61,882 research outputs found
String loop corrections to gauge and Yukawa couplings
We report on the recent progress in computing the effective supergravity
action from superstring scattering amplitudes beyond the tree approximation. We
discuss the moduli-dependent string loop corrections to gauge, gravitational
and Yukawa couplings.Comment: 11 pages, CPTH-C218.0193 (Based on talks presented at the Rome
Workshop STRINGS 92, at the 7th Meeting of the American Physical Society, and
at the 26th Erice Workshop: ``From Superstrings to Supergravity"
Dispersion of biased swimming microorganisms in a fluid flowing through a tube
Classical Taylor-Aris dispersion theory is extended to describe the transport
of suspensions of self-propelled dipolar cells in a tubular flow. General
expressions for the mean drift and effective diffusivity are determined exactly
in terms of axial moments, and compared with an approximation a la Taylor. As
in the Taylor-Aris case, the skewness of a finite distribution of biased
swimming cells vanishes at long times. The general expressions can be applied
to particular models of swimming microorganisms, and thus be used to predict
swimming drift and diffusion in tubular bioreactors, and to elucidate competing
unbounded swimming drift and diffusion descriptions. Here, specific examples
are presented for gyrotactic swimming algae.Comment: 20 pages, 4 figures. Published version available at
http://rspa.royalsocietypublishing.org/content/early/2010/02/09/rspa.2009.0606.short?rss=
Duality of N=2 Heterotic -- Type I Compactifications in Four Dimensions
We discuss type I -- heterotic duality in four-dimensional models obtained as
a Coulomb phase of the six-dimensional U(16) orientifold model compactified on
T^2 with arbitrary SU(16) Wilson lines. We show that Kahler potentials, gauge
threshold corrections and the infinite tower of higher derivative F-terms agree
in the limit that corresponds to weak coupling, large T^2 heterotic
compactifications. On the type I side, all these quantities are completely
determined by the spectrum of N=2 BPS states that originate from D=6 massless
superstring modes.Comment: 22 pages, LaTeX; typos corrected and references adde
The Arecibo Galaxy Environment Survey VII : A Dense Filament With Extremely Long HI Streams
We present completed observations of the NGC 7448 galaxy group and background
volume as part of the blind neutral hydrogen Arecibo Galaxy Environment Survey
(AGES). Our observations cover a region spanning 5x4 degrees, over a redshift
range of approximately -2,000 < cz < 20,000 km/s. A total of 334 objects are
detected, mostly in three overdensities at cz 7,500, cz 9,600 and
cz 11,400 km/s. The galaxy density is extremely high (15 per square
degree) and many (24%) show signs of extended HI emission, including some
features as much as 800 kpc in projected length. We describe the overall
characteristics of this environment : kinematics, typical galaxy colours and
mass to light ratios, and substructure. To aid in the cataloguing of this data
set, we present a new FITS viewer (FRELLED : Fits Realtime Explorer of Low
Latency in Every Dimension). This incorporates interactive source cataloguing
tools which increase our source extraction speed by approximately a factor of
50.Comment: 15 pages, 13 figures, accepted for publication in MNRA
Spectrum of an open disordered quasi-two-dimensional electron system: strong orbital effect of the weak in-plane magnetic field
The effect of an in-plane magnetic field upon open quasi-two-dimensional
electron and hole systems is investigated in terms of the carrier ground-state
spectrum. The magnetic field, classified as weak from the viewpoint of
correlation between size parameters of classical electron motion and the gate
potential spatial profile is shown to efficiently cut off extended modes from
the spectrum and to change singularly the mode density of states (MDOS). The
reduction in the number of current-carrying modes, right up to zero in magnetic
fields of moderate strength, can be viewed as the cause of
magnetic-field-driven metal-to-insulator transition widely observed in
two-dimensional systems. Both the mode number reduction and the MDOS
singularity appear to be most pronounced in the mode states dephasing
associated with their scattering by quenched-disorder potential. This sort of
dephasing is proven to dominate the dephasing which involves solely the
magnetic field whatever level of the disorder.Comment: RevTeX-4 class, 12 pages, 5 eps figure
Credit Availability for Potential Irrigators in North Dakota
Agricultural Finance, Resource /Energy Economics and Policy,
Confinement of supernova explosions in a collapsing cloud
We analyze the confining effect of cloud collapse on an expanding supernova
shockfront. We solve the differential equation for the forces on the shockfront
due to ram pressure, supernova energy, and gravity. We find that the expansion
of the shockfront is slowed and in fact reversed by the collapsing cloud.
Including radiative losses and a potential time lag between supernova explosion
and cloud collapse shows that the expansion is reversed at smaller distances as
compared to the non-radiative case. We also consider the case of multiple
supernova explosions at the center of a collapsing cloud. For instance, if we
scale our self-similar solution to a single supernova of energy 10^51 ergs
occurring when a cloud of initial density 10^2 H/cm^3 has collapsed by 50%, we
find that the shockfront is confined to ~15 pc in ~1 Myrs. Our calculations are
pertinent to the observed unusually compact non-thermal radio emission in blue
compact dwarf galaxies (BCDs). More generally, we demonstrate the potential of
a collapsing cloud to confine supernovae, thereby explaining how dwarf galaxies
would exist beyond their first generation of star formation.Comment: 3 pages, 4 figure
Duality Symmetries in N=2 Heterotic Superstring
We review the derivation and the basic properties of the perturbative
prepotential in N=2 compactifications of the heterotic superstring. We discuss
the structure of the perturbative monodromy group and the embedding of rigidly
supersymmetric monodromies associated with enhanced gauge groups, at both
perturbative and non-perturbative level.Comment: Based on talks presented at several conferences. 12 pages, LaTe
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