1,173 research outputs found
de Sitter gravity from lattice gauge theory
We investigate a lattice model for Euclidean quantum gravity based on
discretization of the Palatini formulation of General Relativity. Using Monte
Carlo simulation we show that while a naive approach fails to lead to a vacuum
state consistent with the emergence of classical spacetime, this problem may be
evaded if the lattice action is supplemented by an appropriate counter term. In
this new model we find regions of the parameter space which admit a ground
state which can be interpreted as (Euclidean) de Sitter space.Comment: 16 pages, 11 figures. email address update
Universality Class of Models
We point out that existing numerical data on the correlation length and
magnetic susceptibility suggest that the two dimensional model with
standard action has critical exponent , which is inconsistent with
asymptotic freedom. This value of is also different from the one of the
Wess-Zumino-Novikov-Witten model that is supposed to correspond to the
model at .Comment: 8 pages, with 3 figures included, postscript. An error concerning the
errors has been correcte
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.
General duality for abelian-group-valued statistical-mechanics models
We introduce a general class of statistical-mechanics models, taking values
in an abelian group, which includes examples of both spin and gauge models,
both ordered and disordered. The model is described by a set of ``variables''
and a set of ``interactions''. A Gibbs factor is associated to each variable
and to each interaction. We introduce a duality transformation for systems in
this class. The duality exchanges the abelian group with its dual, the Gibbs
factors with their Fourier transforms, and the interactions with the variables.
High (low) couplings in the interaction terms are mapped into low (high)
couplings in the one-body terms. The idea is that our class of systems extends
the one for which the classical procedure 'a la Kramers and Wannier holds, up
to include randomness into the pattern of interaction. We introduce and study
some physical examples: a random Gaussian Model, a random Potts-like model, and
a random variant of discrete scalar QED. We shortly describe the consequence of
duality for each example.Comment: 26 pages, 2 Postscript figure
On the question of universality in \RPn and \On Lattice Sigma Models
We argue that there is no essential violation of universality in the
continuum limit of mixed \RPn and \On lattice sigma models in 2 dimensions,
contrary to opposite claims in the literature.Comment: 16 pages (latex) + 3 figures (Postscript), uuencode
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
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
Defining New Parameters for Green Engineering Design of Treatment Reactors
This study proposes a green way to design Plug Flow Reactors (PFR) that use biodegradable polymer solutions, capable of contaminant retaining, for industrial wastewater treatment. Usually, to the design of a reactor, the reaction rate is determined by tests on a Continuous Stirred-Tank (CST), these generate toxic effluents and also increase the cost of the design. In this work, empirical expressions (called âslip functionsâ), in terms of the average concentration of the contaminant, were developed through the study of the transport behaviour of CrVI into solutions of xanthan gum. âIn situâ XRÎŒF was selected as a no-invasive micro-technique to determine local concentrations. Slip functions were used with laboratory experiments planned in similar conditions using Plug Flow Reactors, to obtain useful dimensionless parameters for the industrial design
Random Walks with Long-Range Self-Repulsion on Proper Time
We introduce a model of self-repelling random walks where the short-range
interaction between two elements of the chain decreases as a power of the
difference in proper time. Analytic results on the exponent are obtained.
They are in good agreement with Monte Carlo simulations in two dimensions. A
numerical study of the scaling functions and of the efficiency of the algorithm
is also presented.Comment: 25 pages latex, 4 postscript figures, uses epsf.sty (all included)
IFUP-Th 13/92 and SNS 14/9
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
- âŠ