive black holes; various kinds of flares occur from the Sun, stellar coronae, X-ray binaries and active galactic nuclei; shocks and particle acceleration exist in supernova remnants, gamma-ray bursts, clusters of galaxies, etc. In this report I summarize briefly these phenomena and possible physical mechanisms responsible for them. I emphasize the importance of using the Sun as an astrophysical laboratory in studying these physical processes, especially the roles magnetic fields play in them; it is quite likely that magnetic activities dominate the fundamental physical processes in all of these systems. As a case study, I show that X-ray lightcurves from solar flares, black hole binaries and gamma-ray bursts exhibit a common scaling law of non-linear dynamical properties, over a dynamical range of several orders of magnitudes in intensities, implying that many basic X-ray emission nodes or elements are inter-connected over multi-scales. A future high timing and imaging resolution solar X-ray instrument, aimed at isolating and resolving the fundamental elements of solar X-ray lightcurves, may shed new lights onto the fundamental physical mechanisms, which are common in astrophysical systems with vastly different mass and spatial scales. Using the Sun as an astrophysical laboratory, "Applied Solar Astrophysics" will deepen our understanding of many important astrophysical problems.Comment: 22 pages, 13 figures, invited discourse for the 26th IAU GA, Prague, Czech Republic, Aug. 2006, to be published in Vol. 14 IAU Highlights of Astronomy, Ed. K.A. van der Hucht. Revised slightly to match the final submitted version, after incorporating comments and suggestions from several colleagues. A full-resolution version is available on request from the author at [email protected]

    Double radiative pion capture on hydrogen and deuterium and the nucleon's pion cloud

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    We report measurements of double radiative capture in pionic hydrogen and pionic deuterium. The measurements were performed with the RMC spectrometer at the TRIUMF cyclotron by recording photon pairs from pion stops in liquid hydrogen and deuterium targets. We obtained absolute branching ratios of (3.02±0.27(stat.)±0.31(syst.))×10−5(3.02 \pm 0.27 (stat.) \pm 0.31 (syst.)) \times 10^{-5} for hydrogen and (1.42±0.120.09(stat.)±0.11(syst.))×10−5(1.42 \pm ^{0.09}_{0.12} (stat.) \pm 0.11 (syst.)) \times 10^{-5} for deuterium, and relative branching ratios of double radiative capture to single radiative capture of (7.68±0.69(stat.)±0.79(syst.))×10−5(7.68 \pm 0.69(stat.) \pm 0.79(syst.)) \times 10^{-5} for hydrogen and (5.44±0.460.34(stat.)±0.42(syst.))×10−5(5.44 \pm^{0.34}_{0.46}(stat.) \pm 0.42(syst.)) \times 10^{-5} for deuterium. For hydrogen, the measured branching ratio and photon energy-angle distributions are in fair agreement with a reaction mechanism involving the annihilation of the incident π−\pi^- on the π+\pi^+ cloud of the target proton. For deuterium, the measured branching ratio and energy-angle distributions are qualitatively consistent with simple arguments for the expected role of the spectator neutron. A comparison between our hydrogen and deuterium data and earlier beryllium and carbon data reveals substantial changes in the relative branching ratios and the energy-angle distributions and is in agreement with the expected evolution of the reaction dynamics from an annihilation process in S-state capture to a bremsstrahlung process in P-state capture. Lastly, we comment on the relevance of the double radiative process to the investigation of the charged pion polarizability and the in-medium pion field.Comment: 44 pages, 7 tables, 13 figures, submitted to Phys. Rev.

    Gluon dominance model and cluster production

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    Gluon dominance model (GDM) studies multiparticle production in lepton and hadron processes. It is based on the QCD and phenomenological scheme of hadronization. The model describes well multiplicity distributions and their moments. It has revealed an active role of gluons in multiparticle production, it also has confirmed the fragmentation mechanism of hadronization in e+e- annihilation and its change to recombination mechanism in hadron and nucleus interactions. The GDM explains the shoulder structure of multiplicity distributions. The agreement with Au+Au peripheral collisions data for hadron-pion ratio has been also obtained with this model. Development of GDM allows one to research the multiplicity behavior of ppbar annihilation at tens of GeV. The mechanism of soft photons production and estimates of their emission region have been offered. The experimental data (project "Thermalization", U-70, IHEP) have confirmed a cluster nature of multiparticle production.Comment: 4 pages, 4 figures, Proceedings of the ISMD06 conference, Paraty, Brazil, 2-9 Sep 2006, to appear in Brazilian Journal of Physic

    Measurement of the top quark-pair production cross section with ATLAS in pp collisions at √s=7 TeV

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    A measurement of the production cross-section for top quark pairs (tt) in pp collisions at √s = 7 TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron e or muon ÎŒ) with large missing transverse energy and at least four jets, and dilepton (ee, ΌΌ or eÎŒ) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb^(−1), 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-tt Standard Model processes are estimated using data-driven methods and determined to be 12.2±3.9 events and 2.5±0.6 events, respectively. The kinematic properties of the selected events are consistent with SM tt production. The inclusive top quark pair production cross-section is measured to be σ _tt=14531(stat)^(+42)_(−27)(syst) pb. The measurement agrees with perturbative QCD calculations
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