180 research outputs found
The orbit structure of Dynkin curves
Let G be a simple algebraic group over an algebraically closed field k;
assume that Char k is zero or good for G. Let \cB be the variety of Borel
subgroups of G and let e in Lie G be nilpotent. There is a natural action of
the centralizer C_G(e) of e in G on the Springer fibre \cB_e = {B' in \cB | e
in Lie B'} associated to e. In this paper we consider the case, where e lies in
the subregular nilpotent orbit; in this case \cB_e is a Dynkin curve. We give a
complete description of the C_G(e)-orbits in \cB_e. In particular, we classify
the irreducible components of \cB_e on which C_G(e) acts with finitely many
orbits. In an application we obtain a classification of all subregular orbital
varieties admitting a finite number of B-orbits for B a fixed Borel subgroup of
G.Comment: 12 pages, to appear in Math
Diagrammatic self-energy approximations and the total particle number
There is increasing interest in many-body perturbation theory as a practical tool for the calculation of ground-state properties. As a consequence, unambiguous sum rules such as the conservation of particle number under the influence of the Coulomb interaction have acquired an importance that did not exist for calculations of excited-state properties. In this paper we obtain a rigorous, simple relation whose fulfilment guarantees particle-number conservation in a given diagrammatic self-energy approximation. Hedin's G(0)W(0) approximation does not satisfy this relation and hence violates the particle-number sum rule. Very precise calculations for the homogeneous electron gas and a model inhomogeneous electron system allow the extent of the nonconservation to be estimated
Search for Global Dipole Enhancements in the HiRes-I Monocular Data above 10^{18.5} eV
Several proposed source models for Ultra-High Energy Cosmic Rays (UHECRs)
consist of dipole distributions oriented towards major astrophysical landmarks
such as the galactic center, M87, or Centaurus A. We use a comparison between
real data and simulated data to show that the HiRes-I monocular data for
energies above 10^{18.5} eV is, in fact, consistent with an isotropic source
model. We then explore methods to quantify our sensitivity to dipole source
models oriented towards the Galactic Center, M87, and Centaurus A.Comment: 17 pages, 31 figure
Observation of the Ankle and Evidence for a High-Energy Break in the Cosmic Ray Spectrum
We have measured the cosmic ray spectrum at energies above eV using
the two air fluorescence detectors of the High Resolution Fly's Eye experiment
operating in monocular mode. We describe the detector, PMT and atmospheric
calibrations, and the analysis techniques for the two detectors. We fit the
spectrum to models describing galactic and extragalactic sources. Our measured
spectrum gives an observation of a feature known as the ``ankle'' near eV, and strong evidence for a suppression near eV.Comment: 14 pages, 9 figures. To appear in Physics Letters B. Accepted versio
A Measurement of Time-Averaged Aerosol Optical Depth using Air-Showers Observed in Stereo by HiRes
Air fluorescence measurements of cosmic ray energy must be corrected for
attenuation of the atmosphere. In this paper we show that the air-showers
themselves can yield a measurement of the aerosol attenuation in terms of
optical depth, time-averaged over extended periods. Although the technique
lacks statistical power to make the critical hourly measurements that only
specialized active instruments can achieve, we note the technique does not
depend on absolute calibration of the detector hardware, and requires no
additional equipment beyond the fluorescence detectors that observe the air
showers. This paper describes the technique, and presents results based on
analysis of 1258 air-showers observed in stereo by the High Resolution Fly's
Eye over a four year span.Comment: 7 pages, 3 figures, accepted for publication by Astroparticle Physics
Journa
Air fluorescence measurements in the spectral range 300-420 nm using a 28.5 GeV electron beam
Measurements are reported of the yield and spectrum of fluorescence, excited
by a 28.5 GeV electron beam, in air at a range of pressures of interest to
ultra-high energy cosmic ray detectors. The wavelength range was 300 - 420 nm.
System calibration has been performed using Rayleigh scattering of a nitrogen
laser beam. In atmospheric pressure dry air at 304 K the yield is 20.8 +/- 1.6
photons per MeV.Comment: 29 pages, 10 figures. Submitted to Astroparticle Physic
Measurement of Pressure Dependent Fluorescence Yield of Air: Calibration Factor for UHECR Detectors
In a test experiment at the Final Focus Test Beam of the Stanford Linear
Accelerator Center, the fluorescence yield of 28.5 GeV electrons in air and
nitrogen was measured. The measured photon yields between 300 and 400 nm at 1
atm and 29 deg C are Y(760 Torr, air) = 4.42 +/- 0.73 and Y(760 Torr, nitrogen)
= 29.2 +/- 4.8 photons per electron per meter. Assuming that the fluorescence
yield is proportional to the energy deposition of a charged particle traveling
through air, good agreement with measurements at lower particle energies is
observed.Comment: 22 pages, 14 figures, 2 tables, submitted to Astroparticle Physic
A Likelihood Method for Measuring the Ultrahigh Energy Cosmic Ray Composition
Air fluorescence detectors traditionally determine the dominant chemical
composit ion of the ultrahigh energy cosmic ray flux by comparing the averaged
slant depth of the shower maximum, , as a function of energy to the
slant depths expect ed for various hypothesized primaries. In this paper, we
present a method to make a direct measurement of the expected mean number of
protons and iron by comparing the shap es of the expected
distributions to the distribution for data. The advantages of this method
includes the use of information of the full distribution and its ability to
calculate a flux for various cosmic ray compositi ons. The same method can be
expanded to marginalize uncertainties due to choice of spectra, hadronic models
and atmospheric parameters. We demonstrate the technique with independent
simulated data samples from a parent sample of protons and iron. We accurately
predict the number of protons and iron in the parent sample and show that the
uncertainties are meaningful.Comment: 11 figures, 22 pages, accepted by Astroparticle Physic
Alternative Methods to Finding Patterns in HiRes Stereo Data
In this paper Ultra High Energy Cosmic Rays UHECRs data observed by the HiRes
fluorescence detector in stereo mode is analyzed to search for events in the
sky with an arrival direction lying on a great circle. Such structure is known
as the arc structure. The arc structure is expected when the charged cosmic
rays pass through the galactic magnetic field. The arcs searched for could
represent a broad or a small scale anisotropy depending on the proposed source
model for the UHECRs. The Arcs in this paper are looked for using Hough
transform were Hough transform is a technique used to looking for patterns in
images. No statistically significant arcs were found in this study
Milagro limits and HAWC sensitivity for the rate-density of evaporating Primordial Black Holes
postprin
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