14,336 research outputs found
Observing the Geometry of Warped Compactification via Cosmic Inflation
Using DBI inflation as an example, we demonstrate that the detailed geometry
of warped compactification can leave an imprint on the cosmic microwave
background (CMB). We compute CMB observables for DBI inflation in a generic
class of warped throats and find that the results (such as the sign of the tilt
of the scalar perturbations and its running) depend sensitively on the precise
shape of the warp factor. In particular, we analyze the warped deformed
conifold and find that the results can differ from those of other warped
geometries, even when these geometries approximate well the exact metric of the
warped deformed conifold.Comment: 4 pages, 3 figures. v2: References and clarifications adde
High Harmonic Generation in SF: Raman-excited Vibrational Quantum Beats
In a recent experiment (N. Wagner et al., PNAS v103, p13279) on SF, a
high-harmonic generating laser pulse is preceded by a pump pulse which
stimulates Raman-active modes in the molecule. Varying the time delay between
the two pulses modulates high harmonic intensity, with frequencies equal to the
vibration frequencies of the Raman-active modes. We propose an explanation of
this modulation as a quantum interference between competing pathways that occur
via adjacent vibrational states of the molecule. The Raman and high harmonic
processes act as beamsplitters, producing vibrational quantum beats among the
Raman-active vibrational modes that are excited by the first pulse. We
introduce a rigorous treatment of the electron-ion recombination process and
the effect of the ionic Coulomb field in the electron propagation outside the
molecule, improving over the widely-used three-step model.Comment: submitted to PR
Thermal Phase Variations of WASP-12b: Defying Predictions
[Abridged] We report Warm Spitzer full-orbit phase observations of WASP-12b
at 3.6 and 4.5 micron. We are able to measure the transit depths, eclipse
depths, thermal and ellipsoidal phase variations at both wavelengths. The large
amplitude phase variations, combined with the planet's previously-measured
day-side spectral energy distribution, is indicative of non-zero Bond albedo
and very poor day-night heat redistribution. The transit depths in the
mid-infrared indicate that the atmospheric opacity is greater at 3.6 than at
4.5 micron, in disagreement with model predictions, irrespective of C/O ratio.
The secondary eclipse depths are consistent with previous studies. We do not
detect ellipsoidal variations at 3.6 micron, but our parameter uncertainties
-estimated via prayer-bead Monte Carlo- keep this non-detection consistent with
model predictions. At 4.5 micron, on the other hand, we detect ellipsoidal
variations that are much stronger than predicted. If interpreted as a geometric
effect due to the planet's elongated shape, these variations imply a 3:2 ratio
for the planet's longest:shortest axes and a relatively bright day-night
terminator. If we instead presume that the 4.5 micron ellipsoidal variations
are due to uncorrected systematic noise and we fix the amplitude of the
variations to zero, the best fit 4.5 micron transit depth becomes commensurate
with the 3.6 micron depth, within the uncertainties. The relative transit
depths are then consistent with a Solar composition and short scale height at
the terminator. Assuming zero ellipsoidal variations also yields a much deeper
4.5 micron eclipse depth, consistent with a Solar composition and modest
temperature inversion. We suggest future observations that could distinguish
between these two scenarios.Comment: 19 pages, 10 figures, ApJ in press. Improved discussion of gravity
brightenin
BEC-BCS Crossover of a Trapped Two-Component Fermi Gas with Unequal Masses
We determine the energetically lowest lying states in the BEC-BCS crossover
regime of s-wave interacting two-component Fermi gases under harmonic
confinement by solving the many-body Schrodinger equation using two distinct
approaches. Essentially exact basis set expansion techniques are applied to
determine the energy spectrum of systems with N=4 fermions. Fixed-node
diffusion Monte Carlo methods are applied to systems with up to N=20 fermions,
and a discussion of different guiding functions used in the Monte Carlo
approach to impose the proper symmetry of the fermionic system is presented.
The energies are calculated as a function of the s-wave scattering length a_s
for N=2-20 fermions and different mass ratios \kappa of the two species. On the
BEC and BCS sides, our energies agree with analytically-determined first-order
correction terms. We extract the scattering length and the effective range of
the dimer-dimer system up to \kappa = 20. Our energies for the
strongly-interacting trapped system in the unitarity regime show no shell
structure, and are well described by a simple expression, whose functional form
can be derived using the local density approximation, with one or two
parameters. The universal parameter \xi for the trapped system for various
\kappa is determined, and comparisons with results for the homogeneous system
are presented.Comment: 11 pages, 6 figures, extended versio
Asymptotic Statistics of Poincar\'e Recurrences in Hamiltonian Systems with Divided Phase Space
By different methods we show that for dynamical chaos in the standard map
with critical golden curve the Poincar\'e recurrences P(\tau) and correlations
C(\tau) asymptotically decay in time as P ~ C/\tau ~ 1/\tau^3. It is also
explained why this asymptotic behavior starts only at very large times. We
argue that the same exponent p=3 should be also valid for a general chaos
border.Comment: revtex, 4 pages, 3 ps-figure
Formation of atomic tritium clusters and condensates
We present an extensive study of the static and dynamic properties of systems
of spin-polarized tritium atoms. In particular, we calculate the two-body
|F,m_F>=|0,0> s-wave scattering length and show that it can be manipulated via
a Feshbach resonance at a field strength of about 870G. Such a resonance might
be exploited to make and control a Bose-Einstein condensate of tritium in the
|0,0> state. It is further shown that the quartet tritium trimer is the only
bound hydrogen isotope and that its single vibrational bound state is a
Borromean state. The ground state properties of larger spin-polarized tritium
clusters are also presented and compared with those of helium clusters.Comment: 5 pages, 3 figure
Chaotic behavior in a Z_2 x Z_2 field theory
We investigate the presence of chaos in a system of two real scalar fields
with discrete Z_2 x Z_2 symmetry. The potential that identify the system is
defined with a real parameter r and presents distinct features for r>0 and for
r<0. For static field configurations, the system supports two topological
sectors for r>0, and only one for r<0. Under the assumption of spatially
homogeneous fields, the system exhibts chaotic behavior almost everywhere in
parameter space. In particular a more complex dynamics appears for r>0; in this
case chaos can decrease for increasing energy, a fact that is absent for r<0.Comment: Revtex, 13 pages, no figures. Version with figures in Int. J. Mod.
Phys. A14 (1999) 496
The Radio Quiescence of Active Galaxies with High Accretion Rates
We present 6 cm Very Large Array observations of the Greene & Ho (2004)
sample of 19 low-mass active galaxies with high accretion rates. This is one of
the only studies of a uniform sample of narrow-line Seyfert 1 (NLS1) galaxies
with such high sensitivity and resolution. Although we detect only one source,
the entire sample is very radio-quiet down to strong limits. GH10 was found to
have a radio power of 8.5 x 10^21 W/Hz, and a ratio R = f(6 cm)/f(4400 A) of
2.8. The 3 sigma upper limits for the remaining nondetections correspond to
radio powers from 3 x 10^20 to 8 x 10^21 W/Hz and 0.47 < R <9.9. Stacking all
nondetections yields an even stronger upper limit of R < 0.27. An assessment of
existing observations in the literature confirms our finding that NLS1s are
consistently radio-quiet, with a radio-loud fraction of 0%-6%, which is
significantly lower than the 10%-20% observed in the general quasar population.
By analogy with stellar-mass black holes, we argue that AGNs undergo a state
transition at L_bol/L_Edd~0.01. Below this value a radiatively inefficient
accretion flow effectively drives an outflow, which disappears when the flow
turns into an optically thick, geometrically thin disk, or a radiation
pressure-dominated slim disk at still higher L_bol/L_Edd.Comment: To appear in ApJ; 8 pages, 3 figures; uses emulateapj5.st
Siegert pseudostates: completeness and time evolution
Within the theory of Siegert pseudostates, it is possible to accurately
calculate bound states and resonances. The energy continuum is replaced by a
discrete set of states. Many questions of interest in scattering theory can be
addressed within the framework of this formalism, thereby avoiding the need to
treat the energy continuum. For practical calculations it is important to know
whether a certain subset of Siegert pseudostates comprises a basis. This is a
nontrivial issue, because of the unusual orthogonality and overcompleteness
properties of Siegert pseudostates. Using analytical and numerical arguments,
it is shown that the subset of bound states and outgoing Siegert pseudostates
forms a basis. Time evolution in the context of Siegert pseudostates is also
investigated. From the Mittag-Leffler expansion of the outgoing-wave Green's
function, the time-dependent expansion of a wave packet in terms of Siegert
pseudostates is derived. In this expression, all Siegert pseudostates--bound,
antibound, outgoing, and incoming--are employed. Each of these evolves in time
in a nonexponential fashion. Numerical tests underline the accuracy of the
method
Summing up D-Instantons
We investigate quantum corrections to the moduli space for hypermultiplets
for type IIA near a conifold singularity. We find a unique quantum deformation
based on symmetry arguments which is consistent with a recent conjecture. The
correction can be interpreted as an infinite sum coming from multiple wrappings
of the Euclidean D-branes around the vanishing cycle.Comment: 8 pages, harvmac; a reference added, typoes correcte
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