9,982 research outputs found
On Quantum Special Kaehler Geometry
We compute the effective black hole potential V of the most general N=2, d=4
(local) special Kaehler geometry with quantum perturbative corrections,
consistent with axion-shift Peccei-Quinn symmetry and with cubic leading order
behavior. We determine the charge configurations supporting axion-free
attractors, and explain the differences among various configurations in
relations to the presence of ``flat'' directions of V at its critical points.
Furthermore, we elucidate the role of the sectional curvature at the
non-supersymmetric critical points of V, and compute the Riemann tensor (and
related quantities), as well as the so-called E-tensor. The latter expresses
the non-symmetricity of the considered quantum perturbative special Kaehler
geometry.Comment: 1+43 pages; v2: typo corrected in the curvature of Jordan symmetric
sequence at page 2
Heterotic compactifications and nearly-Kahler manifolds
We propose that under certain conditions heterotic string compactifications
on half-flat and nearly-Kahler manifolds are equivalent. Based on this
correspondence we argue that the moduli space of the nearly-Kahler manifolds
under discussion consists only of the Kahler deformations moduli space and
there is no correspondent for the complex structure deformations.Comment: 5 pages, references added, typos correcte
Head-on collisions of unequal mass black holes in D=5 dimensions
We study head-on collisions of unequal mass black hole binaries in D=5
space-time dimensions, with mass ratios between 1:1 and 1:4. Information about
gravitational radiation is extracted by using the Kodama-Ishibashi
gauge-invariant formalism and details of the apparent horizon of the final
black hole. For the first time, we present waveforms, total integrated energy
and momentum for this process. Our results show surprisingly good agreement,
within 5% or less, with those extrapolated from linearized, point-particle
calculations. Our results also show that consistency with the area theorem
bound requires that the same process in a large number of spacetime dimensions
must display new features.Comment: 10 pages, 5 figures, RevTex4. v2: Published versio
Fourier PCA and Robust Tensor Decomposition
Fourier PCA is Principal Component Analysis of a matrix obtained from higher
order derivatives of the logarithm of the Fourier transform of a
distribution.We make this method algorithmic by developing a tensor
decomposition method for a pair of tensors sharing the same vectors in rank-
decompositions. Our main application is the first provably polynomial-time
algorithm for underdetermined ICA, i.e., learning an matrix
from observations where is drawn from an unknown product
distribution with arbitrary non-Gaussian components. The number of component
distributions can be arbitrarily higher than the dimension and the
columns of only need to satisfy a natural and efficiently verifiable
nondegeneracy condition. As a second application, we give an alternative
algorithm for learning mixtures of spherical Gaussians with linearly
independent means. These results also hold in the presence of Gaussian noise.Comment: Extensively revised; details added; minor errors corrected;
exposition improve
Impurity intrusion in radio-frequency micro-plasma jets operated in ambient air
Space and time resolved concentrations of helium metastable atoms in an
atmospheric pressure radio-frequency micro-plasma jet were measured using
tunable diode laser absorption spectroscopy. Spatial profiles as well as
lifetime measurements show significant influences of air entering the discharge
from the front nozzle and of impurities originating from the gas supply system.
Quenching of metastables was used to deduce quantitative concentrations of
intruding impurities. The impurity profile along the jet axis was determined
from optical emission spectroscopy as well as their dependance on the feed gas
flow through the jet.Comment: Journal of Physics D: Applied Physics (accepted), 6 page
Quasinormal modes of black holes in anti-de Sitter space: a numerical study of the eikonal limit
Using series solutions and time-domain evolutions, we probe the eikonal limit
of the gravitational and scalar-field quasinormal modes of large black holes
and black branes in anti-de Sitter backgrounds. These results are particularly
relevant for the AdS/CFT correspondence, since the eikonal regime is
characterized by the existence of long-lived modes which (presumably) dominate
the decay timescale of the perturbations. We confirm all the main qualitative
features of these slowly-damped modes as predicted by Festuccia and Liu
(arXiv:0811.1033) for the scalar-field (tensor-type gravitational)
fluctuations. However, quantitatively we find dimensional-dependent correction
factors. We also investigate the dependence of the QNM frequencies on the
horizon radius of the black hole (brane) and the angular momentum (wavenumber)
of vector- and scalar-type gravitational perturbations.Comment: 5 pages, RevTex4. v2: References added and minor typos corrected.
Published versio
Black Holes in Supergravity and String Theory
We give an elementary introduction to black holes in supergravity and string
theory. The focus is on BPS solutions in four- and higher-dimensional
supergravity and string theory. Basic ideas and techniques are explained in
detail, including exercises with solutions.Comment: 64 pages, based on lectures given at the school of the TMR network
'Quantum aspects of gauge theories, supersymmetry and unification' in Torino,
January 26 - February 2, 2000. To be published in Class. Quant. Grav. (Some
typos corrected, two references added.
The Monitor Project: Stellar rotation at 13~Myr: I. A photometric monitoring survey of the young open cluster h~Per
We aim at constraining the angular momentum evolution of low mass stars by
measuring their rotation rates when they begin to evolve freely towards the
ZAMS, i.e. after the disk accretion phase has stopped. We conducted a
multi-site photometric monitoring of the young open cluster h Persei that has
an age of ~13 Myr. The observations were done in the I-band using 4 different
telescopes and the variability study is sensitive to periods from less than 0.2
day to 20 days. Rotation periods are derived for 586 candidate cluster members
over the mass range 0.4<=M/Msun<=1.4. The rotation period distribution
indicates a sligthly higher fraction of fast rotators for the lower mass
objects, although the lower and upper envelopes of the rotation period
distribution, located respectively at ~0.2-0.3d and ~10d, are remarkably flat
over the whole mass range. We combine this period distribution with previous
results obtained in younger and older clusters to model the angular momentum
evolution of low mass stars during the PMS. The h Per cluster provides the
first statistically robust estimate of the rotational period distribution of
solar-type and lower mass stars at the end of the PMS accretion phase (>10
Myr). The results are consistent with models that assume significant
core-envelope decoupling during the angular momentum evolution to the ZAMS.Comment: 39 pages, 19 figures, light curves in appendix, 1 long tabl
Black Holes, Space-Filling Chains and Random Walks
Many approaches to a semiclassical description of gravity lead to an integer
black hole entropy. In four dimensions this implies that the Schwarzschild
radius obeys a formula which describes the distance covered by a Brownian
random walk. For the higher-dimensional Schwarzschild-Tangherlini black hole,
its radius relates similarly to a fractional Brownian walk. We propose a
possible microscopic explanation for these random walk structures based on
microscopic chains which fill the interior of the black hole.Comment: 18 pages, 4 figures, 2 tables; v2 and v3: minor changes and refs.
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Comments on Heterotic Flux Compactifications
In heterotic flux compactification with supersymmetry, three different
connections with torsion appear naturally, all in the form .
Supersymmetry condition carries , the Dirac operator has , and
higher order term in the effective action involves . With a view toward
the gauge sector, we explore the geometry with such torsions. After reviewing
the supersymmetry constraints and finding a relation between the scalar
curvature and the flux, we derive the squared form of the zero mode equations
for gauge fermions. With \d H=0, the operator has a positive potential term,
and the mass of the unbroken gauge sector appears formally positive definite.
However, this apparent contradiction is avoided by a no-go theorem that the
compactification with and \d H=0 is necessarily singular, and the
formal positivity is invalid. With \d H\neq 0, smooth compactification
becomes possible. We show that, at least near smooth supersymmetric solution,
the size of should be comparable to that of \d H and the consistent
truncation of action has to keep term. A warp factor equation of
motion is rewritten with contribution included precisely, and
some limits are considered.Comment: 31 pages, a numerical factor correcte
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