11,133 research outputs found
Similar phenomena at different scales: Black Holes, the Sun, Gamma-ray Bursts, Supernovae, Galaxies and Galaxy Clusters
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
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 for hydrogen and for deuterium, and
relative branching ratios of double radiative capture to single radiative
capture of for hydrogen
and 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 on the 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
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 âs=7 TeV
A measurement of the production cross-section for top quark pairs (tt) in pp collisions at âs = 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^(â1), 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 Ï
_tt=14531(stat)^(+42)_(â27)(syst) pb. The measurement agrees with perturbative QCD calculations
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