655 research outputs found
Evaluation of a 3-D rockfall module within a forest patch model
Many slopes in the Alps are prone to rockfall and forests play a vital role in protecting objects such as (rail) roads and infrastructure against rockfall. Decision support tools are required to assess rockfall processes and to quantify the rockfall protection effect of forest stands. This paper presents results of an iterative sequence of tests and improvements of a coupled rockfall and forest dynamics model with focus on the rockfall module. As evaluation data a real-size rockfall experiment in the French Alps and two 2-D rockfall trajectories from Austria and Switzerland were used. Modification of the rebound algorithm and the inclusion of an algorithm accounting for the sudden halt of falling rocks due to surface roughness greatly improved the correspondence between simulated and observed key rockfall variables like run-out distances, rebound heights and jump lengths for the real-size rockfall experiment. Moreover, the observed jump lengths and run-out distances of the 2-D trajectories were well within the stochastic range of variation yielded by the simulations. Based on evaluation results it is concluded that the rockfall model can be employed to assess the protective effect of forest vegetation
Magneto-optics of massive Dirac fermions in bulk Bi2Se3
We report on magneto-optical studies of Bi2Se3, a representative member of
the 3D topological insulator family. Its electronic states in bulk are shown to
be well described by a simple Dirac-type Hamiltonian for massive particles with
only two parameters: the fundamental bandgap and the band velocity. In a
magnetic field, this model implies a unique property - spin splitting equal to
twice the cyclotron energy: Es = 2Ec. This explains the extensive
magneto-transport studies concluding a fortuitous degeneracy of the spin and
orbital split Landau levels in this material. The Es = 2Ec match differentiates
the massive Dirac electrons in bulk Bi2Se3 from those in quantum
electrodynamics, for which Es = Ec always holds.Comment: 5 pages, 3 figures and Supplementary materials, to be published in
Physical Review Letter
A quasi classical approach to electron impact ionization
A quasi classical approximation to quantum mechanical scattering in the
Moeller formalism is developed. While keeping the numerical advantage of a
standard Classical--Trajectory--Monte--Carlo calculation, our approach is no
longer restricted to use stationary initial distributions. This allows one to
improve the results by using better suited initial phase space distributions
than the microcanonical one and to gain insight into the collision mechanism by
studying the influence of different initial distributions on the cross section.
A comprehensive account of results for single, double and triple differential
cross sections for atomic hydrogen will be given, in comparison with experiment
and other theories.Comment: 21 pages, 10 figures, submitted to J Phys
Anomaly and a QCD-like phase diagram with massive bosonic baryons
We study a strongly coupled lattice gauge theory with two flavors of
quarks, invariant under an exact symmetry which is the same as QCD with
two flavors of quarks without an anomaly. The model also contains a coupling
that can be used to break the symmetry and thus mimic the QCD
anomaly. At low temperatures and small baryon chemical potential
the model contains massless pions and massive bosonic baryons similar to QCD
with an even number of colors. In this work we study the phase
diagram of the model and show that it contains three phases : (1) A chirally
broken phase at low and , (2) a chirally symmetric baryon superfluid
phase at low and high , and (3) a symmetric phase at high . We
find that the nature of the finite temperature chiral phase transition and in
particular the location of the tricritical point that seperates the first order
line from the second order line is affected significantly by the anomaly.Comment: 22 pages, 16 figures, 5 tables, references adde
The a-theorem and conformal symmetry breaking in holographic RG flows
We study holographic models describing an RG flow between two fixed points
driven by a relevant scalar operator. We show how to introduce a spurion field
to restore Weyl invariance and compute the anomalous contribution to the
generating functional in even dimensional theories. We find that the
coefficient of the anomalous term is proportional to the difference of the
conformal anomalies of the UV and IR fixed points, as expected from anomaly
matching arguments in field theory. For any even dimensions the coefficient is
positive as implied by the holographic a-theorem. For flows corresponding to
spontaneous breaking of conformal invariance, we also compute the two-point
functions of the energy-momentum tensor and the scalar operator and identify
the dilaton mode. Surprisingly we find that in the simplest models with just
one scalar field there is no dilaton pole in the two-point function of the
scalar operator but a stronger singularity. We discuss the possible
implications.Comment: 50 pages. v2: minor changes, added references, extended discussion.
v3: we have clarified some of the calculations and assumptions, results
unchanged. v4: published version in JHE
One and two dimensional analysis of 3pi correlations measured in Pb+Pb interactions
3pi- correlations from Pb+Pb collisions at 158 GeV/c per nucleon are
presented as measured by the focusing spectrometer of the NA44 experiment at
CERN. The three-body effect is found to be stronger for PbPb than for SPb. The
two-dimensional three-particle correlation function is also measured and the
longitudinal extension of the source is larger than the transverse extension
CORUM: the comprehensive resource of mammalian protein complexes
Protein complexes are key molecular entities that integrate multiple gene products to perform cellular functions. The CORUM (http://mips.gsf.de/genre/proj/corum/index.html) database is a collection of experimentally verified mammalian protein complexes. Information is manually derived by critical reading of the scientific literature from expert annotators. Information about protein complexes includes protein complex names, subunits, literature references as well as the function of the complexes. For functional annotation, we use the FunCat catalogue that enables to organize the protein complex space into biologically meaningful subsets. The database contains more than 1750 protein complexes that are built from 2400 different genes, thus representing 12% of the protein-coding genes in human. A web-based system is available to query, view and download the data. CORUM provides a comprehensive dataset of protein complexes for discoveries in systems biology, analyses of protein networks and protein complex-associated diseases. Comparable to the MIPS reference dataset of protein complexes from yeast, CORUM intends to serve as a reference for mammalian protein complexes
Singular values of the Dirac operator in dense QCD-like theories
We study the singular values of the Dirac operator in dense QCD-like theories
at zero temperature. The Dirac singular values are real and nonnegative at any
nonzero quark density. The scale of their spectrum is set by the diquark
condensate, in contrast to the complex Dirac eigenvalues whose scale is set by
the chiral condensate at low density and by the BCS gap at high density. We
identify three different low-energy effective theories with diquark sources
applicable at low, intermediate, and high density, together with their
overlapping domains of validity. We derive a number of exact formulas for the
Dirac singular values, including Banks-Casher-type relations for the diquark
condensate, Smilga-Stern-type relations for the slope of the singular value
density, and Leutwyler-Smilga-type sum rules for the inverse singular values.
We construct random matrix theories and determine the form of the microscopic
spectral correlation functions of the singular values for all nonzero quark
densities. We also derive a rigorous index theorem for non-Hermitian Dirac
operators. Our results can in principle be tested in lattice simulations.Comment: 3 references added, version published in JHE
Effects of 20â100 nm particles on liquid clouds in the clean summertime Arctic
Observations addressing effects of aerosol par- ticles on summertime Arctic clouds are limited. An air- borne study, carried out during July 2014 from Resolute Bay, Nunavut, Canada, as part of the Canadian NETCARE project, provides a comprehensive in situ look into some effects of aerosol particles on liquid clouds in the clean environment of the Arctic summer. Median cloud droplet number concentrations (CDNC) from 62 cloud samples are 10 cmâ3 for low-altitude cloud (clouds topped below 200 m) and 101 cmâ3 for higher-altitude cloud (clouds based above 200m). The lower activation size of aerosol particles is â€50nm diameter in about 40% of the cases. Particles as small as 20 nm activated in the higher-altitude clouds consis- tent with higher supersaturations (S) for those clouds inferred from comparison of the CDNC with cloud condensation nu- cleus (CCN) measurements. Over 60 % of the low-altitude cloud samples fall into the CCN-limited regime of Mauritsen et al. (2011), within which increases in CDNC may increase liquid water and warm the surface. These first observations of that CCN-limited regime indicate a positive association of the liquid water content (LWC) and CDNC, but no associ- ation of either the CDNC or LWC with aerosol variations. Above the Mauritsen limit, where aerosol indirect cooling may result, changes in particles with diameters from 20 to 100nm exert a relatively strong influence on the CDNC. Within this exceedingly clean environment, as defined by low carbon monoxide and low concentrations of larger parti- cles, the background CDNC are estimated to range between 16 and 160 cmâ3, where higher values are due to activation of particles †50 nm that likely derive from natural sources. These observations offer the first wide-ranging reference for the aerosol cloud albedo effect in the summertime Arctic
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