2,121 research outputs found
Exclusive Production of Neutral Vector Mesons at the Electron-Proton Collider HERA
The first five years of operation of the multi-purpose experiments ZEUS and
H1 at the electron-proton storage ring facility HERA have opened a new era in
the study of vector-meson production in high-energy photon-proton interactions.
The high center-of-mass energy available at this unique accelerator complex
allows investigations in hitherto unexplored kinematic regions, providing
answers to long-standing questions concerning the energy-dependence of the rho,
omega, phi, and J/psi production cross sections. The excellent angular
acceptance of these detectors, combined with that of specialized tagging
detectors at small production angles, has permitted measurements of elastic and
inelastic production processes for both quasi-real photons and those of
virtuality exceeding the squared mass of the vector meson. This report provides
a quantitative picture of the present status of these studies, comparing them
to the extensive measurements in this field at lower energies and summarizing
topical developments in theoretical work motivated by the new data.Comment: This replacement serves to correct an error in Eq. 3.41. An improved
version of this review will appear in book form as Nr. 140 in the series
Springer Tracts in Modern Physics on 6.October. 90 pages including 34 figure
Flutter of asymmetrically swept wings
Two formulations of the oblique wing flutter problem are presented; one formulation allows only simple wing bending deformations and rigid body roll as degrees of freedom, while the second formulation includes a more complex bending-torsional deformation together with the roll freedom. Flutter is found to occur in two basic modes. The first mode is associated with wing bending-aircraft roll coupling and occurs at low values of reduced frequency. The second instability mode closely resembles a classical bending-torsion wing flutter event. This latter mode occurs at much higher reduced frequencies than the first. The occurrence of the bending-roll coupling mode is shown to lead to lower flutter speeds while the bending-torsion mode is associated with higher flutter speeds. The ratio of the wing mass moment of inertia in roll to the fuselage roll moment of inertia is found to be a major factor in the determination of which of the two instabilities is critical
Cross-Correlation of the Cosmic Microwave Background with Radio Sources: Constraints on an Accelerating Universe
We present a new limit on the cosmological constant based on the absence of
correlations between the cosmic microwave background (CMB) and the distribution
of distant radio sources. In the cosmological constant-cold dark matter models
currently favored, such correlations should have been produced via the
integrated Sachs-Wolfe effect, assuming that radio sources trace the local
(z=1) matter density. We find no evidence of correlations between the COBE 53Hz
microwave map and the NVSS 1.4 GHz radio survey. The implied 95% CL limit on
the cosmological constant is Lambda < 0.74, in marginal agreement with the
values suggested by recent measurements of the CMB anisotropy and type-IA
supernovae observations, 0.6 < Lambda < 0.7. If the cosmological model does lie
in this range, then the integrated Sachs-Wolfe effect should be detectable with
upcoming CMB maps and radio surveys.Comment: 5 pages; 3 figures; submitted to PR
Cosmic concordance and the fine structure constant
Recent measurements of a peak in the angular power spectrum of the cosmic
microwave background appear to suggest that geometry of the universe is close
to being flat. But if other accepted indicators of cosmological parameters are
also correct then the best fit model is marginally closed, with the peak in the
spectrum at larger scales than in a flat universe. Such observations can be
reconciled with a flat universe if the fine structure constant had a lower
value at earlier times, which would delay the recombination of electrons and
protons and also act to suppress secondary oscillations as observed. We discuss
evidence for a few percent increase in the fine structure constant between the
time of recombination and the present.Comment: 12 Page
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