6,216 research outputs found
The scalar glueball spectrum
We discuss scenarios for scalar glueballs using arguments based on sum rules,
spectral decomposition, the approximation, the scales of the
strong interaction and the topology of the flux tubes. We analyze the
phenomenological support of those scenarios and their observational
implications. Our investigations hint a rich low lying glueball spectrum.Comment: 11 pages: New title, figure, table and a more detailed comparison
with experiment
An SU(5)Z_{13} Grand Unification Model
We propose an SU(5) grand unified model with an invisible axion and the
unification of the three coupling constants which is in agreement with the
values, at , of , , and . A discrete,
anomalous, symmetry implies that the Peccei-Quinn symmetry is an
automatic symmetry of the classical Lagrangian protecting, at the same time,
the invisible axion against possible semi-classical gravity effects. Although
the unification scale is of the order of the Peccei-Quinn scale the proton is
stabilized by the fact that in this model the standard model fields form the
SU(5) multiplets completed by new exotic fields and, also, because it is
protected by the symmetry.Comment: 14 pages, more typos correcte
Geometric structures on loop and path spaces
Is is known that the loop space associated to a Riemannian manifold admits a
quasi-symplectic structure. This article shows that this structure is not
likely to recover the underlying Riemannian metric by proving a result that is
a strong indication of the "almost" independence of the quasi-symplectic
structure with respect to the metric. Finally conditions to have contact
structures on these spaces are studied.Comment: Final version. To appear in Proceedings of Math. Sci. Indian Academy
of Science
N-Delta(1232) axial form factors from weak pion production
The N-Delta axial form factors are determined from neutrino induced pion
production ANL & BNL data by using a state of the art theoretical model, which
accounts both for background mechanisms and deuteron effects. We find
violations of the off diagonal Goldberger-Treiman relation at the level of 2
sigma which might have an impact in background calculations for T2K and
MiniBooNE low energy neutrino oscillation precision experiments.Comment: 4 pages, 1 figur
Anharmonicity and asymmetry of Landau levels for a two-dimensional electron gas
We calculate the density of states of a two dimensional electron gas located
at the interface of a GaAlAs/GaAs heterojunction. The disorder potential which
is generally created by a single doping layer behind a spacer, is here enhanced
by the presence of a second delta doped layer of scatterers which can be
repulsive or attractive impurities. We have calculated the density of states by
means of the Klauder's approximation, in the presence of a magnetic field of
arbitrary strength. At low field either band tails or impurity bands are
observed for attractive potentials, depending on the impurity concentration. At
higher field, impurity bands are observed for both repulsive and attractive
potentials. We discuss the effect of such an asymmetrical density of states on
the transport properties in the quantum Hall effect regime.Comment: 22 pages, 12 figures. submitted to Phys. Rev.
production in the reaction
We discuss the mechanisms that lead to production in the
reaction. The problem has gained renewed interest
after different works converge to the conclusion that there are two resonances
around the region of 1400 MeV, rather than one, and that they couple
differently to the and channels. We look at the dynamics
of that reaction and find two mechanisms which eventually filter each one of
the resonances, leading to very different shapes of the invariant
mass distributions. The combination of the two mechanisms leads to a shape of
this distribution compatible with the experimental measurements.Comment: RevTeX4, 10 pages, 8 figures, 2 tables, Version to appear in Phys.
Rev.
A note on the violation of the Einstein relation in a driven moderately dense granular gas
The Einstein relation for a driven moderately dense granular gas in
-dimensions is analyzed in the context of the Enskog kinetic equation. The
Enskog equation neglects velocity correlations but retains spatial correlations
arising from volume exclusion effects. As expected, there is a breakdown of the
Einstein relation relating diffusion and
mobility , being the temperature of the impurity. The kinetic theory
results also show that the violation of the Einstein relation is only due to
the strong non-Maxwellian behavior of the reference state of the impurity
particles. The deviation of from unity becomes more significant as
the solid volume fraction and the inelasticity increase, especially when the
system is driven by the action of a Gaussian thermostat. This conclusion
qualitatively agrees with some recent simulations of dense gases [Puglisi {\em
et al.}, 2007 {\em J. Stat. Mech.} P08016], although the deviations observed in
computer simulations are more important than those obtained here from the
Enskog kinetic theory. Possible reasons for the quantitative discrepancies
between theory and simulations are discussed.Comment: 6 figure
A solvable model of the genesis of amino-acid sequences via coupled dynamics of folding and slow genetic variation
We study the coupled dynamics of primary and secondary structure formation
(i.e. slow genetic sequence selection and fast folding) in the context of a
solvable microscopic model that includes both short-range steric forces and and
long-range polarity-driven forces. Our solution is based on the diagonalization
of replicated transfer matrices, and leads in the thermodynamic limit to
explicit predictions regarding phase transitions and phase diagrams at genetic
equilibrium. The predicted phenomenology allows for natural physical
interpretations, and finds satisfactory support in numerical simulations.Comment: 51 pages, 13 figures, submitted to J. Phys.
Statistical Physics of Irregular Low-Density Parity-Check Codes
Low-density parity-check codes with irregular constructions have been
recently shown to outperform the most advanced error-correcting codes to date.
In this paper we apply methods of statistical physics to study the typical
properties of simple irregular codes.
We use the replica method to find a phase transition which coincides with
Shannon's coding bound when appropriate parameters are chosen.
The decoding by belief propagation is also studied using statistical physics
arguments; the theoretical solutions obtained are in good agreement with
simulations. We compare the performance of irregular with that of regular codes
and discuss the factors that contribute to the improvement in performance.Comment: 20 pages, 9 figures, revised version submitted to JP
High-dimensional order-free multivariate spatial disease mapping
Despite the amount of research on disease mapping in recent years, the use of
multivariate models for areal spatial data remains limited due to difficulties
in implementation and computational burden. These problems are exacerbated when
the number of small areas is very large. In this paper, we introduce an
order-free multivariate scalable Bayesian modelling approach to smooth
mortality (or incidence) risks of several diseases simultaneously. The proposal
partitions the spatial domain into smaller subregions, fits multivariate models
in each subdivision and obtains the posterior distribution of the relative
risks across the entire spatial domain. The approach also provides posterior
correlations among the spatial patterns of the diseases in each partition that
are combined through a consensus Monte Carlo algorithm to obtain correlations
for the whole study region. We implement the proposal using integrated nested
Laplace approximations (INLA) in the R package bigDM and use it to jointly
analyse colorectal, lung, and stomach cancer mortality data in Spanish
municipalities. The new proposal permits the analysis of big data sets and
provides better results than fitting a single multivariate model
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