3,425 research outputs found
Convex Rank Tests and Semigraphoids
Convex rank tests are partitions of the symmetric group which have desirable
geometric properties. The statistical tests defined by such partitions involve
counting all permutations in the equivalence classes. Each class consists of
the linear extensions of a partially ordered set specified by data. Our methods
refine existing rank tests of non-parametric statistics, such as the sign test
and the runs test, and are useful for exploratory analysis of ordinal data. We
establish a bijection between convex rank tests and probabilistic conditional
independence structures known as semigraphoids. The subclass of submodular rank
tests is derived from faces of the cone of submodular functions, or from
Minkowski summands of the permutohedron. We enumerate all small instances of
such rank tests. Of particular interest are graphical tests, which correspond
to both graphical models and to graph associahedra
Toward Realistic Intersecting D-Brane Models
We provide a pedagogical introduction to a recently studied class of
phenomenologically interesting string models, known as Intersecting D-Brane
Models. The gauge fields of the Standard-Model are localized on D-branes
wrapping certain compact cycles on an underlying geometry, whose intersections
can give rise to chiral fermions. We address the basic issues and also provide
an overview of the recent activity in this field. This article is intended to
serve non-experts with explanations of the fundamental aspects, and also to
provide some orientation for both experts and non-experts in this active field
of string phenomenology.Comment: 85 pages, 8 figures, Latex, Bibtex, v2: refs added, typos correcte
Non-Minimal and Non-Universal Supersymmetry
I motivate and discuss non-minimal and non-universal models of supersymmetry
and supergravity consistent with string unification at GeV.Comment: 10 pages, Latex. Plenary talk given at 6th Workshop in High Energy
Physics Phenomenology (WHEPP 6), Chennai (Madras), India, 3-15 Jan 200
3-D Model of Broadband Emission from Supernova Remnants Undergoing Non-linear Diffusive Shock Acceleration
We present a 3-dimensional model of supernova remnants (SNRs) where the
hydrodynamical evolution of the remnant is modeled consistently with nonlinear
diffusive shock acceleration occuring at the outer blast wave. The model
includes particle escape and diffusion outside of the forward shock, and
particle interactions with arbitrary distributions of external ambient
material, such as molecular clouds. We include synchrotron emission and
cooling, bremsstrahlung radiation, neutral pion production, inverse-Compton
(IC), and Coulomb energy-loss. Boardband spectra have been calculated for
typical parameters including dense regions of gas external to a 1000 year old
SNR. In this paper, we describe the details of our model but do not attempt a
detailed fit to any specific remnant. We also do not include magnetic field
amplification (MFA), even though this effect may be important in some young
remnants. In this first presentation of the model we don't attempt a detailed
fit to any specific remnant. Our aim is to develop a flexible platform, which
can be generalized to include effects such as MFA, and which can be easily
adapted to various SNR environments, including Type Ia SNRs, which explode in a
constant density medium, and Type II SNRs, which explode in a pre-supernova
wind. When applied to a specific SNR, our model will predict cosmic-ray spectra
and multi-wavelength morphology in projected images for instruments with
varying spatial and spectral resolutions. We show examples of these spectra and
images and emphasize the importance of measurements in the hard X-ray, GeV, and
TeV gamma-ray bands for investigating key ingredients in the acceleration
mechanism, and for deducing whether or not TeV emission is produced by IC from
electrons or neutral pions from protons.Comment: 12 pages, 9 figures, accepted by Apj, 24 June 200
Top Quarks as a Window to String Resonances
We study the discovery potential of string resonances decaying to
final state at the LHC. We point out that top quark pair production is a
promising and an advantageous channel for studying such resonances, due to
their low Standard Model background and unique kinematics. We study the
invariant mass distribution and angular dependence of the top pair production
cross section via exchanges of string resonances. The mass ratios of these
resonances and the unusual angular distribution may help identify their
fundamental properties and distinguish them from other new physics. We find
that string resonances for a string scale below 4 TeV can be detected via the
channel, either from reconstructing the semi-leptonic
decay or recent techniques in identifying highly boosted tops.Comment: 22 pages, 6 figure
Oblique Parameter Constraints on Large Extra Dimensions
We consider the Kaluza-Klein scenario in which gravity propagates in the
dimensional bulk of spacetime and the Standard Model particles are
confined to a 3-brane. We calculate the gauge boson self-energy corrections
arising from the exchange of virtual gravitons and present our results in the
-formalism. We find that the new physics contributions to , and
decouple in the limit that the string scale goes to infinity. The oblique
parameters constrain the lower limit on . Taking the quantum gravity
cutoff to be ,
-parameter constraints impose TeV for at the 1
level. -parameter constraints impose TeV for .Comment: Version to appear in PR
Possible Origin of Fermion Chirality and Gut Structure From Extra Dimensions
The fundamental chiral nature of the observed quarks and leptons and the
emergence of the gauge group itself are most puzzling aspects of the standard
model. Starting from general considerations of topological properties of gauge
field configurations in higher space-time dimensions, it is shown that the
existence of non-trivial structures in ten dimensions would determine a class
of models corresponding to a grand unified GUT structure with complex fermion
representations with respect to . The
discussion is carried out within the framework of string theories with
characteristic energy scales below the Planck mass. Avoidance of topological
obstructions upon continuous deformation of field configurations leads to
global chiral symmetry breaking of the underlying fundamental theory, imposes
rigorous restrictions on the structure of the vacuum and space-time itself and
determines uniquely the gauge structure and matter content.Comment: final version to appear in Phys. Rev.
Calibration and Irradiation Study of the BGO Background Monitor for the BEAST II Experiment
Beam commissioning of the SuperKEKB collider began in 2016. The Beam Exorcism
for A STable experiment II (BEAST II) project is particularly designed to
measure the beam backgrounds around the interaction point of the SuperKEKB
collider for the Belle II experiment. We develop a system using bismuth
germanium oxide (BGO) crystals with optical fibers connecting to a multianode
photomultiplier tube (MAPMT) and a field-programmable gate array (FPGA)
embedded readout board for monitoring the real-time beam backgrounds in BEAST
II. The overall radiation sensitivity of this system is estimated to be
Gy/ADU (analog-to-digital unit) with the standard
10 m fibers for transmission and the MAPMT operating at 700 V. Our -ray
irradiation study of the BGO system shows that the exposure of BGO crystals to
Co -ray doses of 1 krad has led to immediate light output
reductions of 25--40%, and the light outputs further drop by 30--45% after the
crystals receive doses of 2--4 krad. Our findings agree with those of the
previous studies on the radiation hard (RH) BGO crystals grown by the low
thermal gradient Czochralski (LTG Cz) technology. The absolute dose from the
BGO system is also consistent with the simulation, and is estimated to be about
1.18 times the equivalent dose. These results prove that the BGO system is able
to monitor the background dose rate in real time under extreme high radiation
conditions. This study concludes that the BGO system is reliable for the beam
background study in BEAST II
Inhomogeneous Fragmentation of the Rolling Tachyon
Dirac-Born-Infeld type effective actions reproduce many aspects of string
theory classical tachyon dynamics of unstable Dp-branes. The inhomogeneous
tachyon field rolling from the top of its potential forms topological defects
of lower codimensions. In between them, as we show, the tachyon energy density
fragments into a p-dimensional web-like high density network evolving with
time. We present an analytic asymptotic series solution of the non-linear
equations for the inhomogeneous tachyon and its stress energy. The generic
solution for a tachyon field with a runaway potential in arbitrary dimensions
is described by the free streaming of noninteracting massive particles whose
initial velocities are defined by the gradients of the initial tachyon profile.
Thus, relativistic particle mechanics is a dual picture of the tachyon field
effective action. Implications of this picture for inflationary models with a
decaying tachyon field are discussed.Comment: 10 pages, 1 figur
- âŠ