8,727 research outputs found
Physical state representations and gauge fixing in string theory
We re-examine physical state representations in the covariant quantization of
bosonic string. We especially consider one parameter family of gauge fixing
conditions for the residual gauge symmetry due to null states (or BRST exact
states), and obtain explicit representations of observable Hilbert space which
include those of the DDF states. This analysis is aimed at giving a necessary
ingredient for the complete gauge fixing procedures of covariant string field
theory such as temporal or light-cone gauge.Comment: 16 page
Prompt GeV-TeV Emission of Gamma-Ray Bursts Due to High-Energy Protons, Muons and Electron-Positron Pairs
In the framework of the internal shock scenario, we model the broadband
prompt emission of gamma-ray bursts (GRBs) with emphasis on the GeV-TeV bands,
utilizing Monte Carlo simulations that include various processes associated
with electrons and protons accelerated to high energies. While inverse Compton
emission from primary electrons is often dominant, different proton-induced
mechanisms can also give rise to distinct high-energy components, such as
synchrotron emission from protons, muons or secondary electrons/positrons
injected via photomeson interactions. In some cases, they give rise to double
spectral breaks that can serve as unique signatures of ultra-high-energy
protons. We discuss the conditions favorable for such emission, and how they
are related to the production of ultra-high-energy cosmic rays and neutrinos in
internal shocks. Ongoing and upcoming observations by {\it GLAST}, atmospheric
Cerenkov telescopes and other facilities will test these expectations and
provide important information on the physical conditions in GRB outflows.Comment: 11 pages, 8 figures and 14 appendix figures, accepted for publication
in ApJ vol. 671 with minor revision
A critical analysis of self-supervision, or what we can learn from a single image
We look critically at popular self-supervision techniques for learning deep
convolutional neural networks without manual labels. We show that three
different and representative methods, BiGAN, RotNet and DeepCluster, can learn
the first few layers of a convolutional network from a single image as well as
using millions of images and manual labels, provided that strong data
augmentation is used. However, for deeper layers the gap with manual
supervision cannot be closed even if millions of unlabelled images are used for
training. We conclude that: (1) the weights of the early layers of deep
networks contain limited information about the statistics of natural images,
that (2) such low-level statistics can be learned through self-supervision just
as well as through strong supervision, and that (3) the low-level statistics
can be captured via synthetic transformations instead of using a large image
dataset.Comment: Accepted paper at the International Conference on Learning
Representations (ICLR) 202
Gamma Ray Bursts: recent results and connections to very high energy Cosmic Rays and Neutrinos
Gamma-ray bursts are the most concentrated explosions in the Universe. They
have been detected electromagnetically at energies up to tens of GeV, and it is
suspected that they could be active at least up to TeV energies. It is also
speculated that they could emit cosmic rays and neutrinos at energies reaching
up to the eV range. Here we review the recent developments in
the photon phenomenology in the light of \swift and \fermi satellite
observations, as well as recent IceCube upper limits on their neutrino
luminosity. We discuss some of the theoretical models developed to explain
these observations and their possible contribution to a very high energy cosmic
ray and neutrino background.Comment: 12 pages, 7 figures. Text of a plenary lecture at the PASCOS 12
conference, Merida, Yucatan, Mexico, June 2012; to appear in J.Phys. (Conf.
Series
Observability of surface Andreev bound states in a topological insulator in proximity to an s-wave superconductor
To guide experimental work on the search for Majorana zero-energy modes, we
calculate the superconducting pairing symmetry of a three-dimensional
topological insulator in combination with an s-wave superconductor. In analogy
to the case of nanowires with strong spin-orbit coupling we show how the
pairing symmetry changes across different topological regimes. We demonstrate
that a dominant p-wave pairing relation is not sufficient to realize a Majorana
zero-energy mode useful for quantum computation. Our main result of this paper
is the relation between odd-frequency pairing and Majorana zero energy modes by
using Green functions techniques in three-dimensional topological insulators in
the so-called Majorana regime. We discuss thereafter how the pairing relations
in the different regimes can be observed in the shape of the tunneling
conductance of an s-wave proximized three-dimensional topological insulator. We
will discuss the necessity to incorporate a ferromagnetic insulator to localize
the zero-energy bound state to the interface as a Majorana mode.Comment: Accepted for publication in Journal of Physics: Condensed Matte
Level Truncated Tachyon Potential in Various Gauges
New gauge fixing condition with single gauge parameter proposed by the
authors is applied to the level truncated analysis of tachyon condensation in
cubic open string field theory. It is found that the only one real non-trivial
extremum persists to appear in the well-defined region of the gauge parameter,
while the other solutions are turned out to be gauge-artifacts. Contrary to the
previously known pathology in the Feynman-Siegel gauge, tachyon potential is
remarkably smooth enough around Landau-type gauge.Comment: 13 pages, 5 figures. For associated movie files, see
http://hep1.c.u-tokyo.ac.jp/~kato/sft
Scattering by randomly oriented ellipsoids: Application to aerosol and cloud problems
A program was developed for computing the scattering and absorption by arbitrarily oriented and randomly oriented prolate and oblate spheroids. This permits examination of the effect of particle shape for cases ranging from needles through spheres to platelets. Applications of this capability to aerosol and cloud problems are discussed. Initial results suggest that the effect of nonspherical particle shape on transfer of radiation through aerosol layers and cirrus clouds, as required for many climate studies, can be readily accounted for by defining an appropriate effective spherical particle radius
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