23,249 research outputs found
Test and evaluate passive orbital disconnect struts (PODS 3)
The objectives of the Passive Orbital Disconnect Struts (PODS) test are to evaluate modal resonance of the PODS-III supports to obtain engineering data required for use of PODS-III on flight systems; determine possible performance improvements in large LO2/LH2 space applications. (1) Modal Vibration Tests. A modal resonance survey is performed on a set of six PODS-III struts assembled in a dewar simulator. The survey conditions simulate both launch and orbital loadings of the struts. The orbital load range spans a full to an empty tank. The frequencies surveyed cover the range consistent with Shuttle qualification requirements and the principal resonant modes of the strut system. (2) Benefit study. The benefit of using PODS-III supports on OTV and Space Station LO sub 2 and LH sub 2 reference tanks was compared to nondisconnect supports. Four LO sub 2 and LH sub 2 tanks were studied under various conditions: (1) holding the launch resonance at 35 Hz and varying the orbit resonance; (2) analyzing both full and emtpy tanks at launch; (3) varying orbit boundary temperaure; (4) varying the number of struts; (5) varying orbit times; and (6) using or not using vapor cooling
Ordering dynamics of the driven lattice gas model
The evolution of a two-dimensional driven lattice-gas model is studied on an
L_x X L_y lattice. Scaling arguments and extensive numerical simulations are
used to show that starting from random initial configuration the model evolves
via two stages: (a) an early stage in which alternating stripes of particles
and vacancies are formed along the direction y of the driving field, and (b) a
stripe coarsening stage, in which the number of stripes is reduced and their
average width increases. The number of stripes formed at the end of the first
stage is shown to be a function of L_x/L_y^\phi, with \phi ~ 0.2. Thus,
depending on this parameter, the resulting state could be either single or
multi striped. In the second, stripe coarsening stage, the coarsening time is
found to be proportional to L_y, becoming infinitely long in the thermodynamic
limit. This implies that the multi striped state is thermodynamically stable.
The results put previous studies of the model in a more general framework
Precision computation of the kaon bag parameter
Indirect CP violation in K \rightarrow {\pi}{\pi} decays plays a central role
in constraining the flavor structure of the Standard Model (SM) and in the
search for new physics. For many years the leading uncertainty in the SM
prediction of this phenomenon was the one associated with the nonperturbative
strong interaction dynamics in this process. Here we present a fully controlled
lattice QCD calculation of these effects, which are described by the neutral
kaon mixing parameter B_K . We use a two step HEX smeared clover-improved
Wilson action, with four lattice spacings from a\approx0.054 fm to
a\approx0.093 fm and pion masses at and even below the physical value.
Nonperturbative renormalization is performed in the RI-MOM scheme, where we
find that operator mixing induced by chiral symmetry breaking is very small.
Using fully nonperturbative continuum running, we obtain our main result
B_K^{RI}(3.5GeV)=0.531(6)_{stat}(2)_{sys}. A perturbative 2-loop conversion
yields B_K^{MSbar-NDR}(2GeV)=0.564(6)_{stat}(3)_{sys}(6)_{PT}, which is in good
agreement with current results from fits to experimental data.Comment: 10 pages, 7 figures. v2: Added one reference and one figure, replaced
2 figures for better readability and updated ensembles, conclusions
unchanged. Final, published versio
L-functions with large analytic rank and abelian varieties with large algebraic rank over function fields
The goal of this paper is to explain how a simple but apparently new fact of
linear algebra together with the cohomological interpretation of L-functions
allows one to produce many examples of L-functions over function fields
vanishing to high order at the center point of their functional equation. The
main application is that for every prime p and every integer g>0 there are
absolutely simple abelian varieties of dimension g over Fp(t) for which the BSD
conjecture holds and which have arbitrarily large rank.Comment: To appear in Inventiones Mathematica
The ratio FK/Fpi in QCD
We determine the ratio FK/Fpi in QCD with Nf=2+1 flavors of sea quarks, based
on a series of lattice calculations with three different lattice spacings,
large volumes and a simulated pion mass reaching down to about 190 MeV. We
obtain FK/Fpi=1.192 +/- 0.007(stat) +/- 0.006(syst). This result is then used
to give an updated value of the CKM matrix element |Vus|. The unitarity
relation for the first row of this matrix is found to be well observed.Comment: 15 pages, 4 figures, 2 table
Implications of the Visible and X-Ray Counterparts to GRB970228
The gamma-ray burst source GRB970228 has been observed after a delay of 8--12
hours in X-rays and after one day in visible and near infrared light. This
marks the first detection of emission at lower frequencies following the
gamma-ray observation of a GRB and the first detection of any visible
counterpart to a GRB. We consider possible delayed visible and X-ray emission
mechanisms, and conclude that the intrinsic gamma-ray activity continued at a
much reduced intensity for at least a day. There are hints of such continued
activity in other GRB, and future observations can decide if this is true of
GRB in general. The observed multi-band spectrum of GRB970228 agrees with the
predictions of relativistic shock theory when the flux is integrated over a
time longer than that required for a radiating electron to lose its energy.Comment: 5 pp., tex, 1 figur
FFT for the APE Parallel Computer
We present a parallel FFT algorithm for SIMD systems following the `Transpose
Algorithm' approach. The method is based on the assignment of the data field
onto a 1-dimensional ring of systolic cells. The systolic array can be
universally mapped onto any parallel system. In particular for systems with
next-neighbour connectivity our method has the potential to improve the
efficiency of matrix transposition by use of hyper-systolic communication. We
have realized a scalable parallel FFT on the APE100/Quadrics massively parallel
computer, where our implementation is part of a 2-dimensional hydrodynamics
code for turbulence studies. A possible generalization to 4-dimensional FFT is
presented, having in mind QCD applications.Comment: 17 pages, 13 figures, figures include
Thermodynamics of a black hole in a cavity
We present a unified thermodynamical description of the configurations
consisting on self-gravitating radiation with or without a black hole. We
compute the thermal fluctuations and evaluate where will they induce a
transition from metastable configurations towards stable ones. We show that the
probability of finding such a transition is exponentially small. This indicates
that, in a sequence of quasi equilibrium configurations, the system will remain
in the metastable states till it approaches very closely the critical point
beyond which no metastable configuration exists. Near that point, we relate the
divergence of the local temperature fluctuations to the approach of the
instability of the whole system, thereby generalizing the usual fluctuations
analysis in the cases where long range forces are present. When angular
momentum is added to the cavity, the above picture is slightly modified.
Nevertheless, at high angular momentum, the black hole loses most of its mass
before it reaches the critical point at which it evaporates completely.Comment: 27 pages, latex file, contains 3 figures available on request at
[email protected]
F-theory, Geometric Engineering and N=1 Dualities
We consider geometric engineering of N=1 supersymmetric QFTs with matter in
terms of a local model for compactification of F-theory on Calabi-Yau fourfold.
By bringing 3-branes near 7-branes we engineer N=1 supersymmetric
gauge theory with flavors in the fundamental. We identify the Higgs
branch of this system with the instanton moduli space on the compact four
dimensional space of the 7-brane worldvolume. Moreover we show that the
Euclidean 3-branes wrapped around the compact part of the 7-brane worldvolume
can generate superpotential for as well as lead to quantum
corrections to the moduli space for . Finally we argue that Seiberg's
duality for N=1 supersymmetric QCD may be mapped to T-duality exchanging
7-branes with 3-branes.Comment: 15 page
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