22,206 research outputs found
Probability distribution of the order parameter
The probability distribution of the order parameter is exploited in order to
obtain the criticality of magnetic systems. Monte Carlo simulations have been
employed by using single spin flip Metropolis algorithm aided by finite-size
scaling and histogram reweighting techniques. A method is proposed to obtain
this probability distribution even when the transition temperature of the model
is unknown. A test is performed on the two-dimensional spin-1/2 and spin-1
Ising model and the results show that the present procedure can be quite
efficient and accurate to describe the criticality of the system.Comment: 5 pages, 7 figures, to appear in Braz. J. Phys. 34, June 200
Vorton Formation
In this paper we present the first analytic model for vorton formation. We
start by deriving the microscopic string equations of motion in Witten's
superconducting model, and show that in the relevant chiral limit these
coincide with the ones obtained from the supersonic elastic models of Carter
and Peter. We then numerically study a number of solutions of these equations
of motion and thereby suggest criteria for deciding whether a given
superconducting loop configuration can form a vorton. Finally, using a recently
developed model for the evolution of currents in superconducting strings we
conjecture, by comparison with these criteria, that string networks formed at
the GUT phase transition should produce no vortons. On the other hand, a
network formed at the electroweak scale can produce vortons accounting for up
to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors.
To appear in Phys. Rev.
Probing the gluon density of the proton in the exclusive photoproduction of vector mesons at the LHC: A phenomenological analysis
The current uncertainty on the gluon density extracted from the global parton
analysis is large in the kinematical range of small values of the Bjorken -
variable and low values of the hard scale . An alternative to reduces this
uncertainty is the analysis of the exclusive vector meson photoproduction in
photon - hadron and hadron - hadron collisions. This process offers a unique
opportunity to constrain the gluon density of the proton, since its cross
section is proportional to the gluon density squared. In this paper we consider
current parametrizations for the gluon distribution and estimate the exclusive
vector meson photoproduction cross section at HERA and LHC using the leading
logarithmic formalism. We perform a fit of the normalization of the
cross section and the value of the hard scale for the process and demonstrate
that the current LHCb experimental data are better described by models that
assume a slow increasing of the gluon distribution at small - and low
.Comment: 8 pages, 6 figures, 1 table. Version published in European Physical
Journal
Clustering Properties of Dynamical Dark Energy Models
We provide a generic but physically clear discussion of the clustering
properties of dark energy models. We explicitly show that in quintessence-type
models the dark energy fluctuations, on scales smaller than the Hubble radius,
are of the order of the perturbations to the Newtonian gravitational potential,
hence necessarily small on cosmological scales. Moreover, comparable
fluctuations are associated with different gauge choices. We also demonstrate
that the often used homogeneous approximation is unrealistic, and that the
so-called dark energy mutation is a trivial artifact of an effective, single
fluid description. Finally, we discuss the particular case where the dark
energy fluid is coupled to dark matter.Comment: 5 page
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