59,618 research outputs found
On the complexion of pseudoscalar mesons
A strongly momentum-dependent dressed-quark mass function is basic to QCD. It
is central to the appearance of a constituent-quark mass-scale and an
existential prerequisite for Goldstone modes. Dyson-Schwinger equation (DSEs)
studies have long emphasised this importance, and have proved that QCD's
Goldstone modes are the only pseudoscalar mesons to possess a nonzero leptonic
decay constant in the chiral limit when chiral symmetry is dynamically broken,
while the decay constants of their radial excitations vanish. Such features are
readily illustrated using a rainbow-ladder truncation of the DSEs. In this
connection we find (in GeV): f_{eta_c(1S)}= 0.233, m_{eta_c(2S)}=3.42; and
support for interpreting eta(1295), eta(1470) as the first radial excitations
of eta(548), eta'(958), respectively, and K(1460) as the first radial
excitation of the kaon. Moreover, such radial excitations have electromagnetic
diameters greater than 2fm. This exceeds the spatial length of lattices used
typically in contemporary lattice-QCD.Comment: 7 pages, 2 figures. Contribution to the proceedings of the "10th
International Symposium on Meson-Nucleon Physics and the Structure of the
Nucleon (MENU04)," IHEP, Beijing, China, 30/Aug.-4/Sept./0
Structural, vibrational and thermal properties of densified silicates : insights from Molecular Dynamics
Structural, vibrational and thermal properties of densified sodium silicate
(NS2) are investigated with classical molecular dynamics simulations of the
glass and the liquid state. A systematic investigation of the glass structure
with respect to density was performed. We observe a repolymerization of the
network manifested by a transition from a tetrahedral to an octahedral silicon
environment, the decrease of the amount of non-bridging oxygen atoms and the
appearance of three-fold coordinated oxygen atoms (triclusters). Anomalous
changes in the medium range order are observed, the first sharp diffraction
peak showing a minimum of its full-width at half maximum according to density.
The previously reported vibrational trends in densified glasses are observed,
such as the shift of the Boson peak intensity to higher frequencies and the
decrease of its intensity. Finally, we show that the thermal behavior of the
liquid can be reproduced by the Birch-Murnaghan equation of states, thus
allowing us to compute the isothermal compressibility
Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem
The impulsive phase of a solar flare marks the epoch of rapid conversion of
energy stored in the pre-flare coronal magnetic field. Hard X-ray observations
imply that a substantial fraction of flare energy released during the impulsive
phase is converted to the kinetic energy of mildly relativistic electrons
(10-100 keV). The liberation of the magnetic free energy can occur as the
coronal magnetic field reconfigures and relaxes following reconnection. We
investigate a scenario in which products of the reconfiguration - large-scale
Alfven wave pulses - transport the energy and magnetic-field changes rapidly
through the corona to the lower atmosphere. This offers two possibilities for
electron acceleration. Firstly, in a coronal plasma with beta < m_e/m_p, the
waves propagate as inertial Alfven waves. In the presence of strong spatial
gradients, these generate field-aligned electric fields that can accelerate
electrons to energies on the order of 10 keV and above, including by repeated
interactions between electrons and wavefronts. Secondly, when they reflect and
mode-convert in the chromosphere, a cascade to high wavenumbers may develop.
This will also accelerate electrons by turbulence, in a medium with a locally
high electron number density. This concept, which bridges MHD-based and
particle-based views of a flare, provides an interpretation of the
recently-observed rapid variations of the line-of-sight component of the
photospheric magnetic field across the flare impulsive phase, and offers
solutions to some perplexing flare problems, such as the flare "number problem"
of finding and resupplying sufficient electrons to explain the impulsive-phase
hard X-ray emission.Comment: 31 pages, 6 figure
Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow
We report the discovery of variability in the linear polarization from the
Galactic Center black hole source, Sagittarius A*. New polarimetry obtained
with the Berkeley-Illinois-Maryland Association array at a wavelength of 1.3 mm
shows a position angle that differs by 28 +/- 5 degrees from observations 6
months prior and then remains stable for 15 months. This difference may be due
to a change in the source emission region on a scale of 10 Schwarzschild radii
or due to a change of 3 x 10^5 rad m^-2 in the rotation measure. We consider a
change in the source physics unlikely, however, since we see no corresponding
change in the total intensity or polarized intensity fraction. On the other
hand, turbulence in the accretion region at a radius ~ 10 to 1000 R_s could
readily account for the magnitude and time scale of the position angle change.Comment: accepted for publication in ApJ
A flight test of laminar flow control leading-edge systems
NASA's program for development of a laminar flow technology base for application to commercial transports has made significant progress since its inception in 1976. Current efforts are focused on development of practical reliable systems for the leading-edge region where the most difficult problems in applying laminar flow exist. Practical solutions to these problems will remove many concerns about the ultimate practicality of laminar flow. To address these issues, two contractors performed studies, conducted development tests, and designed and fabricated fully functional leading-edge test articles for installation on the NASA JetStar aircraft. Systems evaluation and performance testing will be conducted to thoroughly evaluate all system capabilities and characteristics. A simulated airline service flight test program will be performed to obtain the operational sensitivity, maintenance, and reliability data needed to establish that practical solutions exist for the difficult leading-edge area of a future commercial transport employing laminar flow control
Meson spectroscopy and properties using Dyson-Schwinger equations
We study pseudoscalar and scalar mesons using a practical and symmetry
preserving truncation of QCD's Dyson-Schwinger equations. We investigate and
compare properties of ground and radially excited meson states. In addition to
exact results for radial meson excitations we also present results for meson
masses and decay constants from the chiral limit up to the charm-quark mass, e.
g., the mass of the chi_{c0} (2P) meson.Comment: 8 pages, 3 figures, To appear in the proceedings of 9th International
Workshop on Meson Production, Properties and Interaction (Meson2006), Krakow,
Poland, 9 - 13 June 200
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