22,238 research outputs found
Reconstruction of hidden 3D shapes using diffuse reflections
We analyze multi-bounce propagation of light in an unknown hidden volume and
demonstrate that the reflected light contains sufficient information to recover
the 3D structure of the hidden scene. We formulate the forward and inverse
theory of secondary and tertiary scattering reflection using ideas from energy
front propagation and tomography. We show that using careful choice of
approximations, such as Fresnel approximation, greatly simplifies this problem
and the inversion can be achieved via a backpropagation process. We provide a
theoretical analysis of the invertibility, uniqueness and choices of
space-time-angle dimensions using synthetic examples. We show that a 2D streak
camera can be used to discover and reconstruct hidden geometry. Using a 1D high
speed time of flight camera, we show that our method can be used recover 3D
shapes of objects "around the corner"
A Step Beyond the Bounce: Bubble Dynamics in Quantum Phase Transitions
We study the dynamical evolution of a phase interface or bubble in the
context of a \lambda \phi^4 + g \phi^6 scalar quantum field theory. We use a
self-consistent mean-field approximation derived from a 2PI effective action to
construct an initial value problem for the expectation value of the quantum
field and two-point function. We solve the equations of motion numerically in
(1+1)-dimensions and compare the results to the purely classical evolution. We
find that the quantum fluctuations dress the classical profile, affecting both
the early time expansion of the bubble and the behavior upon collision with a
neighboring interface.Comment: 12 pages, multiple figure
Results From Core-Collapse Simulations with Multi-Dimensional, Multi-Angle Neutrino Transport
We present new results from the only 2D multi-group, multi-angle calculations
of core-collapse supernova evolution. The first set of results from these
calculations was published in Ott et al. (2008). We have followed a nonrotating
and a rapidly rotating 20 solar mass model for ~400 ms after bounce. We show
that the radiation fields vary much less with angle than the matter quantities
in the region of net neutrino heating. This obtains because most neutrinos are
emitted from inner radiative regions and because the specific intensity is an
integral over sources from many angles at depth. The latter effect can only be
captured by multi-angle transport. We then compute the phase relationship
between dipolar oscillations in the shock radius and in matter and radiation
quantities throughout the postshock region. We demonstrate a connection between
variations in neutrino flux and the hydrodynamical shock oscillations, and use
a variant of the Rayleigh test to estimate the detectability of these neutrino
fluctuations in IceCube and Super-K. Neglecting flavor oscillations,
fluctuations in our nonrotating model would be detectable to ~10 kpc in
IceCube, and a detailed power spectrum could be measured out to ~5 kpc. These
distances are considerably lower in our rapidly rotating model or with
significant flavor oscillations. Finally, we measure the impact of rapid
rotation on detectable neutrino signals. Our rapidly rotating model has strong,
species-dependent asymmetries in both its peak neutrino flux and its light
curves. The peak flux and decline rate show pole-equator ratios of up to ~3 and
~2, respectively.Comment: 13 pages, 9 figures, ApJ accepted. Replaced with accepted versio
Neutrino Emission as Diagnostics of Core-Collapse Supernovae
With myriads of detection events from a prospective Galactic core-collapse
supernova, current and future neutrino detectors will be able to sample
detailed, time-dependent neutrino fluxes and spectra. This offers enormous
possibilities for inferring supernova physics from the various phases of the
neutrino signal from the neutronization burst through the accretion and early
explosion phase to the cooling phase. The signal will constrain the time
evolution of bulk parameters of the young proto-neutron star like its mass and
radius as well as the structure of the progenitor, probe multi-dimensional
phenomena in the supernova core, and constrain thedynamics of the early
explosion phase. Aside from further astrophysical implications, supernova
neutrinos may also shed further light on the properties of matter at
supranuclear densities and on open problems in particle physics.Comment: 26 pages, 5 figures. Accepted for publication in Annual Review of
Nuclear and Particle Science, vol. 69. Non-copyedited version prepared by the
autho
Diffuse Supernova Neutrino Background from extensive core-collapse simulations of - progenitors
We revisit the diffuse supernova neutrino background in light of recent
systematic studies of stellar core collapse that reveal the quantitative
impacts of the progenitor conditions on the collapse process. In general, the
dependence of the progenitor on the core-collapse neutrino emission is not
monotonic in progenitor initial mass, but we show that it can, at first order,
be characterized by the core compactness. For the first time, we incorporate
the detailed variations in the neutrino emission over the entire mass range
-, based on (i) a long-term simulation of the core
collapse of a O-Ne-Mg core progenitor, (ii) over 100
simulations of iron core collapse to neutron stars, and (iii) half a dozen
simulations of core collapse to black holes (the "failed channel"). The
fraction of massive stars that undergo the failed channel remains uncertain,
but in view of recent simulations which reveal high compactness to be conducive
to collapse to black holes, we characterize the failed fraction by considering
a threshold compactness above which massive stars collapse to black holes and
below which the final remnant is a neutron star. We predict that future
detections of the diffuse supernova neutrino background may have the power to
reveal this threshold compactness, if its value is relatively small as
suggested by interpretations of several recent astronomical observations.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Selection of dune shapes and velocities. Part 1: Dynamics of sand, wind and barchans
Almost fifty years of investigations of barchan dunes morphology and dynamics
is reviewed, with emphasis on the physical understanding of these objects. The
characteristics measured on the field (shape, size, velocity) and the physical
problems they rise are presented. Then, we review the dynamical mechanisms
explaining the formation and the propagation of dunes. In particular a complete
and original approach of the sand transport over a flat sand bed is proposed
and discussed. We conclude on open problems by outlining future research
directions.Comment: submitted to Eur. Phys. J. B, 20 pages, 20 figure
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