156 research outputs found
Is Quantum Einstein Gravity Nonperturbatively Renormalizable?
We find considerable evidence supporting the conjecture that four-dimensional
Quantum Einstein Gravity is ``asymptotically safe'' in Weinberg's sense. This
would mean that the theory is likely to be nonperturbatively renormalizable and
thus could be considered a fundamental (rather than merely effective) theory
which is mathematically consistent and predictive down to arbitrarily small
length scales. For a truncated version of the exact flow equation of the
effective average action we establish the existence of a non-Gaussian
renormalization group fixed point which is suitable for the construction of a
nonperturbative infinite cutoff-limit. The truncation ansatz includes the
Einstein-Hilbert action and a higher derivative term.Comment: 18 pages, latex, 3 figure
Correlation Structures of Correlated Binomial Models and Implied Default Distribution
We show how to analyze and interpret the correlation structures, the
conditional expectation values and correlation coefficients of exchangeable
Bernoulli random variables. We study implied default distributions for the
iTraxx-CJ tranches and some popular probabilistic models, including the
Gaussian copula model, Beta binomial distribution model and long-range Ising
model. We interpret the differences in their profiles in terms of the
correlation structures. The implied default distribution has singular
correlation structures, reflecting the credit market implications. We point out
two possible origins of the singular behavior.Comment: 16 pages, 7 figure
Small-scale anisotropy of cosmic rays above 10^19eV observed with the Akeno Giant Air Shower Array
With the Akeno Giant Air Shower Array (AGASA), 581 cosmic rays above 10^19eV,
47 above 4 x 10^19eV, and 7 above 10^20eV are observed until August 1998.
Arrival direction distribution of these extremely high energy cosmic rays has
been studied. While no significant large-scale anisotropy is found on the
celestial sphere, some interesting clusters of cosmic rays are observed. Above
4 x 10^19eV, there are one triplet and three doublets within separation angle
of 2.5^o and the probability of observing these clusters by a chance
coincidence under an isotropic distribution is smaller than 1 %. Especially the
triplet is observed against expected 0.05 events. The cos(\theta_GC)
distribution expected from the Dark Matter Halo model fits the data as well as
an isotropic distribution above 2 x 10^19eV and 4 x 10^19eV, but is a poorer
fit than isotropy above 10^19eV. Arrival direction distribution of seven
10^20eV cosmic rays is consistent with that of lower energy cosmic rays and is
uniform. Three of seven are members of doublets above about 4 x 10^19eV.Comment: 40 pages, 12 figure, AASTeX *** Authors found a typo on Table 2 --
Energy of event 94/07/06 **
Infectious Default Model with Recovery and Continuous Limit
We introduce an infectious default and recovery model for N obligors.
Obligors are assumed to be exchangeable and their states are described by N
Bernoulli random variables S_{i} (i=1,...,N). They are expressed by multiplying
independent Bernoulli variables X_{i},Y_{ij},Y'_{ij}, and default and recovery
infections are described by Y_{ij} and Y'_{ij}. We obtain the default
probability function P(k) for k defaults. Taking its continuous limit, we find
two nontrivial probability distributions with the reflection symmetry of S_{i}
\leftrightarrow 1-S_{i}. Their profiles are singular and oscillating and we
understand it theoretically. We also compare P(k) with an implied default
distribution function inferred from the quotes of iTraxx-CJ. In order to
explain the behavior of the implied distribution, the recovery effect may be
necessary.Comment: 13 pages, 7 figure
Yard-Sale exchange on networks: Wealth sharing and wealth appropriation
Yard-Sale (YS) is a stochastic multiplicative wealth-exchange model with two
phases: a stable one where wealth is shared, and an unstable one where wealth
condenses onto one agent. YS is here studied numerically on 1d rings, 2d square
lattices, and random graphs with variable average coordination, comparing its
properties with those in mean field (MF). Equilibrium properties in the stable
phase are almost unaffected by the introduction of a network. Measurement of
decorrelation times in the stable phase allow us to determine the critical
interface with very good precision, and it turns out to be the same, for all
networks analyzed, as the one that can be analytically derived in MF. In the
unstable phase, on the other hand, dynamical as well as asymptotic properties
are strongly network-dependent. Wealth no longer condenses on a single agent,
as in MF, but onto an extensive set of agents, the properties of which depend
on the network. Connections with previous studies of coalescence of immobile
reactants are discussed, and their analytic predictions are successfully
compared with our numerical results.Comment: 10 pages, 7 figures. Submitted to JSTA
Extension of the Cosmic-Ray Energy Spectrum Beyond the Predicted Greisen-Zatsepin-Kuz'min Cutoff
The cosmic-ray energy spectrum above 10^{18.5} eV is reported using the
updated data set of the Akeno Giant Air Shower Array (AGASA) from February 1990
to October 1997. The energy spectrum extends beyond 10^{20} eV and the energy
gap between the highest energy event and the others is being filled up with
recently observed events. The spectral shape suggests the absence of the 2.7 K
cutoff in the energy spectrum or a possible presence of a new component beyond
the 2.7 K cutoff.Comment: to be published in PRL, 3 figures, REVTEX forma
Observation of an Orbital Selective Electron-Mode Coupling in Fe-Based High- Superconductors
We have performed an angle-resolved photoemission spectroscopy study of the
new superconductor BaKFeAs in the low energy range. We
report the observation of an anomaly around 25 meV in the dispersion of
superconducting BaKFeAs samples that nearly vanishes
above . The energy scale of the related mode (132 meV) and its strong
dependence on orbital and temperature indicates that it is unlikely related to
phonons. Moreover, the momentum locations of the kink can be connected by the
antiferromagnetic wavevector. Our results point towards an unconventional
electronic origin of the mode and the superconducting pairing in the Fe-based
superconductors, and strongly support the anti-phase s-wave pairing symmetry.Comment: 4 pages, 3 figure
Electronic structure of optimally doped pnictide BaKFeAs: a comprehensive ARPES investigation
We have conducted a comprehensive angle-resolved photoemission study on the
normal state electronic structure of the Fe-based superconductor
BaKFeAs. We have identified four dispersive bands which
cross the Fermi level and form two hole-like Fermi surfaces around and
two electron-like Fermi surfaces around M. There are two nearly nested Fermi
surface pockets connected by an antiferromagnetic (, ) wavevector.
The observed Fermi surfaces show small dispersion and a total volume
consistent with Luttinger theorem. Compared to band structure calculations, the
overall bandwidth is reduced by a factor of 2. However, many low energy
dispersions display stronger mass renormalization by a factor of 4,
indicating possible orbital (energy) dependent correlation effects. Using an
effective tight banding model, we fitted the band structure and the Fermi
surfaces to obtain band parameters reliable for theoretical modeling and
calculations of the important physical quantities, such as the specific heat
coefficient.Comment: 13 pages, 4 figure
Size-Dependency of Income Distributions and Its Implications
This paper highlights the size-dependency of income distributions, i.e. the
income distribution curves versus the population of a country systematically.
By using the generalized Lotka-Volterra model to fit the empirical income data
in the United States during 1996-2007, we found an important parameter
can scale with a power of the size (population) of U.S. in
that year. We pointed out that the size-dependency of the income distributions,
which is a very important property but seldom addressed by previous studies,
has two non-trivial implications: (1) the allometric growth pattern, i.e. the
power law relationship between population and GDP in different years, which can
be mathematically derived from the size-dependent income distributions and also
supported by the empirical data; (2) the connection with the anomalous scaling
for the probability density function in critical phenomena since the re-scaled
form of the income distributions has the exactly same mathematical expression
for the limit distribution of the sum of many correlated random variables
asymptotically.Comment: 4 figures, 4 page
An statistical analysis of stratification and inequity in the income distribution
The analysis of the USA 2001 income distribution shows that it can be
described by at least two main components, which obey the generalized Tsallis
statistics with different values of the q parameter. Theoretical calculations
using the gas kinetics model with a distributed saving propensity factor and
two ensembles reproduce the empirical data and provide further information on
the structure of the distribution, which shows a clear stratification. This
stratification is amenable to different interpretations, which are analyzed.
The distribution function is invariant with the average individual income,
which implies that the inequity of the distribution cannot be modified by
increasing the total income.Comment: 22 pages, 3 figure
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