815 research outputs found
Lattice energy-momentum tensor with Symanzik improved actions
We define the energy-momentum tensor on lattice for the and
for the nonlinear -model Symanzik tree-improved actions, using Ward
identities or an explicit matching procedure. The resulting operators give the
correct one loop scale anomaly, and in the case of the sigma model they can
have applications in Monte Carlo simulations.Comment: Self extracting archive fil
Composite boson dominance in relativistic field theories
We apply a new bosonization technique to relativistic field theories of fermions whose partition function is dominated by bosonic composites, and derive the effective action for these bosons. The derivation respects all symmetries, including gauge invariance, with the exception of Euclidean invariance which must be checked a posteriori. We use a lattice regularization which should make applications to gauge theories easier. We test the method on a fermion field theory with quartic interaction in the limit when the number of flavours N_f is large, and show that it reproduces the exact results in the bosonic sector, namely condensation of a compositeboson with the right mass which breaks the discrete chiral invariance of the model. Moreover we determine the structure function of the condensed composite, whose spatial part turns out to be identical to that of the Cooper pairs of the BCS model of superconductivity
Status and perspectives of 2Δ, ΔÎČ+ and 2ÎČ+ decays
This paper reviews the main experimental techniques and the most significant results in the searches for the 2Δ, ΔÎČ+ and 2ÎČ+ decay modes. Efforts related to the study of these decay modes are important, since they can potentially offer complementary information with respect to the cases of 2ÎČâ decays, which allow a better constraint of models for the nuclear structure calculations. Some positive results that have been claimed will be mentioned, and some new perspectives will be addressed shortly
Grassmann Integral Representation for Spanning Hyperforests
Given a hypergraph G, we introduce a Grassmann algebra over the vertex set,
and show that a class of Grassmann integrals permits an expansion in terms of
spanning hyperforests. Special cases provide the generating functions for
rooted and unrooted spanning (hyper)forests and spanning (hyper)trees. All
these results are generalizations of Kirchhoff's matrix-tree theorem.
Furthermore, we show that the class of integrals describing unrooted spanning
(hyper)forests is induced by a theory with an underlying OSP(1|2)
supersymmetry.Comment: 50 pages, it uses some latex macros. Accepted for publication on J.
Phys.
Modelling the shrub encroachment in a grassland with a Cellular Automata Model
Abstract. Arid and semi-arid grasslands of southwestern North America have changed dramatically over the last 150 years as a result of shrub encroachment, i.e. the increase in density, cover and biomass of indigenous shrubby plants in grasslands. Numerous studies have documented the expansion of shrublands in the southwestern American grasslands; in particular shrub encroachment has occurred strongly in part of the northern Chihuahuan desert since 1860. This encroachment has been simulated using an ecohydrological Cellular Automata model, CATGraSS. It is a spatially distributed model driven by spatially explicit irradiance and runs on a fine-resolution gridded domain. Plant competition is modelled by keeping track of mortality and establishment of plants; both are calculated probabilistically based on soil moisture stress. For this study CATGraSS has been improved with a stochastic fire module and a grazing function. The model has been implemented in a small area in Sevilleta National Wildlife Refuge (SNWR), characterized by two vegetation types (grass savanna and creosote bush shrub), considering as encroachment causes the fire return period increase, the grazing increase, the seed dispersal caused by animals, the role of wind direction and plant type competition. The model is able to reproduce the encroachment that has occurred in SNWR, simulating an increase of the shrub from 2% in 1860 to the current shrub percentage, 42%, and highlighting among the most influential factors the reduced fire frequency and the increased grazing intensity
Strong Earthquakes in North-Western Africa in the Second Half of the 17th Century, AD: A Critical Reappraisal of the Historical Evidence
The recent seismological literature recorded three strong earthquakes in Algeria, Libya and Tunisia between 1656 and 1694 AD. The historical evidence for these derives from European sources only (gazettes, journalistic pamphlets, missionary literature). Considering the kind of sources involved, their likely biases and the geographical distances that divided their places of production from the places
that they spoke about, it is possible that some of these accounts could be less than
reliable, and therefore have little use as materials from which to assess earthquake
parameters. To answer these doubts, we have retrieved, cross-checked and critically
analysed the original historical sources quoted in previous compilations and studies
Assessing the benefits of Andean crop diversity on farmers' livelihood: insights from a development programme in Bolivia and Peru
This paper analyses the impact of a development programme designed at promoting the sustainable use of Andean Grain diversity. Results demonstrate that knowledge-sharing on agronomic practices, on benefits derived from consumption, and improving Andean Grain quality had a positive impact on income generation and farmer livelihoods. These results demonstrate the effectiveness of programmes aimed at improving rural livelihoods through greater knowledge transfer and use of local agrobiodiversity, wherein private benefits may incentivise the public benefits of agrobiodiversity use and conservation. Findings warrant the need to further monitor and evaluate the potential of agrobiodiversity to improve the well-being of rural communities
Explicit characterization of the identity configuration in an Abelian Sandpile Model
Since the work of Creutz, identifying the group identities for the Abelian
Sandpile Model (ASM) on a given lattice is a puzzling issue: on rectangular
portions of Z^2 complex quasi-self-similar structures arise. We study the ASM
on the square lattice, in different geometries, and a variant with directed
edges. Cylinders, through their extra symmetry, allow an easy determination of
the identity, which is a homogeneous function. The directed variant on square
geometry shows a remarkable exact structure, asymptotically self-similar.Comment: 11 pages, 8 figure
The 1561 Earthquake(s) in Southern Italy: New Insights into a Complex Seismic Sequence
In the summer of 1561, a strong seismic sequence struck southern Italy, then the Spanish-ruled
Kingdom of Naples. Both the Italian seismological tradition and the latest catalogues locate it in
the Vallo di Diano (Diano Valley), a low-seismicity intermontane basin 100 km south-east of
Naples. We explore the hypothesis that current perception of the 1561 earthquake is distorted by
the nature of the historical dataset from which its parameters have been assessed, and which mostly
derive from a singleâalbeit very detailedâprimary source. We present and discuss several previously
unconsidered original accounts. Our results cast doubts on the traditional interpretation of
the earthquake, which could have been either one Vallo di Diano mainshock or several strong
earthquakes within a time/space window compact enough for contemporary viewers to perceive
them as one. Unquestionably, there is much more to the 1561 earthquake(s) than previously
appeared. We hope that this groundbreaking effort will rekindle the interest of the seismological
community in this seismic episode, our knowledge of which is still far from complete
Unquenched Numerical Stochastic Perturbation Theory
The inclusion of fermionic loops contribution in Numerical Stochastic
Perturbation Theory (NSPT) has a nice feature: it does not cost so much
(provided only that an FFT can be implemented in a fairly efficient way).
Focusing on Lattice SU(3), we report on the performance of the current
implementation of the algorithm and the status of first computations
undertaken.Comment: 3 pages, 3 figures, Lattice2002(algor
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