137,140 research outputs found
On avoiding Ostrogradski instabilities within Asymptotic Safety
We study the renormalization group flow of gravity coupled to scalar matter
using functional renormalization group techniques. The novel feature is the
inclusion of higher-derivative terms in the scalar propagator. Such terms give
rise to Ostrogradski ghosts which signal an instability of the system and are
therefore dangerous for the consistency of the theory. Since it is expected
that such terms are generated dynamically by the renormalization group flow
they provide a potential threat when constructing a theory of quantum gravity
based on Asymptotic Safety. Our work then establishes the following picture:
upon incorporating higher-derivative terms in the scalar propagator the flow of
the gravity-matter system possesses a fixed point structure suitable for
Asymptotic Safety. This structure includes an interacting renormalization group
fixed point where the Ostrogradski ghosts acquire an infinite mass and decouple
from the system. Tracing the flow towards the infrared it is found that there
is a subset of complete renormalization group trajectories which lead to stable
renormalized propagators. This subset is in one-to-one correspondence to the
complete renormalization group trajectories obtained in computations which do
not track of the higher-derivative terms. Thus our asymptotically safe
gravity-matter systems are not haunted by Ostrogradski ghosts.Comment: 35 pages, 10 figure
A new microsimulation model for the evaluation of traffic safety performances
Some papers have been recently presented (Cunto and Saccomanno 2007, Cunto and Saccomanno 2008, Saccomanno et al. 2008) on the potential of traffic microsimulation for the analysis of road safety. In particular, studies have confirmed that the reproduction by simulation of user behaviour under different flow and geometry conditions, can identify a potential incident hazard and allow to take appropriate countermeasures at specific points of the road network.
The objective of this paper is to assess the validity of this approach; for this reason a microsimulation model and an automatic video detection system have been developed. The microscopic model allows the estimation of road safety performance through a series of indicators (Deceleration Rate to Avoid Crash, Time to Collision, Proportion of Stopping Distance), representing interactions in real time, between different pairs of vehicles belonging to the traffic stream. When these indicators take a certain critical value, a possible accident scenario is identified.
The microscopic simulation model is used combined with a new video image traffic detection algorithm to calculate vehicle trajectories. Microscopic traffic flow parameters obtained by video detection are used to calibrate the microsimulation model, and the safety performance indicators obtained by the real vehicles trajectories can be compared with simulated scenarios where safety performance indicators are obtained on the simulated trajectories.
Results indicate that the methodology can be useful in the estimation of safety performance indicators and in evaluating traffic control measures
Quantal Andreev billiards: Semiclassical approach to mesoscale oscillations in the density of states
Andreev billiards are finite, arbitrarily-shaped, normal-state regions,
surrounded by superconductor. At energies below the superconducting gap,
single-quasiparticle excitations are confined to the normal region and its
vicinity, the essential mechanism for this confinement being Andreev
reflection. This Paper develops and implements a theoretical framework for the
investigation of the short-wave quantal properties of these
single-quasiparticle excitations. The focus is primarily on the relationship
between the quasiparticle energy eigenvalue spectrum and the geometrical shape
of the normal-state region, i.e., the question of spectral geometry in the
novel setting of excitations confined by a superconducting pair-potential.
Among the central results of this investigation are two semiclassical trace
formulas for the density of states. The first, a lower-resolution formula,
corresponds to the well-known quasiclassical approximation, conventionally
invoked in settings involving superconductivity. The second, a
higher-resolution formula, allows the density of states to be expressed in
terms of: (i) An explicit formula for the level density, valid in the
short-wave limit, for billiards of arbitrary shape and dimensionality. This
level density depends on the billiard shape only through the set of
stationary-length chords of the billiard and the curvature of the boundary at
the endpoints of these chords; and (ii) Higher-resolution corrections to the
level density, expressed as a sum over periodic orbits that creep around the
billiard boundary. Owing to the fact that these creeping orbits are much longer
than the stationary chords, one can, inter alia, hear the stationary chords of
Andreev billiards.Comment: 52 pages, 15 figures, 1 table, RevTe
Chiral critical behavior in two dimensions from five-loop renormalization-group expansions
We analyse the critical behavior of two-dimensional N-vector spin systems
with noncollinear order within the five-loop renormalization-group
approximation. The structure of the RG flow is studied for different N leading
to the conclusion that the chiral fixed point governing the critical behavior
of physical systems with N = 2 and N = 3 does not coincide with that given by
the 1/N expansion. We show that the stable chiral fixed point for ,
including N = 2 and N = 3, turns out to be a focus. We give a complete
characterization of the critical behavior controlled by this fixed point, also
evaluating the subleading crossover exponents. The spiral-like approach of the
chiral fixed point is argued to give rise to unusual crossover and
near-critical regimes that may imitate varying critical exponents seen in
numerous physical and computer experiments.Comment: 17 pages, 12 figure
On Localization and Regularization
Different regularizations are studied in localization of path integrals. We
discuss the effect of the choice of regularization by evaluating the partition
functions for the harmonic oscillator and the Weyl character for SU(2). In
particular, we solve the Weyl shift problem that arises in path integral
evaluation of the Weyl character by using the Atiyah-Patodi-Singer
-invariant and the Borel-Weil theory.Comment: 15 pages, LaTe
Evaluating regional emission estimates using the TRACE-P observations
Measurements obtained during the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) experiment are used in conjunction with regional modeling analysis to evaluate emission estimates for Asia. A comparison between the modeled values and the observations is one method to evaluate emissions. Based on such analysis it is concluded that the inventory performs well for the light alkanes, CO, ethyne, SO2, and NOâ. Furthermore, based on model skill in predicting important photochemical species such as Oâ, HCHO, OH, HOâ, and HNOâ, it is found that the emissions inventories are of sufficient quality to support preliminary studies of ozone production. These are important finding in light of the fact that emission estimates for many species (such as speciated NMHCs and BC) for this region have only recently been estimated and are highly uncertain. Using a classification of the measurements built upon trajectory analysis, we compare observed species distributions and ratios of species to those modeled and to ratios estimated from the emissions inventory. It is shown that this technique can reconstruct a spatial distribution of propane/benzene that looks remarkably similar to that calculated from the emissions inventory. A major discrepancy between modeled and observed behavior is found in the Yellow Sea, where modeled values are systematically underpredicted. The integrated analysis suggests that this may be related to an underestimation of emissions from the domestic sector. The emission is further tested by comparing observed and measured species ratios in identified megacity plumes. Many of the model derived ratios (e.g., BC/CO, SOâ/CâHâ) fall within âŒ25% of those observed and all fall outside of a factor of 2.5. (See Article file for details of the abstract.)Department of Civil and Environmental EngineeringAuthor name used in this publication: Wang, T
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