403 research outputs found
Euclidean Wormholes in String Theory
We show that toroidal compactification of type II string theory to six
dimensions admits axionic euclidean wormhole solutions. These wormholes can be
inserted into backgrounds, which have a
well-defined CFT dual. AdS/CFT duality then suggests that the wormhole
solutions cannot be interpreted using parameters as originally
suggested by Coleman.Comment: 18 pages. ver. 2: typos corrected, references adde
Correlating low energy impact damage with changes in modal parameters: a preliminary study on composite beams
This paper is an experimental study of the effects of multi-site damage on the vibration response of a composite beam damaged by low energy impact. The variation of the modal parameters with different levels of impact energy and density of impact is studied. Specimens are impacted symmetrically in order to induce a global rate of damage. A damage detection tool Damage Index is introduced in order to verify the estimation of damping ratios. Design of Experiments is used to establish the sensitivity of both energy of impact and density of damage. The DOE analysis results (using natural frequency only) indicate that impact energy for 2nd, 3rd and 4th bending modes is the most significant factor contributing to the changes in the modal parameters for this kind of symmetrical dynamic test
Conformational dependence of the intrinsic acidity of the aspartic acid residue sidechain in N-acetyl-L-aspartic acid-N '-methylamide
The sidechain conformational potential energy hypersurfaces (PEHS) for
the gamma(L), beta(L), alpha(L), and alpha(D) backbone conformations of
N-acetyl-L-aspartate-M-methylamide were generated. Of the 81 possible
conformers initially expected for the aspartate residue, only seven
were found after geometric optimizations at the B3LYP/6-31G(d) level of
theory. No stable conformers could be located in the delta(L),
epsilon(L), gamma(D), delta(D), and epsilon(D) backbone conformations.
The 'adiabatic' deprotonation energies for the endo and exo forms of
N-acetyl-L-aspartic acid-N'-methylamide were calculated by comparing
their optimized relative energies against those found for the seven
stable conformers of N-acetyl-L-aspartate-N'-methylamide. Sideehain
conformational PEHSs were also generated for the estimation of
'vertical' deprotonation energies for both endo and exo forms of
N-acetyl-L-aspartic acid-N'-methylamide. All backbone-sidechain
(N-H...-O-C) and backbone-backbone (N-(HO)-O-...=C) hydrogen bond
interactions were analyzed. A total of two backbone-backbone and four
backbone-sidechain interactions were found for
N-acetyl-L-aspartate-N'-methylamide. The deprotonated sidechain of
N-acetyl-L-aspartate-N'-methylamide may allow the aspartyl residue to
form strong hydrogen bond interactions (since it is negatively charged)
which may be significant in such processes as protein-ligand
recognition and ligand binding. As a primary example, the molecular
geometry of the aspartyl residue may be important in peptide folding,
such as that. in the RGD tripeptide. (C) 2002 Elsevier science B.V. All
rights reserved
Hawking Radiation as Tunneling through the Quantum Horizon
Planck-scale corrections to the black-hole radiation spectrum in the
Parikh-Wilczek tunneling framework are calculated. The corrective terms arise
from modifications in the expression of the surface gravity in terms of the
mass-energy of the black hole-emitted particle system. The form of the new
spectrum is discussed together with the possible consequences for the fate of
black holes in the late stages of evaporation.Comment: 13 pages; the contents of this paper overlap somewhat with the
earlier submissions hep-th/0504188 and gr-qc/0505015; (v2) references added
and various cosmetic (but no physics) changes, to appear in JHE
Of Bounces, Branes and Bounds
Some recent studies have considered a Randall-Sundrum-like brane world
evolving in the background of an anti-de Sitter Reissner-Nordstrom black hole.
For this scenario, it has been shown that, when the bulk charge is
non-vanishing, a singularity-free ``bounce'' universe will always be obtained.
However, for the physically relevant case of a de Sitter brane world, we have
recently argued that, from a holographic (c-theorem) perspective, such brane
worlds may not be physically viable. In the current paper, we reconsider the
validity of such models by appealing to the so-called ``causal entropy bound''.
In this framework, a paradoxical outcome is obtained: these brane worlds are
indeed holographically viable, provided that the bulk charge is not too small.
We go on to argue that this new finding is likely the more reliable one.Comment: 15 pages, Revtex; references added and very minor change
On the formation of a Hawking-radiation photosphere around microscopic black holes
We show that once a black hole surpasses some critical temperature
, the emitted Hawking radiation interacts with itself and forms a
nearly thermal photosphere. Using QED, we show that the dominant interactions
are bremsstrahlung and electron-photon pair production, and we estimate
, which when calculated more precisely is
found to be 45 GeV. The formation of the photosphere is
purely a particle physics effect, and not a general relativistic effect, since
the the photosphere forms roughly Schwarzschild radii away from
the black hole. The temperature of the photosphere decreases with distance
from the black hole, and the outer surface is determined by the constraint
(for the QED case), since this is the point at which electrons
and positrons annihilate, and the remaining photons free stream to infinity.
Observational consequences are discussed, and it is found that, although the
QED photosphere will not affect the Page-Hawking limits on primordial black
holes, which is most important for 100MeV black holes, the inclusion of QCD
interactions may significantly effect this limit, since for QCD we estimate
. The photosphere greatly reduces possibility of
observing individual black holes with temperatures greater than ,
since the high energy particles emitted from the black hole are processed
through the photosphere to a lower energy, where the gamma ray background is
much higher. The temperature of the plasma in the photosphere can be extremely
high, and this offers interesting possibilities for processes such as symmetry
restoration.Comment: Latex, 16 pages, 3 postscript figures, submitted to PRD. Also
available at http://fnas08.fnal.gov
On the selection of AGN neutrino source candidates for a source stacking analysis with neutrino telescopes
The sensitivity of a search for sources of TeV neutrinos can be improved by
grouping potential sources together into generic classes in a procedure that is
known as source stacking. In this paper, we define catalogs of Active Galactic
Nuclei (AGN) and use them to perform a source stacking analysis. The grouping
of AGN into classes is done in two steps: first, AGN classes are defined, then,
sources to be stacked are selected assuming that a potential neutrino flux is
linearly correlated with the photon luminosity in a certain energy band (radio,
IR, optical, keV, GeV, TeV). Lacking any secure detailed knowledge on neutrino
production in AGN, this correlation is motivated by hadronic AGN models, as
briefly reviewed in this paper.
The source stacking search for neutrinos from generic AGN classes is
illustrated using the data collected by the AMANDA-II high energy neutrino
detector during the year 2000. No significant excess for any of the suggested
groups was found.Comment: 43 pages, 12 figures, accepted by Astroparticle Physic
Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV
We report a measurement of the longitudinal double-spin asymmetry A_LL and
the differential cross section for inclusive Pi0 production at midrapidity in
polarized proton collisions at sqrt(s) = 200 GeV. The cross section was
measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be
in good agreement with a next-to-leading order perturbative QCD calculation.
The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T <
11 GeV/c and excludes a maximal positive gluon polarization in the proton. The
mean transverse momentum fraction of Pi0's in their parent jets was found to be
around 0.7 for electromagnetically triggered events.Comment: 6 pages, 3 figures, submitted to Phys. Rev. D (RC
High non-photonic electron production in + collisions at = 200 GeV
We present the measurement of non-photonic electron production at high
transverse momentum ( 2.5 GeV/) in + collisions at
= 200 GeV using data recorded during 2005 and 2008 by the STAR
experiment at the Relativistic Heavy Ion Collider (RHIC). The measured
cross-sections from the two runs are consistent with each other despite a large
difference in photonic background levels due to different detector
configurations. We compare the measured non-photonic electron cross-sections
with previously published RHIC data and pQCD calculations. Using the relative
contributions of B and D mesons to non-photonic electrons, we determine the
integrated cross sections of electrons () at 3 GeV/10 GeV/ from bottom and charm meson decays to be = 4.0({\rm
stat.})({\rm syst.}) nb and =
6.2({\rm stat.})({\rm syst.}) nb, respectively.Comment: 17 pages, 17 figure
Evolution of the differential transverse momentum correlation function with centrality in Au+Au collisions at GeV
We present first measurements of the evolution of the differential transverse
momentum correlation function, {\it C}, with collision centrality in Au+Au
interactions at GeV. {\it C} exhibits a strong dependence
on collision centrality that is qualitatively similar to that of number
correlations previously reported. We use the observed longitudinal broadening
of the near-side peak of {\it C} with increasing centrality to estimate the
ratio of the shear viscosity to entropy density, , of the matter formed
in central Au+Au interactions. We obtain an upper limit estimate of
that suggests that the produced medium has a small viscosity per unit entropy.Comment: 7 pages, 4 figures, STAR paper published in Phys. Lett.
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