1,561 research outputs found
Mirage Cosmology on Unstable D3-Brane Universe
We study the geodesic motion of an unstable brane moving in a higher
dimensional bulk spacetime. The tachyon which is coupled to a U(1) gauge field
induces a non-trivial cosmological evolution. Interestingly enough, this system
exhibits a much smoother initial cosmological singularity in comparison with
former works.Comment: 6 pages. Talk at the RTN conference ``The Quest for Unification:
Theory Confronts Experiment", Corfu, Greece, Sept, 11-18, 200
Brane Boxes, Anomalies, Bending and Tadpoles
Certain classes of chiral four-dimensional gauge theories may be obtained as
the worldvolume theories of D5-branes suspended between networks of NS5-branes,
the so-called brane box models. In this paper, we derive the stringy
consistency conditions placed on these models, and show that they are
equivalent to anomaly cancellation of the gauge theories. We derive these
conditions in the orbifold theories which are T-dual to the elliptic brane box
models. Specifically, we show that the expression for tadpoles for unphysical
twisted Ramond-Ramond 4-form fields in the orbifold theory are proportional to
the gauge anomalies of the brane box theory. Thus string consistency is
equivalent to worldvolume gauge anomaly cancellation. Furthermore, we find
additional cylinder amplitudes which give the -functions of the gauge
theory. We show how these correspond to bending of the NS-branes in the brane
box theory.Comment: 14 pages, 3 epsf figures. Minor changes, references adde
Momentum Distributions in
We apply the Green function formalism for production and decay
near threshold in a study of the effects due to the momentum dependent width
for such a system. We point out that these effects are likely to be much
smaller than expected from the reduction of the available phase space. The
Lippmann--Schwinger equation for the QCD chromostatic potential is solved
numerically for partial wave. We compare the results on the total cross
section, top quark intrinsic momentum distributions and on the energy spectra
of bosons from top quark decays with those obtained for the constant width.Comment: 12 pages (without figures) (11 (sub)figures available on request),
Karlsruhe preprint TTP93-11, hep-ph/yymmnn
Loop quantum gravity effects on inflation and the CMB
In loop quantum cosmology, the universe avoids a big bang singularity and
undergoes an early and short super-inflation phase. During super-inflation,
non-perturbative quantum corrections to the dynamics drive an inflaton field up
its potential hill, thus setting the initial conditions for standard inflation.
We show that this effect can raise the inflaton high enough to achieve
sufficient e-foldings in the standard inflation era. We analyze the
cosmological perturbations generated when slow-roll is violated after
super-inflation, and show that loop quantum effects can in principle leave an
indirect signature on the largest scales in the CMB, with some loss of power
and running of the spectral index.Comment: revtex4, 5 pages, 3 figures, significant improvements in explanation
of quantization and perturbation issues; version to appear Classical and
Quantum Gravit
On the connection between the pinch technique and the background field method
The connection between the pinch technique and the background field method is
further explored. We show by explicit calculations that the application of the
pinch technique in the framework of the background field method gives rise to
exactly the same results as in the linear renormalizable gauges. The general
method for extending the pinch technique to the case of Green's functions with
off-shell fermions as incoming particles is presented. As an example, the
one-loop gauge independent quark self-energy is constructed. We briefly discuss
the possibility that the gluonic Green's functions, obtained by either method,
correspond to physical quantities.Comment: 13 pages and 3 figures, all included in a uuencoded file, to appear
in Physical Review
Cosmological evolution of scalar fields and gravitino dark matter in gauge mediation at low reheating temperatures
We consider the dynamics of the supersymmetry-breaking scalar field and the
production of dark matter gravitinos via its decay in a gauge-mediated
supersymmetry breaking model with metastable vacuum. We find that the scalar
field amplitude and gravitino density are extremely sensitive to the parameters
of the hidden sector. For the case of an O'Raifeartaigh sector, we show that
the observed dark matter density can be explained by gravitinos even for low
reheating temperatures T_{R} < 10 GeV. Such low reheating temperatures may be
implied by detection of the NLSP at the LHC if its thermal freeze-out density
is in conflict with BBN.Comment: 11 pages RevTex. Extended discussion and minor corrections,
conclusions unaltered. Version to be published in JCA
Scene Coordinate Regression with Angle-Based Reprojection Loss for Camera Relocalization
Image-based camera relocalization is an important problem in computer vision
and robotics. Recent works utilize convolutional neural networks (CNNs) to
regress for pixels in a query image their corresponding 3D world coordinates in
the scene. The final pose is then solved via a RANSAC-based optimization scheme
using the predicted coordinates. Usually, the CNN is trained with ground truth
scene coordinates, but it has also been shown that the network can discover 3D
scene geometry automatically by minimizing single-view reprojection loss.
However, due to the deficiencies of the reprojection loss, the network needs to
be carefully initialized. In this paper, we present a new angle-based
reprojection loss, which resolves the issues of the original reprojection loss.
With this new loss function, the network can be trained without careful
initialization, and the system achieves more accurate results. The new loss
also enables us to utilize available multi-view constraints, which further
improve performance.Comment: ECCV 2018 Workshop (Geometry Meets Deep Learning
Mars Rover Sample Return: A sample collection and analysis strategy for exobiology
For reasons defined elsewhere it is reasonable to search for biological signatures, both chemical and morphological, of extinct life on Mars. Life on Earth requries the presence of liquid water, therefore, it is important to explore sites on Mars where standing bodies of water may have once existed. Outcrops of layered deposits within the Valles Marineris appear to be ancient lake beds. Because the outcrops are well exposed, relatively shallow core samples would be very informative. The most important biological signature to detect would be organics, microfossils, or larger stromato-like structures, although the presence of cherts, carbonates, clays, and shales would be significant. In spite of the limitations of current robotics and pattern recognition, and the limitations of rover power, computation, Earth communication bandwidth, and time delays, a partial scenario was developed to implement such a scientific investigation. The rover instrumentation and the procedures and decisions and IR spectrometer are described in detail. Preliminary results from a collaborative effort are described, which indicate the rover will be able to autonomously detect stratification, and hence will ease the interpretation burden and lead to greater scientific productivity during the rover's lifetime
Hologrphy and holographic dark energy model
The holographic principle is used to discuss the holographic dark energy
model. We find that the Bekenstein-Hawking entropy bound is far from saturation
under certain conditions. A more general constraint on the parameter of the
holographic dark energy model is also derived.Comment: no figures, use revtex, v2: use iop style, some typos corrected and
references updated, will appear in CQ
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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