14,766 research outputs found
A phase transition for the heights of a fragmentation tree
We provide information about the asymptotic regimes for a homogeneous
fragmentation of a finite set. We establish a phase transition for the
asymptotic behaviours of the shattering times, defined as the first instants
when all the blocks of the partition process have cardinality less than a fixed
integer. Our results may be applied to the study of certain random split trees
Martingales in self-similar growth-fragmentations and their connections with random planar maps
The purpose of the present work is twofold. First, we develop the theory of
general self-similar growth-fragmentation processes by focusing on martingales
which appear naturally in this setting and by recasting classical results for
branching random walks in this framework. In particular, we establish
many-to-one formulas for growth-fragmentations and define the notion of
intrinsic area of a growth-fragmentation. Second, we identify a distinguished
family of growth-fragmentations closely related to stable L\'evy processes,
which are then shown to arise as the scaling limit of the perimeter process in
Markovian explorations of certain random planar maps with large degrees (which
are, roughly speaking, the dual maps of the stable maps of Le Gall & Miermont.
As a consequence of this result, we are able to identify the law of the
intrinsic area of these distinguished growth-fragmentations. This generalizes a
geometric connection between large Boltzmann triangulations and a certain
growth-fragmentation process, which was established in arXiv:1507.02265 .Comment: 50 pages, 5 figures. Final version: to appear in Probab. Theory
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Simulating anthropogenic impacts to bird communities in tropical rain forests
We used an aggregated modelling approach to simulate the impacts ofanthropogenic disturbances on the long-term dynamics of faunal diversityin tropical rain forests. We restricted our study to bird communities eventhough the approach is more general. We developed a model calledBIODIV which simulated the establishment of hypothetical bird speciesin a forest. Our model was based on the results of a simple matrix modelwhich calculated the spatio-temporal dynamics of a tropical rain forest inMalaysia. We analysed the establishment of bird species in a secondaryforest succession and the impacts of 60 different logging scenarios on thediversity of the bird community. Of the three logging parameters(cycle length, method, intensity), logging intensity had the most servereimpact on the bird community. In the worst case the number of bird specieswas reduced to 23% of the species richness found in a primary forest
Dismantling sparse random graphs
We consider the number of vertices that must be removed from a graph G in
order that the remaining subgraph has no component with more than k vertices.
Our principal observation is that, if G is a sparse random graph or a random
regular graph on n vertices with n tending to infinity, then the number in
question is essentially the same for all values of k such that k tends to
infinity but k=o(n).Comment: 7 page
Carbon-Enhanced Metal-Poor Stars, the Cosmic Microwave Background, and the Stellar IMF in the Early Universe
The characteristic mass of stars at early times may have been higher than
today owing to the cosmic microwave background (CMB). This study proposes that
(1) the testable predictions of this "CMB-IMF" hypothesis are an increase in
the fraction of carbon-enhanced metal-poor (CEMP) stars with declining
metallicity and an increase from younger to older populations at a single
metallicity (e.g. disk to halo), and (2) these signatures are already seen in
recent samples of CEMP stars and can be better tested with anticipated data.
The expected spatial variation may explain discrepancies of CEMP frequency
among published surveys. The ubiquity and time dependence of the CMB will
substantially alter the reconstruction of star formation histories in the Local
Group and early Universe.Comment: 7 pages emulateapj format, three figures, accepted for ApJ Letter
Statistical aspects of random fragmentations
AbstractThree random fragmentation of an interval processes are investigated. For each of them, there is a splitting probability and a probability not to split at each step of the fragmentation process whose overall effect is to stabilize the global number of splitting events. Some of their statistical features are studied in each case among which fragments’ size distribution, partition function, structure of the underlying random fragmentation tree, occurrence of a phase transition. In the first homogeneous model, splitting probability does not depend on fragments’ size at each step. In the next two fragmentation models, splitting probability is fragments’ length dependent. In the first such models, fragments further split with probability one if their sizes exceed some cutoff value only; in a second model considered, splitting probability of finite-size objects is assumed to increase algebraically with fragments’ size at each step. The impact of these dependencies on statistical properties of the resulting random partitions are studied. Several examples are supplied
Single electrons from heavy-flavor mesons in relativistic heavy-ion collisions
We study the single electron spectra from and meson semileptonic
decays in Au+Au collisions at 200, 62.4, and 19.2 GeV by
employing the parton-hadron-string dynamics (PHSD) transport approach that has
been shown to reasonably describe the charm dynamics at RHIC and LHC energies
on a microscopic level. In this approach the initial heavy quarks are produced
by using the PYTHIA which is tuned to reproduce the FONLL calculations. The
produced heavy quarks interact with off-shell massive partons in QGP with
scattering cross sections which are calculated in the dynamical quasi-particle
model (DQPM). At energy densities close to the critical energy density the
heavy quarks are hadronized into heavy mesons through either coalescence or
fragmentation. After hadronization the heavy mesons interact with the light
hadrons by employing the scattering cross sections from an effective
Lagrangian. The final heavy mesons then produce single electrons through
semileptonic decay. We find that the PHSD approach well describes the nuclear
modification factor and elliptic flow of single electrons in
d+Au and Au+Au collisions at 200 GeV and the elliptic flow
in Au+Au reactions at 62.4 GeV from the PHENIX
collaboration, however, the large at 62.4 GeV
is not described at all. Furthermore, we make predictions for the
of mesons and of single electrons at the lower energy of 19.2 GeV. Additionally, the medium modification of the azimuthal angle
between a heavy quark and a heavy antiquark is studied. We find that the
transverse flow enhances the azimuthal angular distributions close to 0
because the heavy flavors strongly interact with nuclear medium in relativistic
heavy-ion collisions and almost flow with the bulk matter.Comment: 20 pages, 20 figure
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