176 research outputs found
Sulfur Mineralogy at the Mars Phoenix Landing Site
The Mars Phoenix Scout mission landed at the northernmost location (approx.68deg N) of any lander or rover on the martian surface. This paper compares the S mineralogy at the Phoenix landing site with S mineralogy of soils studied by previous Mars landers. S-bearing phases were not directly detected by the payload onboard the Phoenix spacecraft. Our objective is to derive the possible mineralogy of S-bearing phases at the Phoenix landing site based upon Phoenix measurements in combination with orbital measurements, terrestrial analog and Martian meteorite studies, and telescopic observations
Matrix Cosmology
Exact time-dependent solutions of c=1 string theory are described using the
free fermion formulation. One such class of solutions describes draining of the
Fermi sea and has a spacetime interpretation as closed string tachyon
condensation. A second class of solutions, corresponding to droplets of Fermi
liquid orbiting in phase space, describes closed cosmologies which bounce
through singularities.Comment: 21 pages, 6 figures, v2: added references, minor additions and
correction
Ricci-flat deformation of orbifolds and localized tachyonic modes
We study Ricci-flat deformations of orbifolds in type II theory. We obtain a
simple formula for mass corrections to the twisted modes due to the
deformations, and apply it to originally tachyonic and massless states in
several examples. In the case of supersymmetric orbifolds, we find that
tachyonic states appear when the deformation breaks all the supersymmetries. We
also study nonsupersymmetric orbifolds C^2/Z_{2N(2N+1)}, which is T-dual to N
type 0 NS5-branes. For N>=2, we compute mass corrections for states, which have
string scale tachyonic masses. We find that the corrected masses coincide to
ones obtained by solving the wave equation for the tachyon field in the smeared
type 0 NS5-brane background geometry. For N=1, we show that the unstable mode
representing the bubble creation is the unique tachyonic mode.Comment: 20 pages, minor collection
Moduli Stabilisation and de Sitter String Vacua from Magnetised D7 Branes
Anomalous U(1)'s are ubiquitous in 4D chiral string models. Their presence
crucially affects the process of moduli stabilisation and cannot be neglected
in realistic set-ups. Their net effect in the 4D effective action is to induce
a matter field dependence in the non-perturbative superpotential and a
Fayet-Iliopoulos D-term. We study flux compactifications of IIB string theory
in the presence of magnetised D7 branes. These give rise to anomalous U(1)'s
that modify the standard moduli stabilisation procedure. We consider simple
orientifold models to determine the matter field spectrum and the form of the
effective field theory. We apply our results to one-modulus KKLT and
multi-moduli large volume scenarios, in particular to the Calabi-Yau
P^4_{[1,1,1,6,9]}. After stabilising the matter fields, the effective action
for the Kahler moduli can acquire an extra positive term that can be used for
de Sitter lifting with non-vanishing F- and D-terms. This provides an explicit
realization of the D-term lifting proposal of hep-th/0309187.Comment: 35 pages, 1 figure. v2: Minor changes, references adde
Closed String Tachyon Condensation at c=1
The c=1 matrix model, with or without a type 0 hat, has an exact quantum
solution corresponding to closed string tachyon condensation along a null
surface. The condensation occurs, and spacetime dissolves, at a finite retarded
time on I^+. The outgoing quantum state of tachyon fluctuations in this
time-dependent background is computed using both the collective field and exact
fermion pictures. Perturbative particle production induced by the moving
tachyon wall is shown to be similar to that induced by a soft moving mirror.
Hence, despite the fact that I^+ for the tachyon is geodesicaly incomplete,
quantum correlations in the incoming state are unitarily transmitted to the
outgoing state in perturbation theory. It is also shown that,
non-perturbatively, information can leak across the tachyon wall, and tachyon
scattering is not unitary. Exact unitarity remains intact only in the free
fermion picture.Comment: Minor corrections; References added; 24 pages, 2 figures, harvma
Yang-Mills Duals for Semiclassical Strings
We consider a semiclassical multiwrapped circular string pulsating on S_5,
whose center of mass has angular momentum J on an S_3 subspace. Using the
AdS/CFT correspondence we argue that the one-loop anomalous dimension of the
dual operator is a simple rational function of J/L, where J is the R-charge and
L is the bare dimension of the operator. We then reproduce this result directly
from a super Yang-Mills computation, where we make use of the integrability of
the one-loop system to set up an integral equation that we solve. We then
verify the results of Frolov and Tseytlin for circular rotating strings with
R-charge assignment (J',J',J). In this case we solve for an integral equation
found in the O(-1) matrix model when J' J.
The latter region starts at J'=L/2 and continues down, but an apparent critical
point is reached at J'=4J. We argue that the critical point is just an artifact
of the Bethe ansatz and that the conserved charges of the underlying integrable
model are analytic for all J' and that the results from the O(-1) model
continue onto the results of the O(+1) model.Comment: 26 Pages, LaTeX; v2 Typos corrected, reference update
Three-Charge Supertubes in a Rotating Black Hole Background
The low velocity scattering of a D0-F1 supertube in the background of a BMPV
black hole has been investigated in the moduli space approximation by Marolf
and Virmani. Here we extend the analysis to the case of the D0-D4-F1 supertube
of Bena and Kraus. We find that, similarly to the two-charge case, there is a
critical value of the supertube circumferential angular momentum; above this
value an adiabatic merger with the black hole cannot occur. By reconsidering
the calculation of supertube angular momentum in the transverse direction,
correspondence between the worldvolume and supergravity descriptions is
established. We also examine dynamical mergers and discuss their implications.Comment: 38 pages, 9 figures. New discussion of moduli space approximation vs.
exact DBI action, references adde
Matrix Model and Time-like Linear Dilaton Matter
We consider a matrix model description of the 2d string theory whose matter
part is given by a time-like linear dilaton CFT. This is equivalent to the c=1
matrix model with a deformed, but very simple fermi surface. Indeed, after a
Lorentz transformation, the corresponding 2d spacetime is a conventional linear
dilaton background with a time-dependent tachyon field. We show that the tree
level scattering amplitudes in the matrix model perfectly agree with those
computed in the world-sheet theory. The classical trajectories of fermions
correspond to the decaying D-branes in the time-like linear dilaton CFT. We
also discuss the ground ring structure. Furthermore, we study the properties of
the time-like Liouville theory by applying this matrix model description. We
find that its ground ring structure is very similar to that of the minimal
string.Comment: 30 pages, harvmac, typos corrected, acknowledgements and comments
added(v2), published version (v3
Low Complexity Regularization of Linear Inverse Problems
Inverse problems and regularization theory is a central theme in contemporary
signal processing, where the goal is to reconstruct an unknown signal from
partial indirect, and possibly noisy, measurements of it. A now standard method
for recovering the unknown signal is to solve a convex optimization problem
that enforces some prior knowledge about its structure. This has proved
efficient in many problems routinely encountered in imaging sciences,
statistics and machine learning. This chapter delivers a review of recent
advances in the field where the regularization prior promotes solutions
conforming to some notion of simplicity/low-complexity. These priors encompass
as popular examples sparsity and group sparsity (to capture the compressibility
of natural signals and images), total variation and analysis sparsity (to
promote piecewise regularity), and low-rank (as natural extension of sparsity
to matrix-valued data). Our aim is to provide a unified treatment of all these
regularizations under a single umbrella, namely the theory of partial
smoothness. This framework is very general and accommodates all low-complexity
regularizers just mentioned, as well as many others. Partial smoothness turns
out to be the canonical way to encode low-dimensional models that can be linear
spaces or more general smooth manifolds. This review is intended to serve as a
one stop shop toward the understanding of the theoretical properties of the
so-regularized solutions. It covers a large spectrum including: (i) recovery
guarantees and stability to noise, both in terms of -stability and
model (manifold) identification; (ii) sensitivity analysis to perturbations of
the parameters involved (in particular the observations), with applications to
unbiased risk estimation ; (iii) convergence properties of the forward-backward
proximal splitting scheme, that is particularly well suited to solve the
corresponding large-scale regularized optimization problem
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