5,808 research outputs found
Spin-Statistics Violations in Superstring Theory
I describe how superstring theory may violate spin-statistics in an
experimentally observable manner. Reviewing the basics of superstring
interactions and how to utilize these to produce a statistical phase, I then
apply these ideas to two specific examples. The first is the case of heterotic
worldsheet linkings, whereby one small closed string momentarily enlarges
sufficiently to pass over another, producing such a statistical phase. The
second is the braneworld model with noncommutative geometry, whereby matter
composed of open strings may couple to a background in which spacetime
coordinates do not commute, modifying the field (anti)commutator algebra. I
conclude with ways to sharpen and experimentally test these exciting avenues to
possibly verify superstring theory.Comment: 18 pages, 3 figures; v2: references added and typos correcte
The Starburst Nature of Lyman-Break Galaxies: Testing UV Extinction with X-rays
We derive the bolometric to X-ray correlation for a local sample of normal
and starburst galaxies and use it, in combination with several UV reddening
schemes, to predict the 2--8 keV X-ray luminosity for a sample of 24
Lyman-break galaxies in the HDF/CDF-N. We find that the mean X-ray luminosity,
as predicted from the Meurer UV reddening relation for starburst galaxies,
agrees extremely well with the Brandt stacking analysis. This provides
additional evidence that Lyman-break galaxies can be considered as scaled-up
local starbursts and that the locally derived starburst UV reddening relation
may be a reasonable tool for estimating the UV extinction at high redshift. Our
analysis shows that the Lyman-break sample can not have far-IR to far-UV flux
ratios similar to nearby ULIGs, as this would predict a mean X-ray luminosity
100 times larger than observed, as well as far-IR luminosities large enough to
be detected in the sub-mm. We calculate the UV reddening expected from the
Calzetti effective starburst attenuation curve and the radiative transfer
models of Witt & Gordon for low metallicity dust in a shell geometry with
homogeneous or clumpy dust distributions and find that all are consistent with
the observed X-ray emission. Finally, we show that the mean X-ray luminosity of
the sample would be under predicted by a factor of 6 if the the far-UV is
unattenuated by dust.Comment: 7 pages, 3 figures. Accepted for publication in A
Large Radius Hagedorn Regime in String Gas Cosmology
We calculate the equation of state of a gas of strings at high density in a
large toroidal universe, and use it to determine the cosmological evolution of
background metric and dilaton fields in the entire large radius Hagedorn
regime, (ln S)^{1/d} << R << S^{1/d} (with S the total entropy). The pressure
in this regime is not vanishing but of O(1), while the equation of state is
proportional to volume, which makes our solutions significantly different from
previously published approximate solutions. For example, we are able to
calculate the duration of the high-density "Hagedorn" phase, which increases
exponentially with increasing entropy, S. We go on to discuss the difficulties
of the scenario, quantifying the problems of establishing thermal equilibrium
and producing a large but not too weakly-coupled universe.Comment: 12 pages, 4 figures, more details presented in string thermodynamics
section, to be published in Physical Review
Cluster, Classify, Regress: A General Method For Learning Discountinous Functions
This paper presents a method for solving the supervised learning problem in
which the output is highly nonlinear and discontinuous. It is proposed to solve
this problem in three stages: (i) cluster the pairs of input-output data
points, resulting in a label for each point; (ii) classify the data, where the
corresponding label is the output; and finally (iii) perform one separate
regression for each class, where the training data corresponds to the subset of
the original input-output pairs which have that label according to the
classifier. It has not yet been proposed to combine these 3 fundamental
building blocks of machine learning in this simple and powerful fashion. This
can be viewed as a form of deep learning, where any of the intermediate layers
can itself be deep. The utility and robustness of the methodology is
illustrated on some toy problems, including one example problem arising from
simulation of plasma fusion in a tokamak.Comment: 12 files,6 figure
STR-924: EFFECT OF LOW TEMPERATURE ON THE SHEAR-FATIGUE PERFORMANCE OF REINFORCED CONCRETE BEAMS
This paper investigates the fatigue behaviour of a reinforced concrete beam (without shear reinforcement) at low temperature (-20°C) compared to a similar beam tested near room temperature (+16°C). Two large-scale steel reinforced beams (200 mm x 400 mm x 4200 mm) were fabricated and tested. The beams had temperature differentials over their depth to simulate solar radiation and in-service temperature of the bridges. The beams were cyclically loaded to failure with a stress range representing the ratio of live to dead loads found in most bridges. This study showed that low temperature increased the fatigue life of the reinforced concrete beam by 51%, and changed the mode of failure of the beam from shear fatigue failure at room temperature to flexural fatigue failure at low temperature. It was observed that the low and room temperature beams maintained 65% and 31% of their original flexural rigidity indicating that low temperature mitigated the stiffness degradation of the reinforced concrete caused by fatigue loading. The strains in the tensile reinforcement of the low temperature beam were much lower than the room temperature beam which could be due to the higher strength of concrete and reduction in concrete softening at low temperature which resulted in lower stresses in the tensile reinforcement. The findings of this study show that the contribution of concrete under either static or cyclic load becomes much higher at low temperature
Covariant Closed String Coherent States
We give the first construction of covariant coherent closed string states,
which may be identified with fundamental cosmic strings. We outline the
requirements for a string state to describe a cosmic string, and using DDF
operators provide an explicit and simple map that relates three different
descriptions: classical strings, lightcone gauge quantum states and covariant
vertex operators. The naive construction leads to covariant vertex operators
whose existence requires a lightlike compactification of spacetime. When the
lightlike compactified states in the underlying Hilbert space are projected out
the resulting coherent states have a classical interpretation and are in
one-to-one correspondence with arbitrary classical closed string loops.Comment: 4 page
Inverted orbital polarization in strained correlated oxide films
Manipulating the orbital occupation of valence electrons via epitaxial strain
in an effort to induce new functional properties requires considerations of how
changes in the local bonding environment affect the band structure at the Fermi
level. Using synchrotron radiation to measure the x-ray linear dichroism of
epitaxially strained films of the correlated oxide CaFeO3, we demonstrate that
the orbital polarization of the Fe valence electrons is opposite from
conventional understanding. Although the energetic ordering of the Fe 3d
orbitals is confirmed by multiplet ligand field theory analysis to be
consistent with previously reported strain-induced behavior, we find that the
nominally higher energy orbital is more populated than the lower. We ascribe
this inverted orbital polarization to an anisotropic bandwidth response to
strain in a compound with nearly filled bands. These findings provide an
important counterexample to the traditional understanding of strain-induced
orbital polarization and reveal a new method to engineer otherwise unachievable
orbital occupations in correlated oxides
Epidural Analgesia Decreases Narcotic Requirements in Low Level Spina Bifida Patients Undergoing Urologic Laparotomy for Neurogenic Bladder and Bowel
Purpose
Concern of anatomical anomalies and worsening neurologic symptoms has prevented widespread use of epidural catheters in patients with low level spina bifida (LLSB). We hypothesize that thoracic epidural placement in the T9-T10 interspace is safe and decreases narcotic requirements in LLSB patients following major open lower urinary tract reconstruction (LUTR).
Materials and Methods
We reviewed consecutive LLSB patients who had LUTR and epidurals for post-operative pain control. Controls were LLSB patients who received single shot transversus abdominis plane (TAP) blocks with similar procedures. Complications from epidural placement, including changes in motor and sensory status were recorded. Opioid consumption was calculated utilizing equivalent IV morphine doses. Mean and maximum pain scores on post-operative day (POD) 0-3 were calculated.
Results
10 LLSB patients who had lower urinary tract reconstruction and epidurals were matched to 10 LLSB patients who had lower urinary tract reconstruction and transverse abdominis plane blocks. Groups were demographically similar. All had full abdominal sensation and functional levels at or below L3. No epidural complications or changes in neurological status were noted. The epidural group had decreased opioid consumption on POD 0-3 (0.75 mg/kg vs. 1.29 mg/kg, p=0.04). Pain scores were similar or improved in the epidural group.
Conclusions
Thoracic epidural analgesia appears to be a safe and effective opioid sparing option to assist with post-operative pain management following lower urinary tract reconstruction in LLSB patients
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