142 research outputs found
Investigation of a Toy Model for Frustration in Abelian Lattice Gauge Theory
We introduce a lattice model with local U(1) gauge symmetry which
incorporates explicit frustration in d >2. The form of the action is inspired
from the loop expansion of the fermionic determinant in standard lattice QED.
We study through numerical simulations the phase diagram of the model,
revealing the existence of a frustrated (antiferromagnetic) phase for d=3 and
d=4, once an appropriate order parameter is identified.Comment: 7 pages, 7 figure
CTA and cosmic-ray diffusion in molecular clouds
Molecular clouds act as primary targets for cosmic-ray interactions and are
expected to shine in gamma-rays as a by-product of these interactions. Indeed
several detected gamma-ray sources both in HE and VHE gamma-rays (HE: 100 MeV <
E 100 GeV) have been directly or indirectly associated with
molecular clouds. Information on the local diffusion coefficient and the local
cosmic-ray population can be deduced from the observed gamma-ray signals. In
this work we concentrate on the capability of the forthcoming Cherenkov
Telescope Array Observatory (CTA) to provide such measurements. We investigate
the expected emission from clouds hosting an accelerator, exploring the
parameter space for different modes of acceleration, age of the source, cloud
density profile, and cosmic ray diffusion coefficient. We present some of the
most interesting cases for CTA regarding this science topic. The simulated
gamma-ray fluxes depend strongly on the input parameters. In some cases, from
CTA data it will be possible to constrain both the properties of the
accelerator and the propagation mode of cosmic rays in the cloud.Comment: In Proceedings of the 2012 Heidelberg Symposium on High Energy
Gamma-Ray Astronomy. All CTA contributions at arXiv:1211.184
Simulation Study of TenTen: A new Multi-TeV IACT array
TenTen is a proposed array of Imaging Atmospheric Cherenkov Telescopes (IACT)
optimized for the gamma ray energy regime of 10 TeV to 100 TeV, but with a
threshold of ~1 to a few TeV. It will offer a collecting area of 10 km2 above
energies of 10 TeV. In the initial phase, a cell of 3 to 5 modest-sized
telescopes, each with 10-30 m2 mirror area, is suggested for an Australian
site. A possible expansion of the array could comprise many such cells. Here we
present work on configuration and technical issues from our simulation studies
of the array. Working topics include array layout, telescope size and optics,
camera field of view, telescope trigger system, electronics, and site surveys.Comment: 4 pages, 7 figures, submitted to Proceedings of the ICRC 2007, pdf
forma
TenTen: A New Array of Multi-TeV Imaging Cherenkov Telescopes
The exciting results from H.E.S.S. point to a new population of gamma-ray
sources at energies E > 10 TeV, paving the way for future studies and new
discoveries in the multi-TeV energy range. Connected with these energies is the
search for sources of PeV cosmic-rays (CRs) and the study of multi-TeV
gamma-ray production in a growing number of astrophysical environments. TenTen
is a proposed stereoscopic array (with a suggested site in Australia) of
modest-sized (10 to 30m^2) Cherenkov imaging telescopes with a wide field of
view (8 to 10deg diameter) optimised for the E~10 to 100 TeV range. TenTen will
achieve an effective area of ~10 km^2 at energies above 10 TeV. We outline here
the motivation for TenTen and summarise key performance parameters.Comment: 4 pages, 2 figures, proceedings of the 30th ICRC, Merida, Mexico,
200
The Complex Langevin method: When can it be trusted?
We analyze to what extent the complex Langevin method, which is in principle
capable of solving the so-called sign problems, can be considered as reliable.
We give a formal derivation of the correctness and then point out various
mathematical loopholes. The detailed study of some simple examples leads to
practical suggestions about the application of the method.Comment: 14 pages, including several eps figures and tables; clarification and
minor corrections added, to appear in PR
Timing analysis techniques at large core distances for multi-TeV gamma ray astronomy
We present an analysis technique that uses the timing information of
Cherenkov images from extensive air showers (EAS). Our emphasis is on distant,
or large core distance gamma-ray induced showers at multi-TeV energies.
Specifically, combining pixel timing information with an improved direction
reconstruction algorithm, leads to improvements in angular and core resolution
as large as ~40% and ~30%, respectively, when compared with the same algorithm
without the use of timing. Above 10 TeV, this results in an angular resolution
approaching 0.05 degrees, together with a core resolution better than ~15 m.
The off-axis post-cut gamma-ray acceptance is energy dependent and its full
width at half maximum ranges from 4 degrees to 8 degrees. For shower directions
that are up to ~6 degrees off-axis, the angular resolution achieved by using
timing information is comparable, around 100 TeV, to the on-axis angular
resolution. The telescope specifications and layout we describe here are geared
towards energies above 10 TeV. However, the methods can in principle be applied
to other energies, given suitable telescope parameters. The 5-telescope cell
investigated in this study could initially pave the way for a larger array of
sparsely spaced telescopes in an effort to push the collection area to >10 km2.
These results highlight the potential of a `sparse array' approach in
effectively opening up the energy range above 10 TeV.Comment: Published in Astroparticle Physic
Gamma-ray emission from star-forming complexes observed by MAGIC: The cases of W51 and HESS J1857+026
Massive star-forming regions assemble a large number of young stars with remnants of stellar evolution and a very dense environment. Therefore, particles accelerated in supernova remnants and pulsar wind nebulae encounter optimal conditions for interacting with target material and photon fields, and thus produce gamma-ray emission. However, observations are challenging because multiple phenomena may appear entangled within the resolution of current gamma-ray telescopes. We report on MAGIC observations aimed to understand the nature of the emission from the star-forming region W51 and the unidentified source HESS J1857+026. While gamma-ray emission from W51 is dominated by the interaction of the supernova remnant W51C with dense molecular clouds, HESS J1857+026 is associated to the pulsar wind nebula from PSR J1856+0245. However, an additional source is resolved north of HESSJ1857+026, with sufficient separation to determine that it cannot be powered by the same pulsar. We search for multiwavelength data to determine the origin of the new source
The Long Journey from Ab Initio Calculations to Density Functional Theory for Nuclear Large Amplitude Collective Motion
At present there are two vastly different ab initio approaches to the
description of the the many-body dynamics: the Density Functional Theory (DFT)
and the functional integral (path integral) approaches. On one hand, if
implemented exactly, the DFT approach can allow in principle the exact
evaluation of arbitrary one-body observable. However, when applied to Large
Amplitude Collective Motion (LACM) this approach needs to be extended in order
to accommodate the phenomenon of surface-hoping, when adiabaticity is strongly
violated and the description of a system using a single (generalized) Slater
determinant is not valid anymore. The functional integral approach on the other
hand does not appear to have such restrictions, but its implementation does not
appear to be straightforward endeavor. However, within a functional integral
approach one seems to be able to evaluate in principle any kind of observables,
such as the fragment mass and energy distributions in nuclear fission. These
two radically approaches can likely be brought brought together by formulating
a stochastic time-dependent DFT approach to many-body dynamics.Comment: 9 page
Two-Dimensional QCD in the Wu-Mandelstam-Leibbrandt Prescription
We find the exact non-perturbative expression for a simple Wilson loop of
arbitrary shape for U(N) and SU(N) Euclidean or Minkowskian two-dimensional
Yang-Mills theory regulated by the Wu-Mandelstam-Leibbrandt gauge prescription.
The result differs from the standard pure exponential area-law of YM_2, but
still exhibits confinement as well as invariance under area-preserving
diffeomorphisms and generalized axial gauge transformations. We show that the
large N limit is NOT a good approximation to the model at finite N and conclude
that Wu's N=infinity Bethe-Salpeter equation for QCD_2 should have no bound
state solutions. The main significance of our results derives from the
importance of the Wu-Mandelstam-Leibbrandt prescription in higher-dimensional
perturbative gauge theory.Comment: 7 pages, LaTeX, REVTE
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