56,496 research outputs found
Chemical interactions in Low Earth Orbit (LEO)
Although several observations of material changes on-orbit have been reported, mechanistic understanding has not yet become clear because new sets of non-intuitive processes are occurring on orbit. Reactant kinetic energy, low collision rates and surface/adsorbate interactions must be considered in the analysis of these observations. The specific example of oxide formation of elemental materials is examined in terms of thermodynamics and possible reaction pathways. On the basis of this approach, a rational trend emerges from the orbital behavior of these samples. The role of reactant kinetic energy as opposed to internal energy in chemiluminescent product formation is also presented. Development of a systematic thermochemical approach may be useful in making screening predictions of long-term material behavior on-orbit
Momentum Analyticity and Finiteness of the 1-Loop Superstring Amplitude
The Type II Superstring amplitude to 1-loop order is given by an integral of
-functions over the moduli space of tori, which diverges for real
momenta. We construct the analytic continuation which renders this amplitude
well defined and finite, and we find the expected poles and cuts in the complex
momentum plane.Comment: 10pp, /UCLA/93/TEP/
Small representations, string instantons, and Fourier modes of Eisenstein series (with an appendix by D. Ciubotaru and P. Trapa)
This paper concerns some novel features of maximal parabolic Eisenstein
series at certain special values of their analytic parameter s. These series
arise as coefficients in the R4 and D4R4 interactions in the low energy
expansion of scattering amplitudes in maximally supersymmetric string theory
reduced to D=10-d dimensions on a torus T^d, d<8. For each d these amplitudes
are automorphic functions on the rank d+1 symmetry group E_d+1. Of particular
significance is the orbit content of the Fourier modes of these series when
expanded in three different parabolic subgroups, corresponding to certain
limits of string theory. This is of interest in the classification of a variety
of instantons that correspond to minimal or next-to-minimal BPS orbits. In the
limit of decompactification from D to D+1 dimensions many such instantons are
related to charged 1/2-BPS or 1/4-BPS black holes with euclidean world-lines
wrapped around the large dimension. In a different limit the instantons give
nonperturbative corrections to string perturbation theory, while in a third
limit they describe nonperturbative contributions in eleven-dimensional
supergravity. A proof is given that these three distinct Fourier expansions
have certain vanishing coefficients that are expected from string theory. In
particular, the Eisenstein series for these special values of s have markedly
fewer Fourier coefficients than typical ones. The corresponding mathematics
involves showing that the wavefront sets of the Eisenstein series are supported
on only certain coadjoint nilpotent orbits - just the minimal and trivial
orbits in the 1/2-BPS case, and just the next-to-minimal, minimal and trivial
orbits in the 1/4-BPS case. Thus as a byproduct we demonstrate that the
next-to-minimal representations occur automorphically for E6, E7, and E8, and
hence the first two nontrivial low energy coefficients are exotic
theta-functions.Comment: v3: 127 pp. Minor changes. Final version to appear in the Special
Issue in honor of Professor Steve Ralli
Mass corrections in string theory and lattice field theory
Kaluza-Klein compactifications of higher dimensional Yang-Mills theories
contain a number of four dimensional scalars corresponding to the internal
components of the gauge field. While at tree-level the scalar zero modes are
massless, it is well known that quantum corrections make them massive. We
compute these radiative corrections at 1-loop in an effective field theory
framework, using the background field method and proper Schwinger-time
regularization. In order to clarify the proper treatment of the sum over
KK--modes in the effective field theory approach, we consider the same problem
in two different UV completions of Yang-Mills: string theory and lattice field
theory. In both cases, when the compactification radius is much bigger than
the scale of the UV completion (), we recover a mass
renormalization that is independent of the UV scale and agrees with the one
derived in the effective field theory approach. These results support the idea
that the value of the mass corrections is, in this regime, universal for any UV
completion that respects locality and gauge invariance. The string analysis
suggests that this property holds also at higher loops. The lattice analysis
suggests that the mass of the adjoint scalars appearing in
Super Yang-Mills is highly suppressed due to an interplay between the
higher-dimensional gauge invariance and the degeneracy of bosonic and fermionic
degrees of freedom.Comment: 27 page
Color enhancement of landsat agricultural imagery: JPL LACIE image processing support task
Color enhancement techniques were applied to LACIE LANDSAT segments to determine if such enhancement can assist analysis in crop identification. The procedure involved increasing the color range by removing correlation between components. First, a principal component transformation was performed, followed by contrast enhancement to equalize component variances, followed by an inverse transformation to restore familiar color relationships. Filtering was applied to lower order components to reduce color speckle in the enhanced products. Use of single acquisition and multiple acquisition statistics to control the enhancement were compared, and the effects of normalization investigated. Evaluation is left to LACIE personnel
String Bit Models for Superstring
We extend the model of string as a polymer of string bits to the case of
superstring. We mainly concentrate on type II-B superstring, with some
discussion of the obstacles presented by not II-B superstring, together with
possible strategies for surmounting them. As with previous work on bosonic
string we work within the light-cone gauge. The bit model possesses a good deal
less symmetry than the continuous string theory. For one thing, the bit model
is formulated as a Galilei invariant theory in dimensional
space-time. This means that Poincar\'e invariance is reduced to the Galilei
subgroup in space dimensions. Naturally the supersymmetry present in the
bit model is likewise dramatically reduced. Continuous string can arise in the
bit models with the formation of infinitely long polymers of string bits. Under
the right circumstances (at the critical dimension) these polymers can behave
as string moving in dimensional space-time enjoying the full
Poincar\'e supersymmetric dynamics of type II-B superstring.Comment: 43 pages, phyzzx require
Eisenstein series for higher-rank groups and string theory amplitudes
Scattering amplitudes of superstring theory are strongly constrained by the
requirement that they be invariant under dualities generated by discrete
subgroups, E_n(Z), of simply-laced Lie groups in the E_n series (n<= 8). In
particular, expanding the four-supergraviton amplitude at low energy gives a
series of higher derivative corrections to Einstein's theory, with coefficients
that are automorphic functions with a rich dependence on the moduli. Boundary
conditions supplied by string and supergravity perturbation theory, together
with a chain of relations between successive groups in the E_n series,
constrain the constant terms of these coefficients in three distinct parabolic
subgroups. Using this information we are able to determine the expressions for
the first two higher derivative interactions (which are BPS-protected) in terms
of specific Eisenstein series. Further, we determine key features of the
coefficient of the third term in the low energy expansion of the
four-supergraviton amplitude (which is also BPS-protected) in the E_8 case.
This is an automorphic function that satisfies an inhomogeneous Laplace
equation and has constant terms in certain parabolic subgroups that contain
information about all the preceding terms.Comment: Latex. 38 pages. 1 figure. v2: minor changes and clarifications. v3:
minor corrections, version to appear in Communications in Number Theory and
Physics. v4: corrections to table
Spectral identification/elimination of molecular species in spacecraft glow
Computer models of molecular electronic and vibrational emission intensities were developed. Known radiative emission rates (Einstein coefficients) permit the determination of relative excited state densities from spectral intensities. These codes were applied to the published spectra of glow above shuttle surface and to the Spacelab 1 results of Torr and Torr. The theoretical high-resolution spectra were convolved with the appropriate instrumental slit functions to allow accurate comparison with data. The published spacelab spectrum is complex but N2+ Meinel emission can be clearly identified in the ram spectrum. M2 First Positive emission does not correlate well with observed features, nor does the CN Red System. Spectral overlay comparisons are presented. The spectrum of glow above shuttle surfaces, in contrast to the ISO data, is not highly structured. Diatomic molecular emission was matched to the observed spectral shape. Source excitation mechanisms such as (oxygen atom)-(surface species) reaction product chemiluminescence, surface recombination, or resonance fluorescent re-emission will be discussed for each tentative assignment. These assignments are the necessary first analytical step toward mechanism identification. Different glow mechanisms will occur above surfaces under different orbital conditions
Combinatorics of Boundaries in String Theory
We investigate the possibility that stringy nonperturbative effects appear as
holes in the world-sheet. We focus on the case of Dirichlet string theory,
which we argue should be formulated differently than in previous work, and we
find that the effects of boundaries are naturally weighted by .Comment: 12 pages, 2 figures, LaTe
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