159 research outputs found
Piecewise Euclidean structures and Eberlein's Rigidity Theorem in the singular case
In this article, we generalize Eberlein's Rigidity Theorem to the singular
case, namely, one of the spaces is only assumed to be a CAT(0) topological
manifold. As a corollary, we get that any compact irreducible but locally
reducible locally symmetric space of noncompact type does not admit a
nonpositively curved (in the Aleksandrov sense) piecewise Euclidean structure.
Any hyperbolic manifold, on the other hand, does admit such a structure.Comment: 28 pages. Published copy, also available at
http://www.maths.warwick.ac.uk/gt/GTVol3/paper13.abs.htm
Trialogue on the number of fundamental constants
This paper consists of three separate articles on the number of fundamental
dimensionful constants in physics. We started our debate in summer 1992 on the
terrace of the famous CERN cafeteria. In the summer of 2001 we returned to the
subject to find that our views still diverged and decided to explain our
current positions. LBO develops the traditional approach with three constants,
GV argues in favor of at most two (within superstring theory), while MJD
advocates zero.Comment: Version appearing in JHEP; 31 pages late
Microwave cavity hidden sector photon threshold crossing
Hidden sector photons are a weakly interacting slim particle arising from an
additional U(1) gauge symmetry predicted by many standard model extensions. We
present and demonstrate a new experimental method using a single microwave
cavity to search for hidden sector photons. Only photons with a great enough
energy are able to oscillate into hidden sector photons of a particular mass.
If our cavity is driven on resonance and tuned over the corresponding threshold
frequency, there is an observable drop in the circulating power signifying the
creation of hidden sector photons. This approach avoids the problems of
microwave leakage and frequency matching inherent in photon regeneration
techniques
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Lessons Learned from Open-label Deep Brain Stimulation for Tourette Syndrome: Eight Cases over 7 Years
Background: Deep brain stimulation (DBS) remains an experimental but promising treatment for patients with severe refractory Gilles de la Tourette syndrome (TS). Controversial issues include the selection of patients (age and clinical presentation), the choice of brain targets to obtain optimal patient-specific outcomes, and the risk of surgery- and stimulation-related serious adverse events. Methods: This report describes our open-label experience with eight patients with severe refractory malignant TS treated with DBS. The electrodes were placed in the midline thalamic nuclei or globus pallidus, pars internus, or both. Tics were clinically assessed in all patients pre- and postoperatively using the Modified Rush Video Protocol and the Yale Global Tic Severity Scale (YGTSS). Results: Although three patients had marked postoperative improvement in their tics (>50% improvement on the YGTSS), the majority did not reach this level of clinical improvement. Two patients had to have their DBS leads removed (one because of postoperative infection and another because of lack of benefit). Discussion Our clinical experience supports the urgent need for more data and refinements in interventions and outcome measurements for severe, malignant, and medication-refractory TS. Because TS is not an etiologically homogenous clinical entity, the inclusion criteria for DBS patients and the choice of brain targets will require more refinement
Photon and Graviton Mass Limits
Efforts to place limits on deviations from canonical formulations of
electromagnetism and gravity have probed length scales increasing dramatically
over time.Historically, these studies have passed through three stages: (1)
Testing the power in the inverse-square laws of Newton and Coulomb, (2) Seeking
a nonzero value for the rest mass of photon or graviton, (3) Considering more
degrees of freedom, allowing mass while preserving explicit gauge or
general-coordinate invariance. Since our previous review the lower limit on the
photon Compton wavelength has improved by four orders of magnitude, to about
one astronomical unit, and rapid current progress in astronomy makes further
advance likely. For gravity there have been vigorous debates about even the
concept of graviton rest mass. Meanwhile there are striking observations of
astronomical motions that do not fit Einstein gravity with visible sources.
"Cold dark matter" (slow, invisible classical particles) fits well at large
scales. "Modified Newtonian dynamics" provides the best phenomenology at
galactic scales. Satisfying this phenomenology is a requirement if dark matter,
perhaps as invisible classical fields, could be correct here too. "Dark energy"
{\it might} be explained by a graviton-mass-like effect, with associated
Compton wavelength comparable to the radius of the visible universe. We
summarize significant mass limits in a table.Comment: 42 pages Revtex4. This version contains corrections and changes
contained in the published version, Rev. Mod. Phys. 82, 939-979 (2010), with
a few addition
Effects of STN and GPi Deep Brain Stimulation on Impulse Control Disorders and Dopamine Dysregulation Syndrome
Impulse control disorders (ICDs) and dopamine dysregulation syndrome (DDS) are important behavioral problems that affect a subpopulation of patients with Parkinson's disease (PD) and typically result in markedly diminished quality of life for patients and their caregivers. We aimed to investigate the effects of subthalamic nucleus (STN) and internal globus pallidus (GPi) deep brain stimulation (DBS) on ICD/DDS frequency and dopaminergic medication usage.A retrospective chart review was performed on 159 individuals who underwent unilateral or bilateral PD DBS surgery in either STN or GPi. According to published criteria, pre- and post-operative records were reviewed to categorize patients both pre- and post-operatively as having ICD, DDS, both ICD and DDS, or neither ICD nor DDS. Group differences in patient demographics, clinical presentations, levodopa equivalent dose (LED), and change in diagnosis following unilateral/bilateral by brain target (STN or GPi DBS placement) were examined.28 patients met diagnostic criteria for ICD or DDS pre- or post-operatively. ICD or DDS classification did not differ by GPi or STN target stimulation. There was no change in DDS diagnosis after unilateral or bilateral stimulation. For ICD, diagnosis resolved in 2 of 7 individuals after unilateral or bilateral DBS. Post-operative development of these syndromes was significant; 17 patients developed ICD diagnoses post-operatively with 2 patients with pre-operative ICD developing DDS post-operatively.Unilateral or bilateral DBS did not significantly treat DDS or ICD in our sample, even though a few cases of ICD resolved post-operatively. Rather, our study provides preliminary evidence that DDS and ICD diagnoses may emerge following DBS surgery
Resonant Regeneration in the Sub-Quantum Regime -- A demonstration of fractional quantum interference
Light shining through wall experiments (in the optical as well as in the
microwave regime) are a powerful tool to search for light particles coupled
very weakly to photons such as axions or extra hidden sector photons. Resonant
regeneration, where a resonant cavity is employed to enhance the regeneration
rate of photons, is one of the most promising techniques to improve the
sensitivity of the next generation of experiments. However, doubts have been
voiced if such methods work at very low regeneration rates where on average the
cavity contains less than one photon. In this note we report on a demonstration
experiment using a microwave cavity driven with extremely low power, to show
that resonant amplification works also in this regime. In accordance with
standard quantum mechanics this is a demonstration that interference also works
at the level of less than one quantum. As an additional benefit this experiment
shows that thermal photons inside the cavity cause no adverse effects.Comment: 14 pages, 5 figure
New hadrons as ultra-high energy cosmic rays
Ultra-high energy cosmic ray (UHECR) protons produced by uniformly
distributed astrophysical sources contradict the energy spectrum measured by
both the AGASA and HiRes experiments, assuming the small scale clustering of
UHECR observed by AGASA is caused by point-like sources. In that case, the
small number of sources leads to a sharp exponential cutoff at the energy
E<10^{20} eV in the UHECR spectrum. New hadrons with mass 1.5-3 GeV can solve
this cutoff problem. For the first time we discuss the production of such
hadrons in proton collisions with infrared/optical photons in astrophysical
sources. This production mechanism, in contrast to proton-proton collisions,
requires the acceleration of protons only to energies E<10^{21} eV. The diffuse
gamma-ray and neutrino fluxes in this model obey all existing experimental
limits. We predict large UHE neutrino fluxes well above the sensitivity of the
next generation of high-energy neutrino experiments. As an example we study
hadrons containing a light bottom squark. These models can be tested by
accelerator experiments, UHECR observatories and neutrino telescopes.Comment: 17 pages, revtex style; v2: shortened, as to appear in PR
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