784 research outputs found
Ward-Takahashi Identity with External Field in Ladder QED
We derive the Ward-Takahashi identity obeyed by the fermion-antifermion-gauge
boson vertex in ladder QED in the presence of a constant magnetic field. The
general structure in momentum space of the fermion mass operator with external
electromagnetic field is discussed. Using it we find the solutions of the
ladder WT identity with magnetic field. The consistency of our results with the
solutions of the corresponding Schwinger-Dyson equation ensures the gauge
invariance of the magnetic field induced chiral symmetry breaking recently
found in ladder QED.Comment: new references(refs.10,11) added, 18 pages, Late
Lepton Flavor Violation and the Tau Neutrino Mass
We point out that, in the left-right symmetric model of weak interaction, if
mass is in the keV to MeV range, there is a strong correlation
between rare decays such as and
the mass. In particular, we point out that a large range of
masses are forbidden by the cosmological constraints on
in combination with the present upper limits on these processes.Comment: UMDHEP 94-30, 14 pages, TeX file, (some new references added
Cooperation and the evolution of intelligence
The high levels of intelligence seen in humans, other primates, certain cetaceans and birds remain a major puzzle for evolutionary biologists, anthropologists and psychologists. It has long been held that social interactions provide the selection pressures necessary for the evolution of advanced cognitive abilities (the ‘social intelligence hypothesis’), and in recent years decision-making in the context of cooperative social interactions has been conjectured to be of particular importance. Here we use an artificial neural network model to show that selection for efficient decision-making in cooperative dilemmas can give rise to selection pressures for greater cognitive abilities, and that intelligent strategies can themselves select for greater intelligence, leading to a Machiavellian arms race. Our results provide mechanistic support for the social intelligence hypothesis, highlight the potential importance of cooperative behaviour in the evolution of intelligence and may help us to explain the distribution of cooperation with intelligence across taxa
Statistical mechanics of complex networks
Complex networks describe a wide range of systems in nature and society, much
quoted examples including the cell, a network of chemicals linked by chemical
reactions, or the Internet, a network of routers and computers connected by
physical links. While traditionally these systems were modeled as random
graphs, it is increasingly recognized that the topology and evolution of real
networks is governed by robust organizing principles. Here we review the recent
advances in the field of complex networks, focusing on the statistical
mechanics of network topology and dynamics. After reviewing the empirical data
that motivated the recent interest in networks, we discuss the main models and
analytical tools, covering random graphs, small-world and scale-free networks,
as well as the interplay between topology and the network's robustness against
failures and attacks.Comment: 54 pages, submitted to Reviews of Modern Physic
The ghosts of forests past and future : deforestation and botanical sampling in the Brazilian Amazon
The remarkable biodiversity of the Brazilian Amazon is poorly documented and threatened by deforestation. When undocumented areas become deforested, in addition to losing the fauna and flora, we lose the opportunity to know which unique species had occupied a habitat. Here we quantify such knowledge loss by calculating how much of the Brazilian Amazon has been deforested and will likely be deforested until 2050 without having its tree flora sufficiently documented. To this end, we analysed 399 147 digital specimens of nearly 6000 tree species in relation to official deforestation statistics and future deforestation scenarios. We find that by 2017, 30% of all the localities where tree specimens had been collected were mostly deforested. Some 300 000 km(2)(12%; 485 25 x 25 km grid cells) of the Brazilian Amazon had been deforested by 2017, without having a single tree specimen recorded. An additional 250 000-900 000 km(2)of severely under-collected rainforest will likely become deforested by 2050. If future tree sampling is to cover this area, sampling effort has to increase two- to six-fold. Nearly 255 000 km(2)or 7% of rainforest in the Brazilian Amazon is easily accessible but does yet but remain under-collected. Our study highlights how progressing deforestation increases the risk of losing undocumented species of a hyper-diverse tree flora.Peer reviewe
Light Sterile Neutrino from extra dimensions and Four-Neutrino Solutions to Neutrino Anomalies
We propose a four-neutrino model which can reconcile the existing data coming
from underground experiments in terms of neutrino oscillations, together with
the hint from the LSND experiment and a possible neutrino contribution to the
hot dark matter of the Universe. It applies the idea that extra compact
dimensions, probed only by gravity and possibly gauge-singlet fields, can lower
the fundamental scales such as the Planck, string or unification scales. Our
fourth light neutrino ( for sterile) is identified with the zero
mode of the Kaluza-Klein states. To first approximation \nu_sterile combines
with the nu_mu in order to form a Dirac neutrino with mass in the eV range
leaving the other two neutrinos massless. The smallness of this mass scale
(suitable for LSND and Hot Dark Matter) arises without appealing neither to a
see-saw mechanism nor to a radiative mechanism, but from the volume factor
associated with the canonical normalization of the wave-function of the bulk
field in the compactified dimensions. % On the other hand the splitting between
\nm and \nu_sterile (atmospheric scale) as well as the mass of the two other
neutrinos (solar mass scale) arise from the violation of the fermion number on
distant branes. We also discuss alternative scenarios involving
flavour-changing interactions. In one of them \ne can be in the electron-volt
range and therefore be probed in beta decay studies.Comment: 12 pages, latex, no figures, title changed, final version to be
published in Phys Rev
Grand unified theory constrained supersymmetry and neutrinoless double beta decay
We analyze the contributions to the neutrinoless double decay
(-decay) coming from the Grand Unified Theory (GUT) constrained
Minimal Supersymmetric Standard Model (MSSM) with trilinear R-parity breaking.
We discuss the importance of two-nucleon and pion-exchange realizations of the
quark-level -decay transitions. In this context, the questions
of reliability of the calculated relevant nuclear matrix elements within the
Renormalized Quasiparticle Random Phase Approximation (pn-RQRPA) for several
medium and heavy open-shell nuclei are addressed. The importance of gluino and
neutralino contributions to -decay is also analyzed. We review
the present experiments and deduce limits on the trilinear R-parity breaking
parameter from the non-observability of -decay
for different GUT constrained SUSY scenarios. In addition, a detailed study of
limits on the MSSM parameter space coming from the processes
by using the recent CLEO and OPAL results is performed. Some studies in respect
to the future -decay project GENIUS are also presented.Comment: 29 pages, 8 figure
The Logic of Fashion Cycles
Many cultural traits exhibit volatile dynamics, commonly dubbed fashions or fads. Here we show that realistic fashion-like dynamics emerge spontaneously if individuals can copy others' preferences for cultural traits as well as traits themselves. We demonstrate this dynamics in simple mathematical models of the diffusion, and subsequent abandonment, of a single cultural trait which individuals may or may not prefer. We then simulate the coevolution between many cultural traits and the associated preferences, reproducing power-law frequency distributions of cultural traits (most traits are adopted by few individuals for a short time, and very few by many for a long time), as well as correlations between the rate of increase and the rate of decrease of traits (traits that increase rapidly in popularity are also abandoned quickly and vice versa). We also establish that alternative theories, that fashions result from individuals signaling their social status, or from individuals randomly copying each other, do not satisfactorily reproduce these empirical observations
Predicting Maximum Tree Heights and Other Traits from Allometric Scaling and Resource Limitations
Terrestrial vegetation plays a central role in regulating the carbon and water cycles, and adjusting planetary albedo. As such, a clear understanding and accurate characterization of vegetation dynamics is critical to understanding and modeling the broader climate system. Maximum tree height is an important feature of forest vegetation because it is directly related to the overall scale of many ecological and environmental quantities and is an important indicator for understanding several properties of plant communities, including total standing biomass and resource use. We present a model that predicts local maximal tree height across the entire continental United States, in good agreement with data. The model combines scaling laws, which encode the average, base-line behavior of many tree characteristics, with energy budgets constrained by local resource limitations, such as precipitation, temperature and solar radiation. In addition to predicting maximum tree height in an environment, our framework can be extended to predict how other tree traits, such as stomatal density, depend on these resource constraints. Furthermore, it offers predictions for the relationship between height and whole canopy albedo, which is important for understanding the Earth's radiative budget, a critical component of the climate system. Because our model focuses on dominant features, which are represented by a small set of mechanisms, it can be easily integrated into more complicated ecological or climate models.National Science Foundation (U.S.) (Research Experience for Undergraduates stipend)Gordon and Betty Moore FoundationNational Science Foundation (U.S.) (Graduate Research Fellowship Program)Massachusetts Institute of Technology. Presidential FellowshipEugene V. and Clare Thaw Charitable TrustEngineering and Physical Sciences Research CouncilNational Science Foundation (U.S.) (PHY0202180)Colorado College (Venture Grant Program
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